Editing Flynn effect (section)

==Proposed explanations==
{{See also|Impact of health on intelligence}}

===Schooling and test familiarity===
The duration of average schooling has increased steadily. However, a criticism of this explanation is that if (in the United States) older and younger subjects, with similar educational levels, are compared together, then the IQ gains appear almost undiminished in each group compared to when they are considered individually.<ref name="Neisser97"/>

Many studies find that children who do not attend school score drastically lower on the tests than their regularly attending peers. During the 1960s, when some [[Virginia]] counties [[Massive resistance|closed their public schools to avoid racial integration]], compensatory private schooling was available only for White children. On average, the scores of African-American children who received no formal education during that period decreased at a rate of about six IQ points per year.<ref name="Neisser97"/>

Another explanation is an increased familiarity of the general population with tests and testing. For example, children who take the very same IQ test a second time usually gain five or six points. However, this seems to set an upper limit on the effects of test sophistication. One problem with this explanation and others related to schooling is that in the US, the groups with greater test familiarity show smaller IQ increases.<ref name="Neisser97"/>

[[Early intervention]] programs have shown mixed results. Some preschool (ages 3–4) intervention programs like "[[Head Start Program|Head Start]]" do not produce lasting changes of IQ, although they may confer other benefits.{{which|date=July 2014}} The "[[Abecedarian Early Intervention Project]]", an all-day program that provided various forms of [[Environmental enrichment (neural)|environmental enrichment]] to children from infancy onward, showed IQ gains that did not diminish over time. The IQ gains in the experimental group compared to the control group was 4.4 points. These gains persisted until at least age 21.<ref>{{cite book |author1=Plomin R. |author1-link=Robert Plomin|author2=DeFries J.C. |author3=Craig I.W. |author4=McGuffin P. |title=Behavioral genetics in the postgenomic era |year=2003 |edition=4th }}</ref>

Citing a high correlation between rising literacy rates and gains in IQ, [[David Marks (psychologist)|David Marks]] has argued that the Flynn effect is caused by changes in literacy rates.<ref>{{Cite journal |last=Marks |first=David Francis |date=2010-06-01 |title=IQ Variations across Time, Race, and Nationality: An Artifact of Differences in Literacy Skills |url=https://journals.sagepub.com/doi/10.2466/pr0.106.3.643-664 |journal=Psychological Reports |language=en |volume=106 |issue=3 |pages=643–664 |doi=10.2466/pr0.106.3.643-664 |pmid=20712152 |issn=0033-2941 |via=[[Sage Publishing]]}}</ref>

===Nutrition===
{{See also|Iodine deficiency#Deficient populations}}
Improved nutrition is another possible explanation. Today's average adult from an industrialized nation is taller than a comparable adult of a century ago. That increase of stature, likely the result of general improvements in nutrition and health, has been at a rate of more than a centimeter per decade. Available data suggest that these gains have been accompanied by analogous increases in head size, and by an increase in the average size of the brain.<ref name="Neisser97"/><ref>{{cite journal |last1=Jantz |first1=R. |last2=Meadows Jantz |first2=L. |year=2000 |title=Secular change in craniofacial morphology |journal=American Journal of Human Biology |volume=12 |issue= 3|pages=327–38 |doi=10.1002/(SICI)1520-6300(200005/06)12:3<327::AID-AJHB3>3.0.CO;2-1 |pmid=11534023|s2cid=22059721 |doi-access=free }}</ref> This argument had been thought to suffer the difficulty that groups who tend to be of smaller overall body size (e.g. women, or people of Asian ancestry) do not have lower average IQs.<ref name=TO1987/>

A 2005 study presented data supporting the nutrition hypothesis, which predicts that gains will occur predominantly at the low end of the IQ distribution, where nutritional deprivation is probably most severe.<ref name="Colom2005"/> An alternative interpretation of [[skewed]] IQ gains could be that improved education has been particularly important for this group.<ref name=TO1987/>

