flynn effect

{{external media | width = 210px | float = right | headerimage=210px | video1 = [https://www.youtube.com/watch?v=9vpqilhW9uI James Flynn: Why our IQ levels are higher than our grandparents'], (18:41), TED talks

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The Flynn effect is named for James Robert Flynn, who did much to document it and promote awareness of its implications. The term was coined by Richard Herrnstein and Charles Murray in their 1994 book The Bell Curve.{{Cite book |author=Flynn, James R. |title=What Is Intelligence: Beyond the Flynn Effect |edition=expanded paperback |location=Cambridge |publisher=Cambridge University Press |isbn=978-0-521-74147-7 |year=2009 |pages=1–2 |quote=The 'Flynn effect' is the name that has become attached to an exciting development, namely, that the twentieth century saw massive IQ gains from one generation to another. To forestall a diagnosis of megalomania, the label was coined by Herrnstein and Murray, the authors of The Bell Curve, and not by myself.}}{{cite web |first=Cosma |last=Shalizi |title=The Domestication of the Savage Mind |type=Review |url=http://www.cscs.umich.edu/~crshalizi/reviews/flynn-beyond/ |website=University of Michigan |date=27 April 2009 |archive-date=July 19, 2012 |archive-url=https://web.archive.org/web/20120719062416/http://www.cscs.umich.edu/~crshalizi/reviews/flynn-beyond/ |access-date=August 13, 2010}}{{Cite book|last1=Herrnstein|first1=Richard J.|url=https://archive.org/details/bellcurveintell00herr/page/307/mode/1up|title=The Bell Curve: Intelligence and Class Structure in American Life|last2=Murray|first2=Charles|publisher=The Free Press|year=1994|isbn=0-02-914673-9|location=New York|pages=307}} Flynn stated that, if asked, he would have named the effect after Read D. Tuddenham{{Cite journal |last=Haig |first=Brian D. |date=2013-07-01 |title=Detecting Psychological Phenomena: Taking Bottom-Up Research Seriously |url=https://scholarlypublishingcollective.org/ajp/article/126/2/135/258002/Detecting-Psychological-Phenomena-Taking-Bottom-Up |journal=The American Journal of Psychology |language=en |volume=126 |issue=2 |pages=135–153 |doi=10.5406/amerjpsyc.126.2.0135 |pmid=23858950 |issn=0002-9556}} who "was the first to present convincing evidence of massive gains on mental tests using a nationwide sample"{{Citation |last=Flynn |first=James R. |title=Secular Changes in Intelligence |url=https://james-flynn.net/wp-content/uploads/2020/12/The-Cambridge-Handbook-of-Intelligence.pdf |work=The Cambridge Handbook of Intelligence |year=2011 |pages=647–665 |access-date=2023-03-27 |doi=10.1017/cbo9780511977244.033 |isbn=978-0-511-97724-4}} in a 1948 article.{{Cite journal |last=Tuddenham |first=R. D. |date=1948 |title=Soldier intelligence in World Wars I and II |url=https://pubmed.ncbi.nlm.nih.gov/18911933/ |journal=The American Psychologist |volume=3 |issue=2 |pages=54–56 |doi=10.1037/h0054962 |issn=0003-066X |pmid=18911933}}

Although the general term for the phenomenon—referring to no researcher in particular—continues to be "secular rise in IQ scores", many textbooks on psychology and IQ testing have now followed the lead of Herrnstein and Murray in calling the phenomenon the Flynn effect.{{cite book |last1=Fletcher |first1=Richard B. |last2=Hattie |first2=John |title=Intelligence and Intelligence Testing |url=https://books.google.com/books?id=8pzDawey6akC |access-date=August 31, 2013 |year=2011 |publisher=Taylor & Francis |isbn=978-1-136-82321-3 |page=26 |quote=Indeed, this effect, now called the 'Flynn effect', is well established. Nations, almost without exception, have shown gains of about 20 IQ points per generation (30 years). These gains are highest for IQ tests that are most related to reasoning and the capacity to figure out novel problems (this is often called 'fluid intelligence', see Chapter 5); and least related to knowledge, which arises from better educational opportunity, a history of persistence and good motivation for learning (this is often called 'crystallized intelligence', see Chapter 5).}}

