Heritability of IQ

(will be reformatted/completed later)

Twin studies[i] have estimated that the heritability[i] of IQ[i] is highly dependent on age, with the heritability of IQ gradually increasing from 30% in children to 85% in adults. The influence of the shared environmental component is also age-dependent, with the variance explained in small children being about 40% and then gradually approaching 0 in adults. Heritability estimates for IQ do not appear to be method-dependent, that is, methods using twins reared apart, and other family members generate similar estimates to those found in twin studies. The best evidence available suggests that SES and race do not change the heritability of traits.

The latest genome-wide association studies (GWAS)[i] suggest that the additive heritability of IQ is about 2-4% - this discrepancy is commonly referred to the missing heritability problem[i].

Method Invariance

Having results that do not depend on certain methodological constraints indicates that the results are robust to these differences. This is important, as some methods rely on assumptions that may not conform to the real world.

Prenatal environment

Some critics of twin studies have argued that the similar prenatal environment that is shared by twins results in them being more similar than what would be expected, leading to inflated estimates of heritability. One famous study that controlled for prenatal environment by comparing dizygotic twins and siblings found that the heritability of IQ is only 48%. However, there is a glaring issue with this study: no attenpt was made to segregate the estimates by the age of the twins. This is relevant because heritability estimates are higher for older individuals and because the difference between the correlation found in dizygotic twins and siblings gradually converges to 0 with age.

The 2nd concern regarding prenatal environment is chorionicity. Twins can be categorized as monochorionic (MC) or dichorionic (DC), where MC twins share a chorion and DC twins don’t. MZ twins typically share a chorion more often than DZ twins, so prenatal environment could be contributing to an increased similarity in MZ twins. Theoretically speaking, twins who share a chorion must compete for resources within the womb, therefore the twins who share a chorion should be less similar than those who do not. Empirical research into this claim has found no substantial differences between twins who share a chorion and twins who do not. In addition, twin studies that take into account chorionicity do not find that it significantly affects similarity between twins in intelligence.

references: https://gwern.net/doc/iq/2013-bouchard.pdf https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2889158/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8280022/ https://www.nature.com/articles/41319 https://link.springer.com/article/10.1007/s10519-015-9745-3 https://sci-hubtw.hkvisa.net/10.1023/a:1010257512183

http://library.lol/main/AE5E723C28C1D85187C43CC69ABDB0C2 words that should be converted to links are later followed with an [i]