A century ago, [[micronutrient|nutritional]] deficiencies may have limited body and organ functionality, including skull volume. The first two years of life are a critical time for nutrition. The consequences of malnutrition can be irreversible and may include poor cognitive development, educability, and future economic productivity.<ref>{{cite web |title=The Lancet Series on Maternal and Child Undernutrition |year=2008 |url=http://www.thelancet.com/series/maternal-and-child-undernutrition |access-date=February 11, 2011 |archive-date=July 17, 2011 |archive-url=https://web.archive.org/web/20110717005704/http://www.thelancet.com/series/maternal-and-child-undernutrition |url-status=live }}</ref> On the other hand, Flynn has pointed to 20-point gains on Dutch military ([[Raven's Progressive Matrices|Raven's]] type) IQ tests between 1952, 1962, 1972, and 1982. In 1962 he observed that Dutch 18-year-olds had a major nutritional handicap. They were either in the womb or were recently born, during the great [[Dutch famine of 1944]]—when German troops monopolized food and 18,000 people died of starvation.<ref>C. Banning (1946). "Food Shortage and Public Health, First Half of 1945". ''Annals of the American Academy of Political and Social Science'' Vol. 245, The Netherlands during German Occupation (May 1946), pp. 93–110</ref> Yet, concludes Flynn, "they do not show up even as a blip in the pattern of Dutch IQ gains. It is as if the famine had never occurred."<ref name="PB101-171">{{cite journal | author = Flynn J.R. | year = 1987 | title = Massive IQ gains in 14 nations: What IQ tests really measure | journal = Psychological Bulletin | volume = 101 | issue = 2| pages = 171–91 |doi=10.1037/0033-2909.101.2.171}}</ref><ref>Flynn, James R. (2009). ''What Is Intelligence?'' (p. 103). Cambridge University Press. Kindle Edition.</ref> It appears that the effects of diet are gradual, taking effect over decades (affecting mother as well as the child) rather than a few months.{{Citation needed|reason=Reliable source needed for the whole sentence|date=February 2015}}

In support of the nutritional hypothesis, it is known that, in the United States, the average height before 1900 was about 10&nbsp;cm (~4&nbsp;inches) shorter than it is today.<ref>{{cite journal |last1=Samaras |first1=Thomas T. |last2=Elrick |first2=Harold |date=May 2002 |title=Group Height, body size, and longevity: is smaller better for the human body? |journal=West J Med |volume=176 |issue=3 |pages=206–08 |pmc=1071721 |pmid=12016250 |doi=10.1136/ewjm.176.3.206}}</ref> Possibly related to the Flynn effect is a similar change of [[cranial vault|skull]] size and shape during the last 150 years. A Norwegian study found that height gains were strongly correlated with intelligence gains until the cessation of height gains in military conscript cohorts towards the end of the 1980s.<ref name="doi10.1016/j.intell.2004.06.004">{{Cite journal | doi = 10.1016/j.intell.2004.06.004| last1 = Sundet | first1 = J. | last2 = Barlaug | first2 = D. | last3 = Torjussen | first3 = T. | journal = Intelligence | volume = 32 | issue = 4 | pages = 349–62 |title=The end of the Flynn effect?: A study of secular trends in mean intelligence test scores of Norwegian conscripts during half a century| year = 2004 }}</ref> Both height and skull size increases probably result from a combination of [[phenotypic plasticity]] and genetic [[Selection (biology)|selection]] over this period.<ref>{{cite journal |vauthors=Jantz RL, Meadows Jantz L |title=Secular change in craniofacial morphology |journal=Am. J. Hum. Biol. |volume=12 |issue=3 |pages=327–38 |date=May 2000 |pmid=11534023 |doi=10.1002/(SICI)1520-6300(200005/06)12:3<327::AID-AJHB3>3.0.CO;2-1|s2cid=22059721 |doi-access=free }}<br />
{{cite journal |author=Jantz RL |title=Cranial change in Americans: 1850–1975 |journal=J. Forensic Sci. |volume=46 |issue=4 |pages=784–87 |date=July 2001 |doi=10.1520/JFS15047J |pmid=11451056 }}</ref> With only five or six human generations in 150 years, time for [[natural selection]] has been very limited, suggesting that increased skeletal size resulting from changes in population [[phenotype]]s is more likely than recent genetic evolution.

It is well known that [[micronutrient]] deficiencies change the development of intelligence. For instance, one study has found that [[iodine deficiency]] causes a fall, on average, of 12 IQ points in China.<ref>{{Cite journal|author=Qian M |title=The effects of iodine on intelligence in children: a meta-analysis of studies conducted in China |journal=Asia Pacific Journal of Clinical Nutrition |volume=14 |issue=1 |pages=32–42 |year=2005 |pmid=15734706|author2=Wang D|author3=Watkins WE|display-authors=3|last4=Gebski|first4=V|last5=Yan|first5=YQ|last6=Li|first6=M|last7=Chen|first7=ZP}}</ref>

