Editing Race and intelligence (section)

===Heritability within and between groups===
[[File:Heritability plants.jpeg|thumb| An environmental factor that varies between groups but not within groups can cause group differences in a trait that is otherwise 100 percent [[heritable]].|330x330px]]

[[Twin studies]] of intelligence have reported high heritability values. However, these studies have been criticized for being based on questionable assumptions.<ref>{{Cite book |last1=Carson |first1=Michael |title='Race', IQ and Genes |last2=Beckwith |first2=Jon |date=2001 |publisher=John Wiley & Sons, Ltd |isbn=978-0-470-01590-2 |pages=1–5 |language=en |doi=10.1002/9780470015902.a0005689.pub3}}</ref><ref>{{Cite journal |last1=Beckwith |first1=Jon |last2=Morris |first2=Corey A. |date=December 2008 |title=Twin Studies of Political Behavior: Untenable Assumptions? |journal=Perspectives on Politics |language=en |volume=6 |issue=4 |pages=785–791 |doi=10.1017/S1537592708081917 |s2cid=55630117 |issn=1541-0986 }}</ref><ref>{{Cite journal |last1=Kamin |first1=Leon J. |last2=Goldberger |first2=Arthur S. |date=February 2002 |title=Twin Studies in Behavioral Research: A Skeptical View |journal=Theoretical Population Biology |volume=61 |issue=1 |pages=83–95 |doi=10.1006/tpbi.2001.1555 |pmid=11895384 |bibcode=2002TPBio..61...83K |issn=0040-5809}}</ref> When used in the context of human [[behavior genetics]], the term "heritability" can be misleading, as it does not necessarily convey information about the relative importance of genetic or environmental factors on the development of a given trait, nor does it convey the extent to which that trait is genetically determined.{{sfn|Moore|Shenk|2016}} Arguments in support of a genetic explanation of racial differences in IQ are sometimes fallacious. For instance, hereditarians have sometimes cited the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races, as evidence that racial differences in IQ are genetic.<ref>{{harvnb|Mackenzie|1984}}</ref>

Psychometricians have found that intelligence is substantially heritable within populations, with 30–50% of variance in IQ scores in early childhood being attributable to genetic factors in analyzed US populations, increasing to 75–80% by late adolescence.{{sfn|Neisser|Boodoo|Bouchard|Boykin|1996}}<ref name="Deary, Johnson & Houlihan 2009">{{harvnb|Deary|Johnson|Houlihan|2009}}</ref> In biology heritability is defined as the ratio of variation attributable to genetic differences in an observable [[trait (biology)|trait]] to the trait's total observable variation. The heritability of a trait describes the proportion of variation in the trait that is attributable to genetic factors within a particular population. A heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. In psychological testing, heritability tends to be understood as the degree of correlation between the results of a test taker and those of their biological parents. However, since high heritability is simply a correlation between child and parents, it does not describe the causes of heritability which in humans can be either genetic or environmental.

Therefore, a high heritability measure does not imply that a trait is genetic or unchangeable. In addition, environmental factors that affect all group members equally will not be measured by heritability, and the heritability of a trait may also change over time in response to changes in the distribution of genetic and environmental factors.{{sfn|Neisser|Boodoo|Bouchard|Boykin|1996}} High heritability does not imply that all of the heritability is genetically determined; rather, it can also be due to environmental differences that affect only a certain genetically defined group (indirect heritability).<ref name="Block 2002">{{harvnb|Block|2002}}</ref>

The figure to the right demonstrates how heritability works. In each of the two gardens the difference between tall and short cornstalks is 100% heritable, as cornstalks that are genetically disposed for growing tall will become taller than those without this disposition. But the difference in height between the cornstalks to the left and those on the right is 100% environmental, as it is due to different nutrients being supplied to the two gardens. Hence, the causes of differences within a group and between groups may not be the same, even when looking at traits that are highly heritable.<ref name="Block 2002" />