• {{cite book |title=Gifted Lives: What Happens when Gifted Children Grow Up |last=Freeman |first=Joan |date=2010 |publisher=Routledge |location=London |isbn=978-0-415-47009-4 |pages=290–91 |quote=A strange new phenomenon has been growing since about 1950, called the 'Flynn Effect' after Professor James Flynn of the University of Otago, New Zealand. In his book What is Intelligence ?, Flynn describes a year-on-year rise in measured intelligence, about three IQ points a decade.}}
• {{cite news |first=Annalisa |last=Barbieri |url=https://www.theguardian.com/lifeandstyle/2010/oct/09/gifted-children-joan-freeman-psychologist |title=Young, gifted and likely to suffer for it |newspaper=The Guardian |date=8 October 2010 |access-date=December 14, 2016 |archive-date=November 11, 2020 |archive-url=https://web.archive.org/web/20201111215132/https://www.theguardian.com/lifeandstyle/2010/oct/09/gifted-children-joan-freeman-psychologist |url-status=live }}
• {{cite book |last=Urbina |first=Susana |title=Essentials of Psychological Testing |date=2004 |publisher=John Wiley & Sons |isbn=978-0-471-41978-5 |page=103 |quote=A puzzling longitudinal trend in the opposite direction, known as the 'Flynn effect', has been well documented in successive revisions of major intelligence tests (like the S-B and the Wechsler scales) that invariably involve the administration of both the old and new versions to a segment of the newer standardization sample, for comparative purposes. Data from revisions of various intelligence tests in the United States as well as in other countries—extensively analyzed by J.R. Flynn (1984, 1987)—show a pronounced, long-term upward trend in the level of performance required to obtain any given IQ score. The Flynn effect presumably reflects population gains over time in the kinds of cognitive performance that intelligence tests sample.}} {{cite book |last=Wasserman |first=John D. |editor1-last=Weiner |editor1-first=Irving B. |editor2-last=Graham |editor2-first=John R. |editor3-last=Naglieri |editor3-first=Jack A. |title=Handbook of Psychology |volume=10: Assessment Psychology |chapter=Chapter 18: Assessment of Intellectual Functioning |year=2012 |publisher=John Wiley & Sons |isbn=978-0-470-89127-8 |page=486 |quote=Both definitions also specify that the intellectual functioning criterion for a diagnosis of intellectual disability is approximately 2 SDs or more below the normative mean, but factors such as test score statistical error (standard error of measurement), test fairness, normative expectations for the population of interest, the Flynn effect, and practice effects from previous testing need to be considered before arriving at any diagnosis.}}
• {{cite book |last=Chamorro-Premuzic |first=Tomas |author-link=Tomas Chamorro-Premuzic |title=Personality and Individual Differences |date=2011 |publisher=Wiley |isbn=978-1-4051-9927-8 |page=221 |quote=Flynn effect The finding by sociologist James Flynn that there are generational increases in IQ across nations.}}

IQ tests are updated periodically. For example, the Wechsler Intelligence Scale for Children (WISC), originally developed in 1949, was updated in 1974, 1991, 2003, and again in 2014. The revised versions are standardized based on the performance of test-takers in standardization samples. A standard score of IQ 100 is defined as the mean performance of the standardization sample. Thus one way to see changes in norms over time is to conduct a study in which the same test-takers take both an old and new version of the same test. Doing so confirms IQ gains over time. Some IQ tests—for example, tests used for military draftees in NATO countries in Europe—report raw scores, and those also confirm a trend of rising scores over time. The average rate of increase seems to be about three IQ points per decade in the United States, as scaled by the Wechsler tests. The increasing test performance over time appears on every major test, in every age range, at every ability level, and in every modern industrialized country, although not necessarily at the same rate as in the United States. The increase was continuous and roughly linear from the earliest days of testing to the mid-1990s.{{cite journal |author=Neisser U |title=Rising Scores on Intelligence Tests |journal=American Scientist |volume=85 |issue=5 |pages=440–47 |year=1997 |url=http://www.americanscientist.org/issues/feature/rising-scores-on-intelligence-tests/99999 |url-status=deviated |archive-url=https://web.archive.org/web/20161104214157/http://www.americanscientist.org/issues/feature/rising-scores-on-intelligence-tests/99999 |archive-date=November 4, 2016 |bibcode=1997AmSci..85..440N }}{{better source needed|date=November 2017}} Though the effect is most associated with IQ increases, a similar effect has been found with increases in attention and of semantic and episodic memory.

Ulric Neisser estimated that using the IQ values of 1997, the average IQ of the United States in 1932, according to the first Stanford–Binet Intelligence Scales standardization sample, was 80. Neisser states that "Hardly any of them would have scored 'very superior', but nearly one-quarter would have appeared to be 'deficient.'" He also wrote that "Test scores are certainly going up all over the world, but whether intelligence itself has risen remains controversial." Quantitative psychologist, Joseph Lee Rodgers argues that the effect occurs outside of families in any case.Joseph Lee Rodgers (2014). "http://www.iapsych.com/iqmr/fe/LinkedDocuments/rodgers2014.pdf Intelligence Are birth order effects on intelligence really Flynn Effects? Reinterpreting Belmont and Marolla 40 years later" (PDF). Intelligence 42: 128-133. "No within-family data exist that document an increase in intelligence over birth order, suggesting that its source derives from outside the family and will only manifest in data and analyses that account for between-family variance (such as cross-sectional data)." (p. 130)