Scientists James Feyrer, Dimitra Politi, and David N. Weil have found in the U.S. that the proliferation of iodized salt increased IQ by 15 points in some areas. Journalist Max Nisen has stated that with this type of salt becoming popular, that "the aggregate effect has been extremely positive."<ref>{{Cite web|last=Nisen|first=Max|title=How Adding Iodine To Salt Resulted In A Decade's Worth Of IQ Gains For The United States|url=https://www.businessinsider.com/iodization-effect-on-iq-2013-7|date=2013-07-22|access-date=2023-01-23|website=Business Insider|language=en-US}}</ref>

Daley et al. (2003) found a significant Flynn effect among children in rural [[Kenya]], and concluded that nutrition was one of the hypothesized explanations that best explained their results (the others were parental literacy and family structure).<ref>{{cite journal|last1=Daley|first1=TC|last2=Whaley|first2=SE|last3=Sigman|first3=MD|last4=Espinosa|first4=MP|last5=Neumann|first5=C|s2cid=12315212|title=IQ on the rise: the Flynn effect in rural Kenyan children.|journal=Psychological Science|date=May 2003|volume=14|issue=3|pages=215–19|pmid=12741743|doi=10.1111/1467-9280.02434}}</ref>

===Generally more stimulating environment===
Still another theory is that the general environment today is much more complex and stimulating. One of the most striking 20th-century changes in the human intellectual environment has come from the increase of exposure to many types of [[visual media]]. From pictures on the wall to movies to television to video games to computers, each successive generation has been exposed to richer optical displays than the one before and may have become more adept at visual analysis. This would explain why visual tests like the Raven's have shown the greatest increases. An increase only of particular forms of intelligence would explain why the Flynn effect has not caused a "cultural renaissance too great to be overlooked."<ref name="Neisser97"/>

In 2001, [[William Dickens]] and James Flynn presented a model for resolving several contradictory findings regarding IQ. They argue that the measure "[[heritability]]" includes both a direct effect of the [[genotype]] on IQ and also indirect effects such that the genotype changes the [[Environment (biophysical)|environment]], thereby affecting IQ. That is, those with a greater IQ tend to seek stimulating environments that further increase IQ. These reciprocal effects result in [[gene environment correlation]]. The direct effect could initially have been very small, but [[feedback]] can create large differences in IQ. In their model, an environmental stimulus can have a very great effect on IQ, even for adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition during early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that any program designed to increase IQ may produce long-term IQ gains if that program teaches children how to replicate the types of cognitively demanding experiences that produce IQ gains outside the program. To maximize lifetime IQ, the programs should also motivate them to continue searching for cognitively demanding experiences after they have left the program.<ref name=Dickens01>{{cite journal |doi=10.1037/0033-295X.108.2.346 |vauthors=Dickens WT, Flynn JR |title=Heritability estimates versus large environmental effects: The IQ paradox resolved |journal=Psychological Review |volume=108 |issue=2 |pages=346–369 |year=2001 |pmid=11381833 |url=https://www.psychologytoday.com/files/u81/Dickens_and_Flynn__2001_.pdf|citeseerx=10.1.1.139.2436 }}</ref><ref>{{cite journal|vauthors=Dickens WT, Flynn JR |title=The IQ Paradox: Still Resolved |journal=Psychological Review |volume=109 |issue=4 |year=2002 |url=http://www.brookings.edu/views/papers/dickens/20020205.pdf |doi=10.1037/0033-295x.109.4.764 |pages=764–71 |url-status=dead |archive-url=https://web.archive.org/web/20070319031706/http://www.brookings.edu/views/papers/dickens/20020205.pdf |archive-date=March 19, 2007  }}</ref>

Flynn, in his 2007 book ''[[What Is Intelligence?]]'', further expanded on this theory. Environmental changes resulting from modernization—such as more intellectually demanding work, greater use of technology, and smaller families—have meant that a much larger proportion of people are more accustomed to manipulating abstract concepts such as hypotheses and categories than a century ago. Substantial portions of IQ tests deal with these abilities. Flynn gives, as an example, the question 'What do a dog and a rabbit have in common?' A modern respondent might say they are both mammals (an abstract, or ''a priori'' answer, which depends only on the meanings of the words ''dog'' and ''rabbit''), whereas someone a century ago might have said that humans catch rabbits with dogs (a concrete, or ''a posteriori'' answer, which depended on what happened to be the case at that time).<ref>{{cite book |last=Flynn |first=James R. |author-link=James Flynn (academic) |title=[[What Is Intelligence?|What Is Intelligence? Beyond the Flynn Effect]] |date=August 2012 |publisher=Cambridge University Press |isbn=9780511605253 |pages=24–29}}</ref><ref>{{cite magazine |last1=Gladwell |first1=Malcolm |title=None of the Above |url=https://www.newyorker.com/magazine/2007/12/17/none-of-the-above |magazine=The New Yorker |access-date=July 6, 2024 |date=December 10, 2007 |url-access=limited}}</ref>{{Better source needed|date=July 2024}}

