Editing Positive feedback

{{Short description|Feedback loop that increases an initial small effect}}
{{Use British English|date=September 2024}}

[[File:Stampede loop.png|thumb|[[Causal loop diagram]] that depicts the causes of a stampede as a positive feedback loop. Alarm or panic can sometimes be spread by positive feedback among a herd of animals to cause a [[stampede]].]]
[[File:Birmingham Northern Rock bank run 2007.jpg|thumb|right|In sociology a [[network effect]] can quickly create the positive feedback of a [[bank run]]. The above photo is of the [[Nationalisation of Northern Rock#Run on the bank|UK Northern Rock 2007 bank run]].]]

'''Positive feedback''' ('''exacerbating feedback''', '''self-reinforcing feedback''') is a process that occurs in a [[feedback loop]] where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation.<ref name=zuckerman>{{cite book
 |title         = Human Population and the Environmental Crisis
 |author1       = Ben Zuckerman
 |author2       = David Jefferson
 |name-list-style = amp
 |publisher     = Jones & Bartlett Learning
 |year          = 1996
 |isbn          = 9780867209662
 |page          = 42
 |url           = https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
 |url-status       = live
 |archive-url    = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
 |archive-date   = 2018-01-06
}}</ref> That is, '''''A''' produces more of '''B''' which in turn produces more of '''A'''''.<ref name="culturalanthropology2nded">Keesing, R.M. (1981). Cultural anthropology: A contemporary perspective (2nd ed.) p.149. Sydney: Holt, Rinehard & Winston, Inc.</ref> In contrast, a system in which the results of a change act to reduce or counteract it has [[negative feedback]].<ref name=zuckerman/><ref name=theorymodelling/> Both concepts play an important role in science and engineering, including biology, chemistry, and [[cybernetics]].

Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling>
{{cite book
 |title       = Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems
 |author1     = Bernard P. Zeigler
 |author2     = Herbert Praehofer
 |author3     = Tag Gon Kim Section
 |publisher   = Academic Press
 |year        = 2000
 |isbn        = 9780127784557
 |page        = 55
 |section     = 3.3.2 Feedback in continuous systems
 |url         = https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
 |quote       = A positive feedback loop is one with an even number of negative influences <nowiki>[around the loop]</nowiki>.
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170103061121/https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
 |archive-date = 2017-01-03
}}</ref>
That is, positive feedback is [[Phase (waves)|in phase with]] the input, in the sense that it adds to make the input larger.<ref>
{{cite book
 |title       = Newnes Dictionary of Electronics
 |edition     = 4th
 |author1     = S W Amos
 |author2     = R W Amos
 |publisher   = Newnes
 |year        = 2002
 |isbn        = 9780750656429
 |page        = 247
 |url         = https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170329045057/https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
 |archive-date = 2017-03-29
}}</ref><ref>
{{cite book
 |title       = Modern Dictionary of Electronics
 |edition     = 7th
 |author      = Rudolf F. Graf
 |publisher   = Newnes
 |year        = 1999
 |isbn        = 9780750698665
 |page        = 276
 |url         = https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170329122152/https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
 |archive-date = 2017-03-29
}}</ref>
Positive feedback tends to cause [[Control theory#Stability|system instability]]. When the loop gain is positive and above 1, there will typically be [[exponential growth]], increasing [[oscillation]]s, [[chaotic behavior]] or other divergences from [[wikt:equilibrium|equilibrium]].<ref name=theorymodelling/> System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system [[Latch (electronics)|latched]] into a new stable state. Positive feedback may be controlled by signals in the system being [[Filter (signal processing)|filtered]], [[damped]], or [[Maxima and minima|limited]], or it can be cancelled or reduced by adding negative feedback.

Positive feedback is used in [[digital electronics]] to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, [[thermal runaway]] is a type of positive feedback that can destroy [[semiconductor junction]]s. Positive feedback in [[chemical reaction]]s can increase the rate of reactions, and in some cases can lead to [[explosion]]s. Positive feedback in mechanical design causes [[Tipping point (physics)|tipping-point]], or over-centre, mechanisms to snap into position, for example in [[Miniature snap-action switch|switches]] and [[locking pliers]]. Out of control, it can cause [[Tacoma Narrows Bridge (1940)|bridges to collapse]]. Positive feedback in economic systems can cause [[Business cycle|boom-then-bust cycles]].   A familiar example of positive feedback is the loud squealing or howling sound produced by [[audio feedback]] in [[public address system]]s: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.

[[File:Positive Feedback Diagram (2).svg|thumb|[[Platelet]] clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended.  The exponential form of the graph illustrates the positive feedback mechanism. ]]

== Overview ==
Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system [[Gain (electronics)|gain]] is increased.<ref>{{cite web|title=Positive feedback|url=http://www.oxforddictionaries.com/definition/english/positive-feedback|work=Oxford English Dictionary|publisher=Oxford University Press|access-date=15 April 2014|url-status=dead|archive-url=https://web.archive.org/web/20140302160045/http://www.oxforddictionaries.com/definition/english/positive-feedback|archive-date=2 March 2014}}</ref> The feedback from the outcome to the originating process can be direct, or it can be via other state variables.<ref name=theorymodelling/> Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results.<ref name=theorymodelling/> Positive feedback reinforces and negative feedback moderates the original process. ''Positive'' and ''negative'' in this sense refer to [[loop gain]]s greater than or less than zero, and do not imply any [[value judgement]]s as to the desirability of the outcomes or effects.<ref>{{cite web|title=Feedback|url=http://metadesigners.org/Feedback-Glossary|work=Glossary|publisher=Metadesigners Network|access-date=15 April 2014|url-status=live|archive-url=https://web.archive.org/web/20140416183720/http://metadesigners.org/Feedback-Glossary|archive-date=16 April 2014}}</ref>  A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction.

=== Basic ===
[[File:Ideal feedback model.svg|thumb|A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary [[causal system|causal]] functions.]]

A simple feedback loop is shown in the diagram.  If the loop gain AB is positive, then a condition of ''positive'' or ''regenerative'' feedback exists.

If the functions A and B are linear and AB is smaller than unity, then the overall system gain from the input to output is finite but can be very large as AB approaches unity.<ref name=smith>Electronics circuits and devices second edition. Ralph J. Smith</ref>  In that case, it can be shown that the overall or loop gain from input to output is:

:<math>G_c = A/(1-AB)</math>

When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite.