Trahan et al. (2014) found that the effect was about 2.93 points per decade,{{clarify|reason=Over what time interval?|date=March 2023}} based on both Stanford–Binet and Wechsler tests; they also found no evidence the effect was diminishing.{{cite journal|last1=Trahan|first1=LH|last2=Stuebing|first2=KK|last3=Fletcher|first3=JM|last4=Hiscock|first4=M|title=The Flynn effect: a meta-analysis.|journal=Psychological Bulletin|date=September 2014|volume=140|issue=5|pages=1332–60|doi=10.1037/a0037173|pmid=24979188|pmc=4152423}} In contrast, Pietschnig and Voracek (2015) reported, in their meta-analysis of studies involving nearly 4 million participants, that the Flynn effect had decreased in recent decades. They also reported that the magnitude of the effect was different for different types of intelligence ("0.41, 0.30, 0.28, and 0.21 IQ points annually for fluid, spatial, full-scale, and crystallized IQ test performance, respectively"), and that the effect was stronger for adults than for children.{{Cite journal |last1=Jakob Pietschnig |last2=Martin Voracek |s2cid=12604392 |date=May 1, 2015 |title=One Century of Global IQ Gains: A Formal Meta-Analysis of the Flynn Effect (1909–2013) |journal=Perspectives on Psychological Science |language=en |volume=10 |issue=3 |pages=282–306 |doi=10.1177/1745691615577701 |pmid=25987509 |issn=1745-6916}}

Raven (2000) found that, as Flynn suggested, data interpreted as showing a decrease in many abilities with increasing age must be re-interpreted as showing that there has been a dramatic increase of these abilities with the date of birth. On many tests this occurs at all levels of ability.{{cite journal | author = Raven John | year = 2000 | title = The Raven's Progressive Matrices: Change and Stability over Culture and Time | url = http://eyeonsociety.co.uk/resources/RPMChangeAndStability.pdf | journal = Cognitive Psychology | volume = 41 | issue = 1 | pages = 1–48 | doi = 10.1006/cogp.1999.0735 | pmid = 10945921 | s2cid = 26363133 | access-date = July 9, 2011 | archive-date = April 28, 2019 | archive-url = https://web.archive.org/web/20190428221444/http://eyeonsociety.co.uk/resources/RPMChangeAndStability.pdf | url-status = live }}

Some studies have found the gains of the Flynn effect to be particularly concentrated at the lower end of the distribution. Teasdale and Owen (1989), for example, found the effect primarily reduced the number of low-end scores, resulting in an increased number of moderately high scores, with no increase in very high scores.{{Cite journal| doi = 10.1016/0160-2896(89)90021-4 | last1 = Teasdale | first1 = T. | title = Continuing secular increases in intelligence and a stable prevalence of high intelligence levels | journal = Intelligence | volume = 13 | issue = 3 | pages = 255–62 | year = 1989 }} In another study, two large samples of Spanish children were assessed with a 30-year gap. Comparison of the IQ distributions indicated that the mean IQ scores on the test had increased by 9.7 points (the Flynn effect), the gains were concentrated in the lower half of the distribution and negligible in the top half, and the gains gradually decreased as the IQ of the individuals increased.{{cite journal | url=http://synapse.princeton.edu/~brained/chapter15/colom_andres-pueyo05_intelligence_Spanish-schoolchildren-nutrition-hypothesis.pdf | title=The generational intelligence gains are caused by decreasing variance in the lower half of the distribution: Supporting evidence for the nutrition hypothesis | author1=Colom, R. | author2=Lluis-Font, J.M. | author3=Andrés-Pueyo, A. | name-list-style=amp | journal=Intelligence | year=2005 | volume=33 | issue=1 | pages=83–91 | doi=10.1016/j.intell.2004.07.010 | access-date=October 4, 2012 | archive-date=August 13, 2012 | archive-url=https://web.archive.org/web/20120813042004/http://synapse.princeton.edu/~brained/chapter15/colom_andres-pueyo05_intelligence_Spanish-schoolchildren-nutrition-hypothesis.pdf | url-status=live }} Some studies have found a reverse Flynn effect with declining scores for those with high IQ.

In 1987, Flynn took the position that the very large increase indicates that IQ tests do not measure intelligence but only a minor sort of "abstract problem-solving ability" with little practical significance. He argued that if IQ gains did reflect intelligence increases, there would have been consequent changes of our society that have not been observed (a presumed non-occurrence of a "cultural renaissance"). By 2012 Flynn no longer endorsed this view of intelligence, having elaborated and refined his view of what rising IQ scores meant.{{Cite news|url=https://www.theguardian.com/technology/2012/sep/23/james-flynn-iq-scores-environment|title=James Flynn: IQ may go up as well as down|last=Tucker|first=Ian|date=2012-09-22|work=The Guardian|access-date=2019-01-21|language=en-GB|issn=0261-3077|archive-date=January 21, 2019|archive-url=https://web.archive.org/web/20190121232649/https://www.theguardian.com/technology/2012/sep/23/james-flynn-iq-scores-environment|url-status=live}}

