Editing Intelligence quotient (section)

===Age===
[[Ronald S. Wilson]] is largely credited with the idea that IQ heritability rises with age.<ref>{{Cite journal |date=May 1987 |title=Ronald S. Wilson (1933–1986) |url=https://link.springer.com/10.1007/BF01065501 |journal=Behavior Genetics |language=en |volume=17 |issue=3 |pages=211–217 |doi=10.1007/BF01065501 |pmid=3307742 |issn=0001-8244}}</ref> Researchers building on this phenomenon dubbed it "The Wilson Effect," named after the behavioral geneticist.<ref name=":14">{{Cite journal |last=Bouchard |first=Thomas J. |date=October 2013 |title=The Wilson Effect: The Increase in Heritability of IQ With Age |url=https://www.cambridge.org/core/product/identifier/S1832427413000546/type/journal_article |journal=Twin Research and Human Genetics |language=en |volume=16 |issue=5 |pages=923–930 |doi=10.1017/thg.2013.54 |pmid=23919982 |issn=1832-4274}}</ref> A paper by [[Thomas J. Bouchard Jr.]], examining twin and adoption studies, including twins "reared apart," finds that IQ "reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood."<ref name=":14" /> IQ can change to some degree over the course of childhood.{{sfn|Kaufman|2009|pp=[https://archive.org/details/iqtestingpsych00phdd/page/n234 220]–222}} In one [[longitudinal study]], the mean IQ scores of tests at ages 17 and 18 were correlated at [[Correlation coefficient|{{nowrap|1=''r'' = 0.86}}]] with the mean scores of tests at ages five, six, and seven and at {{nowrap|1=''r'' = 0.96}}{{Explain|date=October 2020|reason=Please provide context to r correlation values. Are 0.86 and 0.96 good? How do they compare with correlation at older ages?}} with the mean scores of tests at ages 11, 12, and 13.{{sfn|Neisser et al.|1995}}

The current consensus is that [[fluid intelligence]] generally declines with age after early adulthood, while [[crystallized intelligence]] remains intact.{{sfn|Kaufman|2009|loc="Chapter 8"|p={{Page needed|date=January 2011}}}} However, the exact peak age of fluid intelligence or crystallized intelligence remains elusive. Cross-sectional studies usually show that especially fluid intelligence peaks at a relatively young age (often in the early adulthood) while longitudinal data mostly show that intelligence is stable until mid-adulthood or later. Subsequently, intelligence seems to decline slowly.<ref name="DesjardinsWarnke2012">{{cite journal |last1=Desjardins |first1=Richard |last2=Warnke |first2=Arne Jonas |year=2012 |title=Ageing and Skills |url=http://www.oecd-ilibrary.org/education/ageing-and-skills_5k9csvw87ckh-en |journal=OECD Education Working Papers |doi=10.1787/5k9csvw87ckh-en |doi-access=free |hdl-access=free |hdl=10419/57089}}</ref>

For decades, practitioners' handbooks and textbooks on IQ testing have reported IQ declines with age after the beginning of adulthood. However, later researchers pointed out this phenomenon is related to the [[Flynn effect]] and is in part a [[Cohort (statistics)|cohort]] effect rather than a true aging effect. A variety of studies of IQ and aging have been conducted since the norming of the first Wechsler Intelligence Scale drew attention to IQ differences in different age groups of adults. Both cohort effects (the birth year of the test-takers) and practice effects (test-takers taking the same form of IQ test more than once) must be controlled to gain accurate data.{{Inconsistent|date=October 2020|reason=Resolve the distinction between IQ (which, by definition, is age-normalized) and intelligence (which IQ attempts to measure) in this section.}} It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.{{sfn|Kaufman|2009|loc="Chapter 8"|p={{Page needed|date=January 2011}}}}