Thus depending on the feedback, state changes can be convergent, or divergent.  The result of positive feedback is to [[wikt:augment|augment]] changes, so that small perturbations may result in big changes.

A system in equilibrium in which there is positive feedback to any change from its current state may be unstable, in which case the system is said to be in an [[unstable equilibrium]]. The magnitude of the forces that act to move such a system away from its equilibrium is an [[increasing function]] of the ''distance'' of the state from the equilibrium.

Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3|s2cid=2954380}}</ref>

=== Hysteresis ===
{{main|Hysteresis}}

[[File:Hysteresis sharp curve.svg|thumb|Hysteresis causes the output value to depend on the history of the input.]]

[[File:Op-Amp Schmitt Trigger.svg|thumb|In a [[Schmitt trigger]] circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.]]

In the real world, positive feedback loops typically do not cause ever-increasing growth but are modified by limiting effects of some sort. According to [[Donella Meadows]]:

::"Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That's why there are so few of them. Usually, a negative loop will kick in sooner or later."<ref name=meadows>
Donella Meadows, [http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf ''Leverage Points: Places to Intervene in a System''] {{webarchive|url=https://web.archive.org/web/20131008160618/http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf |date=2013-10-08 }}, 1999</ref>

Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit.

Once it reaches the limit, it will be stable. However, if the input goes past the limit,{{clarify|date=June 2012}} then the feedback will change sign{{dubious|date=June 2012}} and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows [[bistability|bistable]] behaviour.

== Terminology ==
The terms ''positive'' and ''negative'' were first applied to feedback before [[World War II]]. The idea of positive feedback was already current in the 1920s with the introduction of the [[regenerative circuit]].<ref name=mindell>
{{Cite book
 |first       = David A.
 |last        = Mindell
 |title       = Between Human and Machine : Feedback, Control, and Computing before Cybernetics
 |date        = 2002
 |publisher   = Johns Hopkins University Press
 |location    = Baltimore, MD
 |url         = https://books.google.com/books?id=sExvSbe9MSsC
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=sExvSbe9MSsC
 |archive-date = 2018-01-06
|isbn        = 9780801868955
 }}</ref>

{{harvtxt|Friis|Jensen|1924}} described regeneration in a set of electronic amplifiers as a case where ''the "feed-back" action is positive'' in contrast to negative feed-back action, which they mention only in passing.<ref name="friis">{{Citation |last1=Friis |first1=H. T. |first2=A. G. |last2=Jensen |title=High Frequency Amplifiers |journal=Bell System Technical Journal |volume=3 |issue= 2|date=April 1924 |pages=181–205 |doi=10.1002/j.1538-7305.1924.tb01354.x}}</ref> [[Harold Stephen Black]]'s classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:
::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black>
{{Citation |first=H. S. |last=Black |title=Stabilized feed-back amplifiers |journal=Electrical Engineering |volume=53 |pages=114–120 |date=January 1934 |doi=10.1109/ee.1934.6540374}}</ref>
According to {{harvtxt|Mindell|2002}} confusion in the terms arose shortly after this:
::"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier's gain. In contrast, Nyquist and Bode, when they built on Black's work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."<ref name=mindell/>{{rp|page=121}}

These confusions, along with the everyday associations of positive with ''good'' and negative with ''bad'', have led many systems theorists to propose alternative terms. For example, Donella Meadows prefers the terms ''reinforcing'' and ''balancing'' feedbacks.<ref>{{cite book |last1=Meadows |first1=Donella H. |title=Thinking in systems : a primer |date=2009 |publisher=Earthscan |location=London |isbn=9786000014056}}</ref>

== Examples and applications ==

=== In electronics ===

[[File:Regenerartive Receiver-S7300056.JPG|thumb|right|A vintage style regenerative radio receiver. Due to the controlled use of positive feedback, sufficient amplification can be derived from a single [[vacuum tube]] or valve (centre).]]

[[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country=US |number=1113149 |title=Wireless receiving system |pubdate=1914-10-06}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|access-date=23 September 2010|url-status=live|archive-url=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archive-date=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback.

The oscillation that can break out in a regenerative radio circuit is used in [[electronic oscillator]]s. By the use of [[tuned circuit]]s or a [[piezoelectric]] [[crystal]] (commonly [[quartz]]), the signal that is amplified by the positive feedback remains linear and [[sinusoidal]]. There are several designs for such [[harmonic oscillator]]s, including the [[Armstrong oscillator]], [[Hartley oscillator]], [[Colpitts oscillator]], and the [[Wien bridge oscillator]]. They all use positive feedback to create oscillations.<ref>{{cite web|title=Sinewave oscillators|url=http://www.educypedia.be/electronics/analogosciltypes.htm|work=EDUCYPEDIA - electronics|access-date=23 September 2010|url-status=dead|archive-url=https://web.archive.org/web/20100927094330/http://www.educypedia.be/electronics/analogosciltypes.htm|archive-date=27 September 2010}}</ref>

Many electronic circuits, especially amplifiers, incorporate [[negative feedback]]. This reduces their gain, but improves their linearity, [[input impedance]], [[output impedance]], and [[Bandwidth (signal processing)|bandwidth]], and stabilises all of these parameters, including the loop gain. These parameters also become less dependent on the details of the amplifying device itself, and more dependent on the feedback components, which are less likely to vary with manufacturing tolerance, age and temperature. The difference between positive and negative feedback for [[Alternating current|AC]] signals is one of [[Phase (waves)|phase]]: if the signal is fed back out of phase, the feedback is negative and if it is in phase the feedback is positive. One problem for amplifier designers who use negative feedback is that some of the components of the circuit will introduce [[phase shift]] in the feedback path. If there is a frequency (usually a high frequency) where the phase shift reaches 180°, then the designer must ensure that the amplifier gain at that frequency is very low (usually by [[low-pass filter]]ing). If the loop gain (the product of the amplifier gain and the extent of the positive feedback) at any frequency is greater than one, then the amplifier will oscillate at that frequency ([[Barkhausen stability criterion]]). Such oscillations are sometimes called [[parasitic oscillation]]s. An amplifier that is stable in one set of conditions can break into parasitic oscillation in another. This may be due to changes in temperature, supply voltage, adjustment of front-panel controls, or even the proximity of a person or other conductive item.