Earlier investigators had discovered rises in raw IQ test scores in some study populations, but had not published general investigations of that issue in particular. Historian Daniel C. Calhoun cited earlier psychology literature on IQ score trends in his book The Intelligence of a People (1973).{{cite book |last=Calhoun |first=Daniel |title=The Intelligence of a People |date=1973 |publisher=Princeton University Press |isbn=978-0-691-04619-8 |url=https://archive.org/details/intelligenceofpe0000calh |url-access=registration}} Robert L. Thorndike – not to be confused with his famous father Edward – drew attention to rises in Stanford-Binet scores in a 1975 review of the history of intelligence testing.{{Cite journal |last=Thorndike |first=Robert L. |author-link=Robert L. Thorndike|title=Mr. Binet's Test 70 Years Later |journal=Educational Researcher |volume=4 |issue=5 |year=1975 |pages=3–7 |issn=0013-189X |doi=10.3102/0013189X004005003 |jstor=1174855 |s2cid=145355731 }} In 1982, Richard Lynn recorded an increase in average IQ among the population of Japan.{{Cite journal |last=Lynn |first=Richard |author-link=Richard Lynn|date=May 1982 |title=IQ in Japan and the United States shows a growing disparity |url=https://www.nature.com/articles/297222a0 |journal=Nature |language=en |volume=297 |issue=5863 |pages=222–223 |doi=10.1038/297222a0 |bibcode=1982Natur.297..222L |s2cid=4331657 |issn=1476-4687}}

{{See also|g factor (psychometrics)|Intelligence (trait)}}

There is debate about whether the rise in IQ scores also corresponds to a rise in general intelligence, or only a rise in special skills related to taking IQ tests. Because children attend school longer now and have become much more familiar with the testing of school-related material, one might expect the greatest gains to occur on such school content-related tests as vocabulary, arithmetic or general information. Just the opposite is the case: abilities such as these have experienced relatively small gains and even occasional decreases over the years. Meta-analytic findings indicate that Flynn effects occur for tests assessing both fluid and crystallized abilities. For example, Dutch conscripts gained 21 points during only 30 years, or 7 points per decade, between 1952 and 1982. This rise in IQ test scores is not wholly explained by an increase in general intelligence. Studies have shown that while test scores have improved over time, the improvement is not fully correlated with latent factors related to intelligence.{{cite journal | vauthors = Must O, Must A, Raudik V | year = 2003 | title = The secular rise in IQs: In Estonia, the Flynn effect is not a Jensen effect | journal = Intelligence | volume = 31 | issue = 5 | pages = 461–71 | doi = 10.1016/S0160-2896(03)00013-8 | url = http://www.iapsych.com/iqmr/fe/LinkedDocuments/Must2003.pdf | access-date = September 13, 2011 | archive-date = October 11, 2017 | archive-url = https://web.archive.org/web/20171011193700/http://www.iapsych.com/iqmr/fe/LinkedDocuments/Must2003.pdf | url-status = live }} Other researchers argue that the IQ gains described by the Flynn effect are due in part to increasing intelligence, and in part to increases in test-specific skills.{{cite journal |author1=Wicherts, J.M. |author2=Dolan, C.V. |author3=Hessen, D.J. |author4=Oosterveld, P. |author5=Baal, G.C.M. van |author6=Boomsma, D.I. |author7=Span, M.M. |name-list-style=amp |year=2004 |title=Are intelligence tests measurement invariant over time? Investigating the nature of the Flynn effect |journal=Intelligence |volume=32 |issue=5 |pages=509537 |url=http://users.fmg.uva.nl/jwicherts/wicherts2004.pdf |doi=10.1016/j.intell.2004.07.002 |quote=The overall conclusion of the present paper is that factorial invariance with respect to cohorts is not tenable . . . . The fact that the gains cannot be explained solely by increases at the level of the latent variables (common factors), which IQ tests purport to measure, should not sit well with explanations that appeal solely to changes at the level of the latent variables. |url-status=dead |archive-url=https://web.archive.org/web/20050529034027/http://users.fmg.uva.nl/jwicherts/wicherts2004.pdf |archive-date=May 29, 2005 |citeseerx=10.1.1.207.4350 }}{{cite journal | vauthors= Te Nijenhuis J, De Jong MJ, Evers A, Van Der Flier H | title= Are cognitive differences between immigrant and majority groups diminishing? | journal= European Journal of Personality | year= 2004 | volume= 18 | issue= 5 | pages= 405–34 | doi= 10.1002/per.511 | s2cid= 4806581 | url= https://research.vu.nl/ws/files/1957002/Nijenhuis%20European%20Journal%20of%20Personality%2018%202004%20u.pdf | access-date= November 10, 2019 | archive-date= February 21, 2021 | archive-url= https://web.archive.org/web/20210221012426/https://research.vu.nl/ws/files/1957002/Nijenhuis%20European%20Journal%20of%20Personality%2018%202004%20u.pdf | url-status= live }}{{cite journal |title=Secular Gains in Fluid Intelligence: Evidence from the Culture-Fair Intelligence Test |vauthors=Colom R, Garcia-Lopez O| journal= Journal of Biosocial Science | year= 2003 | volume= 35 | pages= 33–39 | doi= 10.1017/S0021932003000336 | pmid= 12537154 | issue= 1|s2cid=24493926}} One study suggested that the IQ gains reflected changes in modes of thinking that better reflected cognitive skills assessed by IQ tests rather than raw intelligence itself.{{cite journal |last1=Flynn |first1=James Robert |last2=Te Nijenhuis |author2-link= Jan te Nijenhuis |first2=Jan |last3=Metzen |first3=Daniel |date=May–June 2014 |title=The g beyond Spearman's g: Flynn's paradoxes resolved using four exploratory meta-analyses |url=https://www.sciencedirect.com/science/article/abs/pii/S0160289614000105 |journal=Intelligence |volume=44 |pages=1–10 |doi=10.1016/j.intell.2014.01.009 |access-date=2 July 2023}}

