Editing Natural experiment (section)

==Recent examples==

===Family size===
An aim of a study Angrist and Evans (1998)<ref name=angrist:00>{{cite journal |last1=Angrist |first1=J. |author-link=Joshua Angrist |first2=W. |last2=Evans |year=1998 |title=Children and Their Parents' Labor Supply: Evidence from Exogenous Variation in Family Size |journal=[[American Economic Review]] |volume=88 |issue=3 |pages=450–477 |jstor=116844 }}</ref> was to estimate the effect of family size on the labor market outcomes of the mother. For at least two reasons, the correlations between family size and various outcomes (e.g., earnings) do not inform us about how family size causally affects labor market outcomes. First, both labor market outcomes and family size may be affected by unobserved "third" variables (e.g., personal preferences). Second, labor market outcomes themselves may affect family size (called "reverse causality"). For example, a woman may defer having a child if she gets a raise at work. The authors observed that two-child families with either two boys or two girls are substantially more likely to have a third child than two-child families with one boy and one girl. The sex of the first two children, then, constitutes a kind of natural experiment: it is as if an experimenter had randomly assigned some families to have two children and others to have three. The authors were then able to credibly estimate the causal effect of having a third child on labor market outcomes. Angrist and Evans found that childbearing had a greater impact on poor and less educated women than on highly educated women although the earnings impact of having a third child tended to disappear by that child's 13th birthday. They also found that having a third child had little impact on husbands' earnings.<ref name=angrist:00/>

===Game shows===
Within economics, game shows are a frequently studied form of natural experiment. While game shows might seem to be artificial contexts, they can be considered natural experiments due to the fact that the context arises without interference of the scientist. Game shows have been used to study a wide range of different types of economic behavior, such as decision making under risk<ref>{{cite journal |last1=Post |last2=Van den Assem |last3=Baltussen |last4=Thaler |title=Deal or No Deal? Decision Making under Risk in a Large-Payoff Game Show |journal=[[American Economic Review]] |volume=98 |issue=1 |year=2008 |pages=38–71 |jstor=29729963 |ssrn=636508 |doi=10.1257/aer.98.1.38|s2cid=12816022 |url=https://papers.tinbergen.nl/06009.pdf }}</ref> and cooperative behavior.<ref>{{cite journal |last1=van den Assem |last2=van Dolder |last3=Thaler |title=Split or Steal? Cooperative Behavior When the Stakes Are Large |year=2012 |journal=Management Science |volume=58 |issue=1 |pages= 2|doi=10.1287/mnsc.1110.1413 | ssrn=1592456 |s2cid=1371739 |hdl=1765/31292 |url=https://pure.eur.nl/en/publications/03c29bad-5853-4be1-b013-3f30efe3a938 |hdl-access=free }}</ref>

===Smoking ban===
In [[Helena, Montana]] a [[smoking ban]] was in effect in all public spaces, including bars and restaurants, during the six-month period from June 2002 to December 2002. Helena is geographically isolated and served by only one hospital. The investigators observed that the rate of [[myocardial infarction|heart attacks]] dropped by 40% while the smoking ban was in effect. Opponents of the law prevailed in getting the enforcement of the law suspended after six months, after which the rate of heart attacks went back up.<ref name="Sargent_1">{{cite journal |last1=Sargent |first1=R. P. |last2=Shepard |first2=R. M. |last3=Glantz |first3=S. A. |title=Reduced incidence of admissions for myocardial infarction associated with public smoking ban: before and after study |journal=[[British Medical Journal]] |volume=328 |issue=7446 |pages=977–980 |year=2004 |doi=10.1136/bmj.38055.715683.55 |pmid=15066887 |pmc=404491}}</ref> This study was an example of a natural experiment, called a [[crossover study|case-crossover experiment]], where the exposure is removed for a time and then returned. The study also noted its own weaknesses which potentially suggest that the inability to control variables in natural experiments can impede investigators from drawing firm conclusions.'<ref name="Sargent_1" />

===Nuclear weapons testing===

[[Nuclear weapons testing]] released large quantities of radioactive [[isotope]]s into the atmosphere, some of which could be incorporated into biological tissues. The release stopped after the [[Partial Nuclear Test Ban Treaty]] in 1963, which prohibited atmospheric nuclear tests. This resembled a large-scale [[pulse-chase experiment]], but could not have been performed as a regular experiment in humans due to scientific ethics. Several types of observations were made possible (in people born before 1963), such as determination of the rate of replacement for cells in different human tissues.{{cn|date=April 2025}}

===Vietnam War draft===

An important question in economics research is what determines earnings. Angrist (1990) evaluated the effects of military service on lifetime earnings.<ref name = "Angrist">{{cite journal |last=Angrist |first=Joshua D. |year=1990 |title=Lifetime Earnings and the Vietnam Draft Lottery: Evidence from Social Security Administrative Records |journal=[[American Economic Review]] |volume=80 |issue=3 |pages=313–336 |jstor=2006669 }}</ref> Using statistical methods developed in [[econometrics]],<ref>{{Cite web|url=https://www.youtube.com/watch?v=OUN6Gp_H3q4|title=Natural experiments in econometrics|date=4 August 2013 |via=www.youtube.com}}</ref> Angrist capitalized on the approximate [[random assignment]] of the [[draft lottery (1969)|Vietnam War draft lottery]], and used it as an [[instrumental variable]] associated with eligibility (or non-eligibility) for military service. Because many factors might predict whether someone serves in the military, the draft lottery frames a natural experiment whereby those drafted into the military can be compared against those not drafted because the two groups should not differ substantially prior to military service. Angrist found that the earnings of veterans were, on average, about 15 percent less than the earnings of non-veterans.<ref name = "Angrist"/>

===Industrial melanism===

With the [[Industrial Revolution]] in the nineteenth century, many species of moth, including the well-studied [[peppered moth]], responded to the [[atmospheric pollution]] of [[sulphur dioxide]] and [[soot]] around cities with [[industrial melanism]], a dramatic increase in the frequency of dark forms over the formerly abundant pale, speckled forms. In the twentieth century, as regulation improved and pollution fell, providing the conditions for a large-scale natural experiment, the trend towards industrial melanism was reversed, and melanic forms quickly became scarce. The effect led the evolutionary biologists L. M. Cook and J. R. G. Turner to conclude that "[[natural selection]] is the only credible explanation for the overall decline".<ref name=CookTurner2008>{{cite journal |last1=Cook |first1=L. M. |last2=Turner |first2=J. R. G.|title=Decline of melanism in two British moths: spatial, temporal and inter-specific variation |journal=Heredity |date=2008 |volume=101 |issue=6 |pages=483–489 |doi=10.1038/hdy.2008.105 |pmid=18941471 |doi-access=free |bibcode=2008Hered.101..483C }}</ref>