Amplifiers may oscillate gently in ways that are hard to detect without an [[oscilloscope]], or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.<ref>{{cite book|last=Self|first=Douglas|title=Audio Power Amplifier Design Handbook|year=2009|publisher=Focal Press|isbn=978-0-240-52162-6|pages=254–255|url=https://books.google.com/books?id=Qpmi4ia2nhcC&pg=PA254|url-status=live|archive-url=https://web.archive.org/web/20140129111458/http://books.google.com/books?id=Qpmi4ia2nhcC&pg=PA254&lpg=PA254|archive-date=2014-01-29}}</ref>

[[File:Smitt hysteresis graph.svg|thumb|right|The effect of using a Schmitt trigger (B) instead of a comparator (A)]]

Many common [[digital electronic]] circuits employ positive feedback. While normal simple Boolean [[logic gate]]s usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent [[Boolean logic|Boolean]] '0' and '1', but many more complex gates use feedback. When an input voltage is expected to vary in an [[Analogue electronics|analogue]] way, but sharp thresholds are required for later digital processing, the [[Schmitt trigger]] circuit uses positive feedback to ensure that if the input voltage creeps gently above the threshold, the output is forced smartly and rapidly from one logic state to the other. One of the corollaries of the Schmitt trigger's use of positive feedback is that, should the input voltage move gently down again past the same threshold, the positive feedback will hold the output in the same state with no change. This effect is called [[hysteresis]]: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value. By reducing the extent of the positive feedback, the hysteresis-width can be reduced, but it can not entirely be eradicated. The Schmitt trigger is, to some extent, a [[Latch (electronics)|latching]] circuit.<ref>{{cite web|title=CMOS Schmitt Trigger—A Uniquely Versatile Design Component|url=http://www.fairchildsemi.com/an/AN/AN-140.pdf|work=Fairchild Semiconductor Application Note 140|publisher=Fairchild Semiconductors|access-date=29 September 2010|year=1975|url-status=live|archive-url=https://web.archive.org/web/20101122185614/http://fairchildsemi.com/an/AN/AN-140.pdf|archive-date=22 November 2010}}</ref>

[[File:Positive feedback bistable switch.svg|thumb|Positive feedback is a mechanism by which an output is enhanced, such as protein levels. However, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I), therefore when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concentration of A increases exponentially if d[A]=k [A].]]

[[File:R-S mk2.gif|thumb|right|Illustration of an R-S ('reset-set') flip-flop made from two digital [[NOR gate|nor]] gates with positive feedback. Red and black mean logical '1' and '0', respectively.]]

An electronic [[flip-flop (electronics)|flip-flop]], or "latch", or "bistable [[multivibrator]]", is a circuit that due to high positive feedback is not stable in a balanced or intermediate state. Such a bistable circuit is the basis of one [[bit]] of electronic [[Computer memory|memory]].  The flip-flop uses a pair of amplifiers, transistors, or logic gates connected to each other so that positive feedback maintains the state of the circuit in one of two unbalanced stable states after the input signal has been removed until a suitable alternative signal is applied to change the state.<ref>{{cite web|last=Strandh|first=Robert|title=Latches and flip-flops|url=http://www.labri.fr/perso/strandh/Teaching/AMP/Common/Strandh-Tutorial/flip-flops.html|publisher=Laboratoire Bordelais de Recherche en Informatique|access-date=4 November 2010|url-status=live|archive-url=https://web.archive.org/web/20110716085637/http://www.labri.fr/perso/strandh/Teaching/AMP/Common/Strandh-Tutorial/flip-flops.html|archive-date=16 July 2011}}</ref> Computer [[random access memory]] (RAM) can be made in this way, with one latching circuit for each bit of memory.<ref>{{cite web|last=Wayne|first=Storr|title=Sequential Logic Basics: SR Flip-Flop|url=http://www.electronics-tutorials.ws/sequential/seq_1.html|publisher=Electronics-Tutorials.ws|access-date=29 September 2010|url-status=live|archive-url=https://web.archive.org/web/20100916114700/http://www.electronics-tutorials.ws/sequential/seq_1.html|archive-date=16 September 2010}}</ref>

[[Thermal runaway]] occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.<ref>{{cite web|last=Sharma|first=Bijay Kumar|title=Analog Electronics Lecture 4 Part C RC coupled Amplifier Design Procedure|url=http://cnx.org/content/m31058/latest/|access-date=29 September 2010|year=2009}}</ref>

[[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]]

[[Sound recording and reproduction|Audio]] and [[video]] systems can demonstrate positive feedback. If a [[microphone]] picks up the amplified sound output of [[loudspeaker]]s in the same circuit, then howling and screeching sounds of [[audio feedback]] (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and [[Filter (signal processing)|filtered]] by the characteristics of the audio system and the room.

===Audio and live music===
[[Audio feedback]] (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a [[microphone]] or [[guitar pickup]]) and an audio output (for example, a loudly-amplified [[loudspeaker]]). In this example, a signal received by the microphone is [[Amplifier|amplified]] and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again.  The [[frequency]] of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small [[PA system]]s the sound is readily recognized as a loud squeal or screech.

Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a [[sound reinforcement system]] or [[PA system]]. [[Audio engineer]]s use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, [[electric guitar]] players in [[rock music]] bands using loud [[guitar amplifier]]s and [[distortion (music)|distortion]] effects have intentionally created guitar feedback to create a desirable musical effect.  "[[I Feel Fine]]" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive [[audio feedback|feedback]] note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."<ref>{{cite book |last=Sheff |first=David |date=2000 |title=All We Are Saying |location=New York, New York |publisher=St. Martin's Press |page=[https://archive.org/details/allwearesayingla00lenn/page/173 173] |isbn=978-0-312-25464-3 |url=https://archive.org/details/allwearesayingla00lenn/page/173 }}</ref>

The principles of audio feedback were first discovered by Danish scientist [[Søren Absalon Larsen]]. Microphones are not the only transducers subject to this effect. [[Phone cartridge]]s can do the same, usually in the low-frequency range below about 100&nbsp;Hz, manifesting as a low rumble. [[Jimi Hendrix]] was an innovator in the intentional use of guitar feedback in his [[guitar solo]]s to create unique sound effects. He helped develop the controlled and musical use of audio feedback in [[electric guitar]] playing,<ref>{{cite book|last = Shadwick|first = Keith|title = Jimi Hendrix, Musician|publisher = [[Backbeat Books]]|year = 2003|page = 92|isbn = 978-0-87930-764-6}}</ref> and later [[Brian May]] was a famous proponent of the technique.<ref>{{cite web|last=May|first=Brian|title=Burns Brian May Tri-Sonic Pickups|url=http://www.brianmayguitars.co.uk/accessories/19|publisher=House Music & Duck Productions|access-date=2 February 2011|url-status=live|archive-url=https://web.archive.org/web/20101120063431/http://brianmayguitars.co.uk/accessories/19|archive-date=20 November 2010}}</ref>

[[File:Adam Savage HOPE.jpg|thumb|right|220px|[[Video feedback]]]]

===Video===
Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''.