{{See also|Impact of health on intelligence}}

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.

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 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.

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.

Early intervention programs have shown mixed results. Some preschool (ages 3–4) intervention programs like "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 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.{{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 }}

Citing a high correlation between rising literacy rates and gains in IQ, David Marks has argued that the Flynn effect is caused by changes in literacy rates.{{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}}

{{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.{{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 }} 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.

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. An alternative interpretation of skewed IQ gains could be that improved education has been particularly important for this group.

A century ago, 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.{{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 }} On the other hand, Flynn has pointed to 20-point gains on Dutch military (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.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 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."{{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}}Flynn, James R. (2009). What Is Intelligence? (p. 103). Cambridge University Press. Kindle Edition. 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 cm (~4 inches) shorter than it is today.{{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}} Possibly related to the Flynn effect is a similar change of 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.{{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 }} Both height and skull size increases probably result from a combination of phenotypic plasticity and genetic selection over this period.{{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 }}

{{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 }} 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 phenotypes 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.{{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}}

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."{{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}}

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).{{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}}

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."

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, 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.{{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 }}{{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 }}

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).{{cite book |last=Flynn |first=James R. |author-link=James Flynn (academic) |title=What Is Intelligence? Beyond the Flynn Effect |date=August 2012 |publisher=Cambridge University Press |isbn=9780511605253 |pages=24–29}}{{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}}{{Better source needed|date=July 2024}}

{{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 infectious diseases had lower average IQ. The effect remained after controlling for the effects of wealth and educational variation.{{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}}

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.{{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}} 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.{{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 }}

{{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.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.114.3.806Mingroni, 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. 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.Mackintosh, N.J. (2011). IQ and Human Intelligence. Oxford: Oxford University Press, p. 291.

{{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.{{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 }}

File:Sundet et al 2004 fig 3.svg

Jon Martin Sundet and colleagues (2004) examined scores on intelligence tests given to Norwegian conscripts between the 1950s and 2002. They found that the increase of scores of general intelligence stopped after the mid-1990s and declined in numerical reasoning sub-tests.

Teasdale and Owen (2005) examined the results of IQ tests given to Danish male conscripts. Between 1959 and 1979 the gains were 3 points per decade. Between 1979 and 1989 the increase approached 2 IQ points. Between 1989 and 1998 the gain was about 1.3 points. Between 1998 and 2004 IQ declined by about the same amount as it gained between 1989 and 1998. They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18-year-olds."{{cite journal|last1=Teasdale|first1=Thomas W.|last2=Owen|first2=David R.|year=2005|title=A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse|journal=Personality and Individual Differences|volume=39|issue=4|pages=837–43|doi=10.1016/j.paid.2005.01.029}} The same authors in a more comprehensive 2008 study, again on Danish male conscripts, found that there was a 1.5-point increase between 1988 and 1998, but a 1.5-point decrease between 1998 and 2003/2004.{{cite journal |doi=10.1016/j.intell.2007.01.007 |vauthors=Teasdale TW, Owen DR |title=Secular declines in cognitive test scores: A reversal of the Flynn Effect |journal=Intelligence |volume=36 |issue=2 |pages=121–26 |year=2008 |url=http://www.iapsych.com/iqmr/fe/LinkedDocuments/teasdale2008.pdf |access-date=April 18, 2010 |archive-date=October 15, 2012 |archive-url=https://web.archive.org/web/20121015184832/http://www.iapsych.com/iqmr/fe/LinkedDocuments/teasdale2008.pdf |url-status=live }}

In Australia, the IQ of 6–12-year-olds (as measured by colored progressive matrices) has shown no increase from 1975 to 2003.{{cite journal | last1=Cotton | first1= S.M.| last2= Kiely| first2= P.M.|last3= Crewther|first3= D.P.|last4= Thomson|first4= B.|last5= Laycock|first5= R.|last6= Crewther|first6= S.G. |year=2005|title= A normative and reliability study for the Raven's Colored Progressive Matrices for primary school aged children in Australia|journal= Personality and Individual Differences|volume= 39| issue= 3|pages= 647–60| doi = 10.1016/j.paid.2005.02.015}}