===Infectious diseases===
{{See also|Parasite load#Host stress|Impact of health on intelligence}}
Eppig, Fincher, and Thornhill (2011) conducted a study looking at different US states found that states with a higher prevalence of [[Infection|infectious diseases]] had lower average IQ. The effect remained after controlling for the effects of wealth and educational variation.<ref>{{Cite journal |vauthors=Eppig C, Fincher CL, Thornhill R |title=Parasite prevalence and the distribution of intelligence among the states of the USA |journal=Intelligence |volume=39 |issue=2–3 |pages=155–60 |year=2011 |doi=10.1016/j.intell.2011.02.008}}</ref>

Atheendar Venkataramani (2010) studied the effect of [[malaria]] on IQ in a sample of Mexicans. Malaria eradication during the birth year was associated with increases in IQ. It also increased the probability of employment in a skilled occupation. The author suggests that this may be one explanation for the Flynn effect and that this may be an important explanation for the link between national malaria burden and economic development.<ref>{{cite SSRN |vauthors=Venkataramani A |title=Early Life Exposure to Malaria and Cognition and Skills in Adulthood: Evidence from Mexico |date=September 18, 2010 |ssrn=1679164}}</ref> A literature review of 44 papers states that cognitive abilities and school performance were shown to be impaired in sub-groups of patients (with either cerebral malaria or uncomplicated malaria) when compared with healthy controls. Studies comparing cognitive functions before and after treatment for acute malarial illness continued to show significantly impaired school performance and cognitive abilities even after recovery. Malaria [[prophylaxis]] was shown to improve cognitive function and school performance in clinical trials when compared to placebo groups.<ref>{{cite journal |vauthors=Fernando SD, Rodrigo C, Rajapakse S |title=The 'hidden' burden of malaria: cognitive impairment following infection |journal=Malar. J. |volume=9 |issue=1 |pages=366 |year=2010 |pmid=21171998 |pmc=3018393 |doi=10.1186/1475-2875-9-366 |doi-access=free }}</ref>

===Heterosis===
{{Further|Inbreeding depression#In humans}}
[[Heterosis]], or ''hybrid vigor'', associated with historical reductions of the levels of [[inbreeding]], has been proposed by Michael Mingroni as an alternative explanation of the Flynn effect.<ref>Mingroni, M. A. (2007). "[http://www.iapsych.com/iqmr/fe/LinkedDocuments/mingroni2007.pdf Resolving the IQ paradox: Heterosis as a cause of the Flynn effect and other trends]" (PDF). ''Psychological Review'', 114(3), 806–829. doi:10.1037/0033-295X.114.3.806</ref><ref>Mingroni, M.A. (2004). "[https://www.mingroni.net/_files/ugd/970723_2f1c2ed3946246e0b69a9c1b1396c825.pdf The secular rise in IQ: Giving heterosis a second look]" (PDF). ''Intelligence'', 32, 65-83.</ref> However, James Flynn has pointed out that even if everyone mated with a sibling in 1900, subsequent increases in heterosis would not be a sufficient explanation of the observed IQ gains.<ref>[[Nicholas Mackintosh|Mackintosh, N.J.]] (2011). ''IQ and Human Intelligence''. Oxford: Oxford University Press, p. 291.</ref>

===Reduction of lead in gasoline===
{{See also|Lead abatement|Lead poisoning|Lead–crime hypothesis}}

One study found the drop in blood lead levels in the United States from the 1970s to 2007 correlated with a 4–5 point increase in IQ.<ref>{{cite journal |title=The possible societal impact of the decrease in U.S. blood lead levels on adult IQ |url=https://www.sciencedirect.com/science/article/abs/pii/S0013935114001066 |first1=Alan S. |last1=Kaufman |first2=Xiaobin |last2=Zhou |first3=Matthew R. |last3=Reynolds |first4=Nadeen L. |last4=Kaufman |first5=Garo P. |last5=Green |first6=Lawrence G. |last6=Weisse |doi=10.1016/j.envres.2014.04.015 |journal=Environmental Research |volume=132 |date=July 2014 |pages=413–420|pmid=24853978 |bibcode=2014ER....132..413K }}</ref>