=== Switches ===
In [[electrical switch]]es, including [[bimetallic strip]] based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.<ref>{{cite web|title=Positive Feedback and Bistable Systems|url=http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|publisher=University of Washington|quote=* Non-Hysteretic Switches, Memoryless Switches: These systems have no memory, that is, once the input signal is removed, the system returns to its original state. * Hysteretic Switches, Bistability: Bistable systems, in contrast, have memory. That is, when switched to one state or another, these systems remain in that state unless forced to change back. The light switch is a common example of a bistable system from everyday life. All bistable systems are based around some form of positive feedback loop.|url-status=live|archive-url=https://web.archive.org/web/20150413020657/http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|archive-date=2015-04-13}}</ref>

=== In biology ===

[[File:Positive Feedback- Childbirth (1).svg|thumb|Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin]](3). Oxytocin is then carried via the bloodstream to the [[uterus]] (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]]

==== In physiology ====

A number of examples of positive feedback systems may be found in [[physiology]].
* One example is the onset of [[Contraction (childbirth)|contractions]] in [[childbirth]], known as the [[Ferguson reflex]]. When a contraction occurs, the hormone [[oxytocin]] causes a nerve stimulus, which stimulates the [[hypothalamus]] to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in [[amplitude]] and [[frequency]].<ref name=Guyton1991>Guyton, Arthur C. (1991) ''Textbook of Medical Physiology''. (8th ed). Philadelphia: W.B. Saunders. {{ISBN|0-7216-3994-1}}</ref>{{rp|pages=924–925}}
* Another example is the process of [[blood clotting]]. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.<ref name=Guyton1991/>{{rp|pages=392–394}}
* [[Lactation]] also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the [[pituitary]] gland to produce more [[prolactin]] to produce more milk.<ref name=Guyton1991/>{{rp|page=926}}
* A spike in [[estrogen]] during the [[follicular phase]] of the menstrual cycle causes [[ovulation]].<ref name=Guyton1991/>{{rp|page=907}}
* The generation of [[nerve signal]]s is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on ([[Hodgkin cycle]]). So a slight initial leakage results in an explosion of sodium leakage which creates the nerve [[action potential]].<ref name=Guyton1991/>{{rp|page=59}}
* In [[excitation–contraction coupling]] of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which transport calcium out into the cytosol in a positive feedback physiological response.

In most cases, such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.<ref name=Guyton1991/>

==== In gene regulation ====

Positive feedback is a well-studied phenomenon in gene regulation, where it is most often associated with [[bistability]]. Positive feedback occurs when a gene activates itself directly or indirectly via a double negative feedback loop. Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability.<ref name=Hasty2002/> A classic example of positive feedback is the [[lac operon]] in ''E. coli''. Positive feedback plays an integral role in cellular differentiation, development, and cancer progression, and therefore, positive feedback in gene regulation can have significant physiological consequences. Random motions in [[molecular dynamics]] coupled with positive feedback can trigger interesting effects, such as create population of phenotypically different cells from the same parent cell.<ref name=Veening2008/> This happens because noise can become amplified by positive feedback. Positive feedback can also occur in other forms of [[cell signaling]], such as enzyme kinetics or metabolic pathways.<ref name=Christoph2001/>

==== In evolutionary biology ====

Positive feedback loops have been used to describe aspects of the dynamics of change in biological [[evolution]].  For example, beginning at the macro level, [[Alfred J. Lotka]] (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems.<ref name=Lotka1945/>  At the human level, [[Richard D. Alexander]] (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence.<ref name=Alexander1989/> [[Bernard Crespi|Crespi]] (2004) discussed several other examples of positive feedback loops in evolution.<ref name=Crespi2004/>  The analogy of [[evolutionary arms race]]s provides further examples of positive feedback in biological systems.<ref name=Blindwatchmaker/>

[[File:Phanerozoic Biodiversity.svg|300px|right|thumb|During the Phanerozoic the [[biodiversity]] shows a steady but not monotonic increase from near zero to several thousands of genera.]]
It has been shown that changes in [[biodiversity]] through the [[Phanerozoic]] correlate much better with hyperbolic model (widely used in [[demography]] and [[macrosociology]]) than with [[Exponential growth|exponential]] and [[Logistic function|logistic]] models (traditionally used in [[population biology]] and extensively applied to [[fossil]] [[biodiversity]] as well). The latter models imply that changes in diversity are guided by first-order positive feedback (more ancestors, more descendants) or a [[negative feedback]] arising from resource limitation. The hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the [[world population growth]] has been demonstrated (see below) to arise from second-order positive feedback between the population size and the rate of [[technological growth]]. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of [[biodiversity]] and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.<ref>{{Cite journal |author-link=Andrey Korotayev |doi=10.1016/j.palwor.2007.01.002 |title=Phanerozoic marine biodiversity follows a hyperbolic trend |date=2007 |last1=Markov |first1=Alexander V. |last2=Korotayev |first2=Andrey V. |journal=Palaeoworld |volume=16 |issue=4 |pages=311–318 }}</ref><ref>{{cite journal | last1 = Markov | first1 = A. | last2 = Korotayev | first2 = A. | year = 2008 | title = Hyperbolic growth of marine and continental biodiversity through the Phanerozoic and community evolution | url = http://elementy.ru/genbio/abstracts?artid=177 | journal = Journal of General Biology | volume = 69 | issue = 3 | pages = 175–194 | pmid = 18677962 | url-status = live | archive-url = https://web.archive.org/web/20091225000305/http://elementy.ru/genbio/abstracts?artid=177 | archive-date = 2009-12-25 }}</ref>

