James Thompson of Psychological Comments blogged on a huge meta-analysis of 50 years worth of twin studies published in the journal Nature. (Full text copy here). Although the study covered a wide range of phenotypes, several of them have particular relevance to intelligence, academics, and occupational outcomes. The authors of the study also published an interactive website, MaTCH, where more detailed statistics can be browsed.
I screen captured traits some traits relevant to academic and occupational outcomes below:
Structure of Brain
Higher level cognitive function (general intelligence)
Mental function of language
Not purely environmental
Problems related to negative life events in childhood
Problems related to upbringing
Longer run outcomes
Acquiring and keeping a job
Work and employment
The evidence that intelligence is highly heritable has long been over-whelming. The meta-analysis quantifies and documents this and compiles evidence pertaining to the heritability of several key outcomes (e.g., education, employment, etc) and various other phenotypes that collectively influence them. While IQ alone may only “explain”, say, 34% of the variance (r=0.58) in particular academic outcomes, these outcomes are influenced by other traits that are weakly correlated with IQ and that are also heritable to varying degrees.
There was a paper published recently on the heritability of GCSE scores (one of the UK’s main comprehensive examinations for university admissions) that tried to explain observed heritability with the twin method using intelligence and a variety of other observed phenotypes. I found this interesting because the GCSE is more akin to HS GPA in the US, i.e, if we had more a uniform curriculum and if we had uniform grading standards (which we obviously don’t), and therefore probably a better predictor of everyday “success” (imho) than more cognitively loaded tests like the SAT in some respects (which is both good and bad).
In any event, the GCSE analysis is a good start to understanding how these pieces fit together conceptually with outcomes like academic achievement, professional success, etc. They found that (1) GCSE scores have a heritability of 62%, which is more heritable than intelligence alone (2) they can account for much of the residual heritability with a combination of other heritable traits (e.g., self-efficacy, school environment, etc) (3) the role of the shared environment is quite significant (much more than it is on intelligence).
Note: A = additive genetic heritability; C = shared environment (e.g., family, school, neighborhood, etc); E = non-shared environment (non-systematic environmental influences, noise, etc).
Note: The phenotype correlations in the 2nd column (intelligence) are only weakly correlated with IQ.
Thus it is quite likely that the combined effects of these many and varied heritable traits on academics and occupational outcomes are quite a bit larger than naive estimates of IQ heritability alone might imply (especially low-ball estimates). Because these traits are heritable and because these traits have a pronounced impact on “success”, the children of higher socioeconomic groups are likely to have more of these beneficial traits and fewer deleterious traits, i.e., not only are they (on average) smarter, but they are also, on average, more conscientious, better behaved, and so on.
These traits collectively influence individual outcomes, peer groups, classroom behavior, and the like, so the sorts correlations I found earlier on California’s standardized tests with respect to various measures of SES (e.g., median family income, parental education levels, etc) are not entirely surprising and are probably mostly genetic (especially in a broader gene-environment sense). That being said, I believe there are also some non-trivial systematic environmental influences on objective measures of academic success and the like (see: culture, family, peer groups, etc), i.e., it’s not purely genetic.