In the United Kingdom, a study by Flynn (2009) himself found that tests carried out in 1980 and again in 2008 show that the IQ score of an average 14-year-old dropped by more than two points over the period. For the upper half of the results, the performance was even worse. Average IQ scores declined by six points. However, children aged between five and 10 saw their IQs increase by up to half a point a year over the three decades. Flynn argues that the abnormal drop in British teenage IQ could be due to youth culture having "stagnated" or even dumbed down.{{Cite journal| doi = 10.1016/j.ehb.2009.01.009 | pmid = 19251490| last1 = Flynn | first1 = J.R. | title = Requiem for nutrition as the cause of IQ gains: Raven's gains in Britain 1938–2008 | journal = Economics & Human Biology | volume = 7 | issue = 1 | pages = 18–27 | year = 2009 }}

Bratsberg & Rogeberg (2018) present evidence that the Flynn effect in Norway has reversed between the years 1962–1991, and that both the original rise in mean IQ scores and their subsequent decline within this period can be observed within families consisting of native-born parents and their children, indicating that environmental factors were the likely cause for these changes. Because IQ data was only available for male Norwegians, who were subject to military conscription, years of schooling were used as an approximation for female IQ to support this conclusion.

One possible explanation of a worldwide decline in intelligence is an increase in air pollution; coal burning emits mercury, and intelligence has continued to climb in areas, like the southern United States, where coal burning has declined.{{cite journal|first1=Ivica|last1=Pesovski|first2=Andrea|last2=Kulakov|first3=Vladimir|last3=Trajkovikj|title=Differences in cognitive ability assessment results between Millennial and Generation Z cohorts|url=https://repository.ukim.mk/handle/20.500.12188/25674|date=2022|journal=The 19th International Conference on Informatics and Information Technologies – CIIT 2022}}{{cite journal|first1=Bernt|last1=Bratsberg|first2=Ole|last2=Rogeberg|title=Flynn effect and its reversal are both environmentally caused|journal=Proceedings of the National Academy of Sciences|date=26 June 2018|issn=0027-8424|pages=6674–6678|volume=115|issue=26|pmid=29891660|pmc=6042097|doi=10.1073/pnas.1718793115|doi-access=free |bibcode=2018PNAS..115.6674B }}

Winter et al. (2024) when comparing two WAIS-5 validity studies found a reduced Flynn effect of an increase of 1.2 IQ points per decade rather than the expected 3 IQ point increase per decade. The authors identified various novel factors including social media dependency and the COVID-19 pandemic which may have contributed to a reduced Flynn effect.{{Cite journal |last1=Winter |first1=Emily L. |last2=Trudel |first2=Sierra M. |last3=Kaufman |first3=Alan S. |date=2024-11-15 |title=Wait, Where's the Flynn Effect on the WAIS-5? |journal=Journal of Intelligence |language=en |volume=12 |issue=11 |pages=118 |doi=10.3390/jintelligence12110118 |doi-access=free |issn=2079-3200|pmc=11595985 }}

File:Flynn-–-World-Regions.png

{{see also|Nations and intelligence|Race and intelligence|Sex differences in intelligence}}

If the Flynn effect has ended in developed nations but continues in less developed ones, this would tend to diminish national differences in IQ scores.

Also, if the Flynn effect has ended for the majority in developed nations, it may still continue for minorities, especially for groups like immigrants where many may have received poor nutrition during early childhood or have had other disadvantages. A study in the Netherlands found that children of non-Western immigrants had improvements for g, educational achievements, and work proficiency compared to their parents, although there were still remaining differences compared to ethnic Dutch.

In the United States, the IQ gap between black and white people was gradually closing over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers. For instance, Vincent reported in 1991 that the black–white IQ gap was decreasing among children, but that it was remaining constant among adults.{{Cite journal |last=Vincent |first=Ken R. |date=March 1991 |title=Black/white IQ differences: Does age make the difference? |journal=Journal of Clinical Psychology |volume=47 |issue=2 |pages=266–270 |doi=10.1002/1097-4679(199103)47:2<266::aid-jclp2270470213>3.0.co;2-s |pmid=2030133 }} Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of black people and white people closed by about 5 or 6 IQ points between 1972 and 2002,{{sfn|Dickens|Flynn|2006}} a reduction of about one-third. In the same period, the educational achievement disparity also diminished.Neisser, Ulric (Ed). 1998. The rising curve: Long-term gains in IQ and related measures. Washington, DC, US: American Psychological Association Reviews by Flynn and Dickens,{{Cite journal |last1=Dickens |first1=William T |last2=Flynn |first2=James R |year=2006 |title=Black Americans Reduce the Racial IQ Gap: Evidence from Standardization Samples |url=http://www.brookings.edu/views/papers/dickens/20060619_iq.pdf |url-status=dead |journal=Psychological Science |volume=17 |issue=10 |pages=913–20 |citeseerx=10.1.1.186.2540 |doi=10.1111/j.1467-9280.2006.01802.x |pmid=17100793 |s2cid=6593169 |archive-url=https://web.archive.org/web/20091009095003/http://www.brookings.edu/views/papers/dickens/20060619_IQ.pdf |archive-date=2009-10-09 }} Nicholas Mackintosh,{{Cite book |last=Mackintosh |first=N. J. |author-link=Nicholas Mackintosh|title=IQ and Human Intelligence |publisher=Oxford University Press |year=2011 |isbn=978-0-19-958559-5 |edition=second |location=Oxford}}{{page needed|date=July 2024}} and Nisbett et al.{{Cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |last6=Halpern |first6=Diane F. |last7=Turkheimer |first7=Eric |year=2012a |title=Intelligence: new findings and theoretical developments |url=http://people.virginia.edu/~ent3c/papers2/nisbett2012int.pdf |journal=American Psychologist |volume=67 |pages=130–159 |doi=10.1037/a0026699 |issn=0003-066X |pmid=22233090 |access-date=22 July 2013 |number=2 |archive-date=November 8, 2020 |archive-url=https://web.archive.org/web/20201108132004/http://people.virginia.edu/~ent3c/papers2/nisbett2012int.pdf}}{{cite web |first=Dan |last=Willingham |title=The latest on intelligence |url=http://www.danielwillingham.com/1/post/2012/05/the-latest-on-intelligence.html |website=Daniel Willingham—Science & Education |date=10 May 2012}} all concluded that the gradual closing of the gap was a real phenomenon.