==== Immune system ====

A [[cytokine storm]], or '''hypercytokinemia''' is a potentially fatal immune reaction consisting of a positive feedback loop between [[cytokine]]s and [[immune cell]]s, with highly elevated levels of various cytokines.<ref name="osterholm">{{cite journal | last = Osterholm | first = Michael T. | author-link = Michael Osterholm |title = Preparing for the Next Pandemic | journal = The New England Journal of Medicine | volume = 352 | issue = 18 | pages = 1839–1842 | date = 2005-05-05 | doi = 10.1056/NEJMp058068   | pmid = 15872196 | citeseerx = 10.1.1.608.6200 | s2cid = 45893174 }}</ref> In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.<ref>{{cite journal|last=Paul|first=William E.|title=Infectious Diseases and the Immune System|journal=Scientific American|volume=269|issue=3|date=September 1993|page=93|bibcode=1993SciAm.269c..90P|doi=10.1038/scientificamerican0993-90|pmid=8211095}}</ref>

==== Cell death ====

[[Apoptosis]] is a [[caspase]]-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as [[cancer]] or [[Parkinson's disease]]. The very core of the apoptotic process is the auto-activation of caspases, which may be modelled via a positive-feedback loop. This positive feedback exerts an auto-activation of the [[effector caspase]] by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.<ref name="ReferenceA"/> When this core process is complemented with inhibitors and enhancers of caspases effects, this process presents bistability, thereby modelling the alive and dying states of a cell.<ref>{{cite journal|last=Eissing|first=Thomas |doi=10.1074/jbc.M404893200 |title=Bistability analyses of a caspase activation model for receptor-induced apoptosis|journal=Journal of Biological Chemistry|volume=279 |issue=35 |date=2014|pages=36892–36897|pmid=15208304 |doi-access=free}}</ref>

=== In psychology ===

Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on.<ref name=Winner1996/>  Winner termed this positive feedback loop as a ''rage to master''.  Vandervert (2009a, 2009b) proposed that the [[child prodigy]] can be explained in terms of a positive feedback loop between the output of thinking/performing in [[working memory]], which then is fed to the [[cerebellum]] where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory.<ref name=Vandervert2009a/><ref name=Vandervert2009b/>  Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for [[language]] evolution in working memory.

=== In economics ===

====Markets with social influence====
Product recommendations and information about past purchases have been shown to influence consumers' choices significantly whether it is for music, movie, book, technological, and other type of products. Social influence often induces a rich-get-richer phenomenon ([[Matthew effect]]) where popular products tend to become even more popular.<ref name="altszyler2017">{{cite journal | title= Transient dynamics in trial-offer markets with social influence: Trade-offs between appeal and quality. | author1= Altszyler, E | author2= Berbeglia, F. | author3= Berbeglia, G. | author4= Van Hentenryck, P. | journal= PLOS ONE | year= 2017 | volume= 12 | issue= 7 | doi=10.1371/journal.pone.0180040 |pmid = 28746334| pmc= 5528888 | page=e0180040| bibcode= 2017PLoSO..1280040A | doi-access= free }}</ref>

====Market dynamics====
According to the theory of [[reflexivity (social theory)|reflexivity]] advanced by [[George Soros]], price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.<ref>{{citation |title=Behavioural Technical Analysis |first=Paul V. |last=Azzopardi |publisher=Harriman House Limited |year=2010 |page=116 |isbn=9780857190680 |url=https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116 |url-status=live |archive-url=https://web.archive.org/web/20170329103058/https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116&source=bl&hl=en&sa=X&f=false |archive-date=2017-03-29 }}</ref>

==== In social media ====

Programs such as [[Facebook]] and [[Twitter]] depend on positive feedback to create interest in topics and drive the take-up of the media.<ref>{{cite report |title=Emergent Instabilities in Algorithmic Feedback Loops
|first1=Keith |last1=Burghardt |first2=Kristina |last2=Lerman
|publisher=Cornell University
|date=18 Jan 2022|arxiv=2201.07203
}}</ref><ref>{{cite news |title= The biggest problem with social media has nothing to do with free speech | first=Mike|last=Loukides|date= September 24, 2019 |publisher=Gizmodo |url=https://qz.com/1714598/information-feedback-loops-make-social-media-more-dangerous/ }}</ref>
In the age of smartphones and social media, the feedback loop has created a craze for virtual validation in the form of likes, shares, and FOMO (fear of missing out).<ref>{{cite news |title=Social Media And The Dopamine Feedback Loop: Here's How It Affects You |first=Dorcas |last=Benewaa |url=https://www.digitaltimes.africa/social-media-and-the-dopamine-feedback-loop-heres-how-it-affects-you/ |publisher=Digital Times |date=May 7, 2021 |archive-date=September 30, 2022 |access-date=September 30, 2022 |archive-url=https://web.archive.org/web/20220930202015/https://www.digitaltimes.africa/social-media-and-the-dopamine-feedback-loop-heres-how-it-affects-you/ |url-status=dead }}</ref>
This is intensified by the use of bots which are designed to respond to particular words or themes and transmit posts more widely.
<ref>{{cite news |title= Can We Avoid the Feedback Loop of Social Media? | first=Jayne|last=Reardon|date= Dec 14, 2017 |publisher=2Civility |url=https://www.2civility.org/avoid-feedback-loop-social-media/}}</ref>

What is called negative feedback in social media should often be regarded as positive feedback in this context. Outrageous statements and negative comments often produce much more feedback than positive comments.

==== Systemic risk ====

[[Systemic risk]] is the risk that an amplification or leverage or positive feedback process presents to a system. This is usually unknown, and under certain conditions, this process can amplify exponentially and rapidly lead to destructive or [[Chaos theory|chaotic]] behaviour.  A [[Ponzi scheme]] is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse. [[W. Brian Arthur]] has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990).<ref>{{cite journal | last1 = Arthur | first1 = W. Brian | year = 1990 | title = Positive Feedbacks in the Economy | journal = Scientific American | volume = 262 | issue = 2| page = 80 | doi = 10.1038/scientificamerican0290-92 | bibcode = 1990SciAm.262b..92A }}</ref> [[Hyman Minsky]] proposed a theory that certain credit expansion practices could make a market economy into "a deviation amplifying system" that could suddenly collapse,<ref name="working_paper">[http://www.levy.org/pubs/wp74.pdf The Financial Instability Hypothesis] {{webarchive|url=https://web.archive.org/web/20091009190421/http://www.levy.org/pubs/wp74.pdf |date=2009-10-09 }} by Hyman P. Minsky, Working Paper No. 74, May 1992, pp. 6–8</ref> sometimes called a [[Minsky moment]].

Simple systems that clearly separate the inputs from the outputs are not prone to [[systemic risk]].  This risk is more likely as the complexity of the system increases because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions.  The more efficient a complex system is, the more likely it is to be prone to systemic risks because it takes only a small amount of deviation to disrupt the system.  Therefore, well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities.

The [[2010 Flash Crash]] incident was blamed on the practice of [[high-frequency trading]] (HFT),<ref name="flashreport">{{cite web|title=Findings Regarding the Market Events of May 6, 2010|url=https://www.sec.gov/news/studies/2010/marketevents-report.pdf|date=2010-09-30|url-status=live|archive-url=https://web.archive.org/web/20170815000431/https://www.sec.gov/news/studies/2010/marketevents-report.pdf|archive-date=August 15, 2017}}</ref> although whether HFT really increases systemic risk remains controversial.{{Citation needed|date=June 2014}}

==== Human population growth ====
{{Main|Human population growth}}
Agriculture and human population can be considered to be in a positive feedback mode,<ref name=Brown2003>{{citation|author= Brown, A. Duncan |year=2003 |title= Feed or Feedback: Agriculture, Population Dynamics and the State of the Planet |publisher= International Books |place=Utrecht  |isbn=978-90-5727-048-2}}</ref> which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to [[systemic risk]].

Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent [[hyperbolic growth]] of the human population in the past, instead of a simpler [[exponential growth]].<ref>
{{cite journal | doi = 10.1016/j.ecolmodel.2010.03.028 | volume=221 | title=On the reasons of hyperbolic growth in the biological and human world systems | year=2010 | journal=Ecological Modelling | pages=1702–1709 | last1 = Dolgonosov | first1 = B.M.| issue=13–14 | bibcode=2010EcMod.221.1702D }}</ref>
It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology.<ref name=mgc>
[[Andrey Korotayev|Korotayev A.]] [https://www.academia.edu/32810527/Compact_Mathematical_Models_of_World_System_Development._In_Globalization_as_Evolutionary_Process._London_Routledge_2008._P._133_160 Compact Mathematical Models of World System Development, and How they can Help us to Clarify our Understanding of Globalization Processes] {{webarchive|url=https://web.archive.org/web/20180106192003/http://www.academia.edu/32810527/Compact_Mathematical_Models_of_World_System_Development._In_Globalization_as_Evolutionary_Process._London_Routledge_2008._P._133_160 |date=2018-01-06 }}. ''Globalization as Evolutionary Process: Modeling Global Change''. Edited by [[George Modelski]], [[Tessaleno Devezas]], and William R. Thompson. London: [[Routledge]], 2007. P. 133-160.</ref>{{rp|page=133–160}} Technological growth increases the [[carrying capacity]] of land for people, which leads to a growing population, and this in turn drives further technological growth.<ref name=mgc/>{{rp|page=146}}<ref>[https://www.academia.edu/35548090/Mathematical_Model_of_the_World_System_Growth Korotayev, A. V., & Malkov, A. S. A Compact Mathematical Model of the World System Economic and Demographic Growth, 1 CE–1973 CE // INTERNATIONAL JOURNAL OF MATHEMATICAL MODELS AND METHODS IN APPLIED SCIENCES Volume 10, 2016. P. 200-209] {{webarchive|url=https://web.archive.org/web/20180106192002/http://www.academia.edu/35548090/Mathematical_Model_of_the_World_System_Growth |date=2018-01-06 }}.</ref>

==== Prejudice, social institutions and poverty ====

[[Gunnar Myrdal]] described a [[vicious circle]] of increasing inequalities, and poverty, which is known as [[circular cumulative causation]].<ref>{{cite web|last=Berger|first=Sebastian|title=Circular Cumulative Causation (CCC) à la Myrdal and Kapp — Political Institutionalism for Minimizing Social Costs|url=http://www.kwilliam-kapp.de/pdf/Circular%20Cumulative%20Causation%20a%20la%20Myrdal%20&%20Kapp.pdf|access-date=26 November 2011|url-status=live|archive-url=https://web.archive.org/web/20120426002431/http://www.kwilliam-kapp.de/pdf/Circular%20Cumulative%20Causation%20a%20la%20Myrdal%20%26%20Kapp.pdf|archive-date=26 April 2012}}</ref>

James Moody, Assistant Professor at [[Ohio State University]], states that students who [[Self-segregation|self-segregate]] or grow up in segregated environments have "little meaningful exposure to other races because they never form relationships with students of another race...[; as a result,...] they are viewing other racial groups at a social distance, which can bolster stereotypes," which ultimately causes a positive feedback loop in which segregated groups become more prejudiced, polarized, and segregated against each other, similar to that of [[political polarization]].<ref>{{Cite web |date=2002-05-28 |title=Teen Friendships More Racially Segregated at Moderately Diverse Schools: Integrated Friendships More Likely at Highly Diverse Schools {{!}} NICHD - Eunice Kennedy Shriver National Institute of Child Health and Human Development |url=https://www.nichd.nih.gov/newsroom/releases/self_segregation |access-date=2024-12-23 |website=www.nichd.nih.gov |language=en}}</ref>

=== In meteorology ===

[[Drought]] intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants or causes them to release less water through [[transpiration]]. Both factors limit [[evapotranspiration]], the process by which water vapour is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water. Less water vapour means both low [[dew point]] temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete.<ref>{{cite book|editor1=S.-Y. Simon Wang|editor2=Jin-Ho Yoon|editor3=Christopher C. Funk|editor4=Robert R. Gillies|title=Climate Extremes: Patterns and Mechanisms|date=2017|publisher=Wiley|isbn=9781119068037|pages=81–82|url=https://books.google.com/books?id=dVQrDwAAQBAJ&pg=PA82}}</ref>