Flynn has commented that he never claimed that the Flynn effect has the same causes as observed differences in average IQ test performance between blacks and whites, but that it shows that environmental factors can create IQ differences of a magnitude similar to that gap.{{Cite journal |doi = 10.1016/j.intell.2010.05.001 |last1 = Flynn |first1 = J.R. |title = The spectacles through which I see the race and IQ debate |journal = Intelligence |volume = 38 |issue = 4 |pages = 363–66 |year = 2010 }} Wicherts et al. had previously suggested a similar interpretation in a 2004 paper.{{cite journal |last1=Wicherts |first1=Jelte M. |last2=Dolan |first2=Conor V. |last3=Hessen |first3=David J. |last4=Oosterveld |first4=Paul |last5=van Baal |first5=G. Caroline M. |last6=Boomsma |first6=Dorret I. |last7=Span |first7=Mark M. |date=2004 |title=Are intelligence tests measurement invariant over time? Investigating the nature of the Flynn effect |journal=Intelligence |volume=32 |issue=5 |pages=509–537 |doi=10.1016/j.intell.2004.07.002 |quote=It appears therefore that the nature of the Flynn effect is qualitatively different from the nature of B–W differences in the United States. Each comparison of groups should be investigated separately. IQ gaps between cohorts do not teach us anything about IQ gaps between contemporary groups, except that each IQ gap should not be confused with real (i.e., latent) differences in intelligence.}} Flynn also argued that his findings undermine the so-called Spearman's hypothesis, which hypothesized that differences in g factor are the major driver of the blacks-whites IQ gap.{{Cite journal |doi = 10.1037/0003-066X.54.1.5 |title = Searching for justice: the discovery of IQ gains over time |year = 1999 |last1 = Flynn |first1 = J.R. |journal = American Psychologist |volume=54| pages = 5–9 |url = http://www.stat.columbia.edu/~gelman/stuff_for_blog/flynn.pdf |access-date = 26 October 2017 |archive-date = 25 June 2010 |archive-url = https://web.archive.org/web/20100625085640/http://www.stat.columbia.edu/~gelman/stuff_for_blog/flynn.pdf |url-status = live}}

{{portal|Biology}}

• Academic inflation
• Environment and intelligence
• Gene–environment correlation