=== In climatology ===
{{See also|Climate change feedback|Runaway greenhouse effect}}
{{multiple image | total_width=500
| image1= Earth Energy Budget with GHE.svg |caption1= [[Earth's energy balance]] between space, the atmosphere, and Earth's surface. Human-caused increases in greenhouse gases [[greenhouse effect|stimulate]] positive feedback in [[global warming]].
| image2= 20220726 Feedbacks affecting global warming and climate change - block diagram.svg |caption2= Some effects of global warming can either enhance (positive feedbacks) or inhibit ([[negative feedback]]s) warming.<ref name=NASA_IntegratedSystem>{{cite web |title=The Study of Earth as an Integrated System |url=https://climate.nasa.gov/nasa_science/science/ |website=nasa.gov |publisher=NASA |date=2016 |archive-url=https://web.archive.org/web/20161102022200/https://climate.nasa.gov/nasa_science/science/ |archive-date=November 2, 2016 |url-status=live }}</ref><ref name=IPCC_AR6_SGI_FigTS.17>Fig. TS.17, ''[https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary],'' Sixth Assessment Report (AR6), Working Group I, IPCC, 2021, p. 96. [https://web.archive.org/web/20220721021347/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Archived] from the original on July 21, 2022.</ref>
}}
Climate ''forcings'' may push a climate system in the direction of warming or cooling,<ref>{{citation | author=US NRC | year=2012 | title=Climate Change: Evidence, Impacts, and Choices | url=https://www.scribd.com/doc/98458016/Climate-Change-Lines-of-Evidence | publisher=US National Research Council (US NRC) | url-status=live | archive-url=https://web.archive.org/web/20160503153121/https://www.scribd.com/doc/98458016/Climate-Change-Lines-of-Evidence | archive-date=2016-05-03 }}, p.9. Also available as [http://nas-sites.org/americasclimatechoices/files/2012/06/19014_cvtx_R1.pdf PDF] {{webarchive|url=https://web.archive.org/web/20130220184517/http://nas-sites.org/americasclimatechoices/files/2012/06/19014_cvtx_R1.pdf |date=2013-02-20 }}</ref> for example, increased atmospheric concentrations of [[greenhouse gas]]es cause warming at the surface. Forcings are external to the climate system and feedbacks are internal processes of the system. Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled.<ref>[http://www.nap.edu/openbook.php?record_id=10850&page=16 ''Understanding Climate Change Feedbacks,'' U.S. National Academy of Sciences] {{webarchive|url=https://web.archive.org/web/20120210122555/http://www.nap.edu/openbook.php?record_id=10850&page=16 |date=2012-02-10 }}</ref> Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes. The main positive feedback in [[global warming]] is the tendency of warming to increase the amount of water vapour in the atmosphere, which in turn leads to further warming.<ref>{{cite web |url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6-3-1.html |title=8.6.3.1 Water Vapour and Lapse Rate - AR4 WGI Chapter 8: Climate Models and their Evaluation |access-date=2010-04-23 |url-status=dead |archive-url=https://web.archive.org/web/20100409130123/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch8s8-6-3-1.html |archive-date=2010-04-09 }}</ref> The main negative feedback comes from the [[Stefan–Boltzmann law]], the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere.

Other examples of positive feedback subsystems in climatology include:
* A warmer atmosphere melts ice, changing the [[albedo]] (surface reflectivity), which further warms the atmosphere.
* Methane hydrates can be unstable so that a warming ocean could release more [[methane]], which is also a greenhouse gas. 
* [[Peat]], occurring naturally in [[peat bog]]s, contains carbon.  When peat dries it [[decomposes]], and may additionally burn. Peat also releases [[nitrous oxide]].
* Global warming affects the cloud distribution. Clouds at higher altitudes enhance the greenhouse effects, while low clouds mainly reflect back sunlight, having opposite effects on temperature.

The [[Intergovernmental Panel on Climate Change]] (IPCC) [[Fourth Assessment Report]] states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."<ref>{{cite journal |url=http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf |title=Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Pg 53 |author=IPCC |url-status=live |archive-url=https://web.archive.org/web/20100209153113/http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf |archive-date=2010-02-09 }}</ref>

=== In sociology ===

A [[self-fulfilling prophecy]] is a social positive feedback loop between beliefs and behaviour: if enough people believe that something is true, their behaviour can make it true, and observations of their behaviour may in turn increase belief. A classic example is a [[bank run]].

Another sociological example of positive feedback is the [[network effect]]. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A [[viral video]] is an example of the network effect in which [[hyperlink|links]] to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including [[Ponzi scheme]]s and [[chain letter]]s. In many cases, population size is the limiting factor to the feedback effect.

=== In political science ===

In politics, institutions can reinforce norms, which can subsequently be a source of positive feedback. This rationale is frequently utilized to comprehend public policy processes, which may be dissected into a sequence of events. Self-reinforcing processes are understood to be affected by positive feedback mechanisms (e.g., supportive policy constituencies).<ref>Falleti, T. G., & Mahoney, J. (2015). The comparative sequential method. In J. Mahoney & K. Thelen (Eds.), Advances in Comparative-Historical Analysis (pp. 211–239). chapter, Cambridge: Cambridge University Press.</ref>  Conversely, unsuccessful policy processes encounter negative feedback mechanisms (e.g., veto points with veto power).<ref>Tsebelis, George. 2002. Veto Players: How Political Institutions Work. Princeton, N.J.: Princeton University Press.</ref> 

A comparative illustration of policy feedback can be observed in the economic foreign policies of Brazil and China, particularly in their execution of state capitalism tactics during the 1990s and 2000s.<ref>Do Vale, Helder Ferreira and Costa, Lilian (2024). “State capitalism in a changing global order: Brazil and China’s strategies for greater global influence”. Research in Globalization, Volume 9, 100265, ISSN 2590-051X, https://doi.org/10.1016/j.resglo.2024.100265.</ref> Although both nations initially embraced similar state capitalist ideas, their paths in executing economic policies diverged over time due to distinct incentives. In China, a positive feedback mechanism reinforced previous policies, whereas in Brazil, negative feedback mechanisms compelled the country to abandon state capitalism policies and dynamics.

=== In chemistry ===

If a chemical reaction causes [[Exothermic reaction|the release of heat]], and the reaction itself [[Reaction rate|happens faster]] at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, [[thermal runaway]] can occur and very quickly lead to a chemical [[explosion]].

=== In conservation ===

Many wildlife are hunted for their parts which can be quite valuable. The closer to extinction that targeted species become, the higher the price there is on their parts.<ref>{{cite journal |doi=10.1016/j.jtbi.2017.06.019 |pmid=28669883 |title=High prices for rare species can drive large populations extinct: The anthropogenic Allee effect revisited |journal=Journal of Theoretical Biology |volume=429 |pages=170–180 |year=2017 |last1=Holden |first1=Matthew H. |last2=McDonald-Madden |first2=Eve |arxiv=1703.06736 |bibcode=2017JThBi.429..170H |s2cid=4877874 }}</ref>

== See also ==
{{div col}}
* {{annotated link|Chain reaction}}
* {{annotated link|Twelve leverage points|Donella Meadows' twelve leverage points to intervene in a system}}
* {{annotated link|Hyperbolic growth}}
* {{annotated link|Reflexivity (social theory)}}
* {{annotated link|Stability criterion}}
* {{annotated link|Strategic complements}}
* {{annotated link|System dynamics}}
* {{annotated link|Technological singularity}}
* {{annotated link|Thermal runaway}}
* {{annotated link|Negative feedback}}
* {{annotated link|Virtuous circle and vicious circle|Vicious/virtuous circle}}: in social and financial systems, a complex of events that reinforces itself through a feedback loop.
* Positive [[reinforcement]]: a situation in [[operant conditioning]] where a consequence increases the frequency of a behaviour.
* Praise of performance: a term often applied in the context of [[performance appraisal]],<ref>''Positive feedback occurs when one is told he has done something well or correctly.''
Tom Coens and Mary Jenkins, "Abolishing Performance Appraisals", p116.</ref> although this usage is disputed<!--.<ref>''..."positive feedback" does not mean "praise" and "negative feedback" does not mean "criticism". Positive feedback denotes a self-reinforcing process ... Telling someone your opinion does not constitute feedback unless they act on your suggestions and thus lead you to revise your view.''
John D.Sterman, Business Dynamics: Systems Thinking and Modeling for a Complex World McGraw Hill/Irwin, 2000. p14. {{ISBN|978-0-07-238915-9}}</ref>-->
* Self-reinforcing feedback: a term used in [[systems dynamics]] to avoid confusion with the "praise" usage<!--<ref name="Senge">
{{cite book
|author=Peter M. Senge
|title=The Fifth Discipline: The Art and Practice of the Learning Organization
|url=https://archive.org/details/fifthdisciplineasen00seng
|url-access=registration
|year= 1990
|publisher=Doubleday
|location=New York
|isbn=0-385-26094-6
|pages=[https://archive.org/details/fifthdisciplineasen00seng/page/424 424]
}}</ref>-->
* {{annotated link|Matthew effect}}
* {{annotated link|Self-fulfilling prophecy}}
* 
* {{annotated link|Autocatalysis}}
* {{annotated link|Meander}}
{{div col end}}

== References ==
{{Reflist|30em|refs=

<ref name="Alexander1989">[[Richard D. Alexander|Alexander, R.]] (1989). Evolution of the human psyche.  In P. Millar & C. Stringer (Eds.), The human revolution: Behavioral and biological perspectives on the origins of modern humans (pp. 455-513). Princeton: Princeton University Press.</ref>

<ref name="Blindwatchmaker">Dawkins, R. 1991. ''[[The Blind Watchmaker]]'' London: Penguin. Note: W.W. Norton also published this book, and some citations may refer to that publication. However, the text is identical, so it depends on which book is at hand</ref>

<ref name="Crespi2004">{{cite journal | last1 = Crespi | first1 = B. J. | author-link=Bernard Crespi | year = 2004 | title = Vicious circles: positive feedback in major evolutionary and ecological transitions | journal = Trends in Ecology and Evolution | volume = 19 | issue = 12| pages = 627–633 | doi=10.1016/j.tree.2004.10.001| pmid = 16701324 }}</ref>

<ref name="Lotka1945">{{cite journal | last1 = Lotka | first1 = A | year = 1945 | title = The law of evolution as a maximal principle | journal = Human Biology | volume = 17 | pages = 168–194 }}</ref>

<ref name="Vandervert2009a">Vandervert, L. (2009a). Working memory, the cognitive functions of the cerebellum and the child prodigy. In L.V. Shavinina (Ed.), International handbook on giftedness (pp. 295-316). The Netherlands: Springer Science.</ref>

<ref name="Vandervert2009b">{{cite journal |last=Vandervert |first=L. |year=2009b |title=The emergence of the child prodigy 10,000 years ago: An evolutionary and developmental explanation |journal=[[Journal of Mind and Behavior]] |volume=30 |issue=1–2 |pages=15–32 }}</ref>

<ref name="Winner1996">{{cite book |last=Winner |first=E. |year=1996 |title=Gifted children: Myths and Realities |location=New York |publisher=Basic Books |isbn=978-0465017607 |url=https://archive.org/details/giftedchildrenmy00winn }}</ref>

<ref name="Hasty2002">{{cite journal |last1=Hasty |first1=J. |last2=McMillen |first2=D. |last3=Collins |first3=J. J. |year=2002 |title=Engineered gene circuits |journal=[[Nature (journal)|Nature]] |volume=420 |issue=6912 |pages=224–230 |doi=10.1038/nature01257 |pmid=12432407 |bibcode=2002Natur.420..224H |doi-access=free }}</ref>

<ref name="Christoph2001">{{cite journal |last1=Bagowski |first1=C. P. |last2=Ferrell |first2=J. E. |year=2001 |title=Bistability in the JNK cascade |journal=[[Current Biology]] |volume=11 |issue=15 |pages=1176–1182 |doi=10.1016/S0960-9822(01)00330-X |pmid=11516948 |s2cid=526628 |doi-access=free |bibcode=2001CBio...11.1176B }}</ref>

<ref name="Veening2008">{{cite journal |last1=Veening |first1=J. |last2=Smits |first2=W. K. |last3=Kuipers |first3=O. P. |title=Bistability, Epigenetics, and Bet-Hedging in Bacteria |journal=[[Annual Review of Microbiology]] |volume=62 |issue=1 |pages=193–210 |doi=10.1146/annurev.micro.62.081307.163002 |pmid=18537474 |year=2008|hdl=11370/59bec46a-4434-4eaa-aaae-03461dd02bbb |s2cid=3747871 |url=https://www.rug.nl/research/portal/files/6724790/2008AnnuRevMicrobiolVeening.pdf }}</ref>
}}

== Further reading ==
* [[Norbert Wiener]] (1948), ''Cybernetics or Control and Communication in the Animal and the Machine'', Paris, Hermann et Cie - MIT Press, Cambridge, MA.
* Katie Salen and Eric Zimmerman. ''Rules of Play''. [[MIT Press]]. 2004. {{ISBN|0-262-24045-9}}. Chapter 18: Games as Cybernetic Systems.

== External links ==
* {{Wikiquote-inline}}

{{Systems science}}
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{{DEFAULTSORT:Positive Feedback}}
[[Category:Classical control theory]]
[[Category:Cybernetics]]
[[Category:Electronic feedback]]
[[Category:Feedback]]

[[fr:Rétroaction]]