{{Reflist|30em}}

• te Nijenhuis, Jan, & van der Flier, H. (2013). "Is the Flynn effect on g?: A meta-analysis." Intelligence, 41(6), 802–807. doi:10.1016/j.intell.2013.03.001
• Rodgers, Joseph Lee (1998). "A critique of the Flynn Effect: Massive IQ gains, methodological artifacts, or both?" Intelligence, 26(4), 337–356. doi:10.1016/S0160-2896(99)00004-5
• {{Cite journal |doi=10.1037/0033-2909.95.1.29 |author=Flynn, James R. |title=The Mean IQ of Americans: Massive Gains 1932 to 1978 |journal=Psychological Bulletin |volume=95 |pages=29–51 |year=1984 |url=http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn1984b.pdf |archive-url=https://web.archive.org/web/20120310210720/http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn1984b.pdf |archive-date=2012-03-10 |url-status=live |access-date=May 16, 2013 }}
• {{Cite journal |author=Flynn, James R. |title=Massive IQ Gains in 14 Nations: What IQ Tests Really Measure |journal=Psychological Bulletin |volume=101 |issue=2 |pages=171–91 |date= March 1987 |url=http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn1987.pdf |archive-url=https://web.archive.org/web/20110124091833/http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn1987.pdf |archive-date=2011-01-24 |url-status=live |access-date=May 13, 2013 |doi=10.1037/0033-2909.101.2.171 }}
• {{Cite book |author=Flynn, James R. |title=Are We Getting Smarter? Rising IQ in the Twenty-first century |location=Cambridge |publisher=Cambridge University Press |isbn=978-1-107-60917-4 |year=2012 |url-access=registration |url=https://archive.org/details/arewegettingsmar0000flyn }}
• {{cite journal |first=Lea |last=Winerman |title=Smarter than ever? |url=http://www.apa.org/monitor/2013/03/smarter.aspx |journal=Monitor on Psychology |volume=44 |issue=3 |page=30|date=March 2013}}
• {{Cite book |title=The Rising Curve: Long-Term Gains in IQ and Related Measures |editor-last=Neisser |editor-first=Ulric |author=Ulric Neisser |author2=James R. Flynn |author3=Carmi Schooler |author4=Patricia M. Greenfield |author5=Wendy M. Williams |author6=Marian Sigman |author7=Shannon E. Whaley |author8=Reynaldo Martorell |author9=Richard Lynn |author10=Robert M. Hauser |author11=David W. Grissmer |author12=Stephanie Williamson |author13=Sheila Nataraj Kirby |author14=Mark Berends |author15=Stephen J. Ceci |author16=Tina B. Rosenblum |author17=Matthew Kumpf |author18=Min-Hsiung Huang |author19=Irwin D. Waldman |author20=Samuel H. Preston |author21=John C. Loehlin |year=1998 |publisher=American Psychological Association |location=Washington DC |isbn=978-1-55798-503-3 |series=APA Science Volume Series |url=https://archive.org/details/risingcurvelongt00neis |url-access=registration}}
• {{citation |last=Tuddenham |first=Read D. |title=Soldier intelligence in World Wars I and II |journal=American Psychologist |volume=3 |issue=2 |pages=54–56 |date=1948 |doi=10.1037/h0054962 |pmid=18911933 }}

• {{Cite web |last=Flynn |first=James Robert |author-link=James Flynn (academic) |date=2006-12-15 |title=Beyond the Flynn Effect |url=https://www.psychometrics.cam.ac.uk/about-us/directory/beyond-the-flynn-effect |url-status=live |archive-url=https://web.archive.org/web/20171011193938/https://www.psychometrics.cam.ac.uk/about-us/directory/beyond-the-flynn-effect |archive-date=2017-10-11 |access-date=2024-05-30 |website=University of Cambridge}}
• {{Cite web |last=Graham |first=Charles |date=2001 |title=The Flynn Effect |url=http://www.indiana.edu/%7Eintell/flynneffect.shtml |url-status=dead |archive-url=https://web.archive.org/web/20051025230525/http://www.indiana.edu/%7Eintell/flynneffect.shtml |archive-date=2005-10-25 |access-date=2024-05-30 |website=Indiana University}}
• {{Cite journal |last=Holloway |first=Marguerite |date=1999-01-01 |title=Flynn's Effect |url=http://www.sciam.com/article.cfm?articleID=00037F65-D9C0-1C6A-84A9809EC588EF21 |url-status=dead |journal=Scientific American |volume=280 |issue=1 |pages=37–38 |doi=10.1038/scientificamerican0199-37 |bibcode=1999SciAm.280a..37H |archive-url=https://web.archive.org/web/20050910211047/http://www.sciam.com/article.cfm?articleID=00037F65-D9C0-1C6A-84A9809EC588EF21 |archive-date=2005-09-10 }}
• {{Cite journal |last1=Dickens |first1=William Theodore |author-link=William Dickens |last2=Flynn |first2=James Robert |author-link2=James Flynn (academic) |date=2001 |title=Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved |url=https://psycnet.apa.org/doiLanding?doi=10.1037%2F0033-295X.108.2.346 |journal=Psychological Review |language=en |volume=108 |issue=2 |pages=346–369 |doi=10.1037/0033-295X.108.2.346 |issn=0033-295X |eissn=1939-1471 |via=American Psychological Association}}
• {{Cite magazine |last=Johnson |first=Steven |date=2005-05-01 |title=Dome Improvement |url=https://www.wired.com/2005/05/flynn-2/ |url-status=live |magazine=Wired |issn=1059-1028 |eissn=1078-3148 |archive-url=https://web.archive.org/web/20161019222711/https://www.wired.com/2005/05/flynn-2/ |archive-date=2016-10-19 }}
• {{Cite web |last=Heylighen |first=Francis Paul |author-link=Francis Heylighen |date=2000-08-22 |title=Increasing intelligence: the Flynn effect |url=http://pespmc1.vub.ac.be/FLYNNEFF.html |url-status=live |archive-url=https://web.archive.org/web/20070704094614/http://pespmc1.vub.ac.be/FLYNNEFF.html |archive-date=2007-07-04 |access-date=2024-05-30 |website=Principia Cybernetica}}

{{Human intelligence topics}}

{{Evolutionary psychology}}

Category:Intelligence quotient

Category:Race and intelligence controversy