Many thanks to commenter Paige Harden for feeding in more recent, large sample studies on the heritability of intelligence and the influence of socio-economic status. To reiterate, socio-economic status conflates two sources of variance: genetic and environmental. Your status in society is usually a blend of your efforts and the opportunities open to you in a particular culture. It is not a pure measure of random allocation to occupations, nor even of capricious allocation, though in some non-democratic societies the way upwards is blocked for some groups, often for religious or clan reasons. In modern welfare-state open economies social status is much more likely to be affected by intelligence and personality. The one partial exception is that if you yourself have intelligence and diligence then you are likely to be wealthier than average, and can shield your children to some extent from the inevitable effects of regression to the mean. You can cushion their fall for a while, but others will rise above them.
In the papers below I have highlighted the sample sizes and the dates at which the original data were collected, since both are highly relevant to these sorts of investigations.
Childhood Socioeconomic Status Amplifies Genetic Effects on Adult Intelligence
Timothy C. Bates, Gary J. Lewis, and Alexander Weiss (2013)
Studies of intelligence in children reveal significantly higher heritability among groups with high socioeconomic status (SES) than among groups with low SES. These interaction effects, however, have not been examined in adults, when between-families environmental effects are reduced. Using 1,702 adult twins (aged 24–84) for whom intelligence assessment data were available (data collected 2004), we tested for interactions between childhood SES and genetic effects, between-families environmental effects, and unique environmental effects. Higher SES was associated with higher mean intelligence scores. Moreover, the magnitude of genetic influences on intelligence was proportional to SES. By contrast, environmental influences were constant. These results suggest that rather than setting lower and upper bounds on intelligence, genes multiply environmental inputs that support intellectual growth. This mechanism implies that increasing SES may raise average intelligence but also magnifies individual differences in intelligence.
Comment: Notice that these authors report IQ means by SES (fig 2, page 4) which allows readers to understand what is going on. There is a gradual rise in intelligence with social class, but nothing major, though there would be significant impacts on social class representation at IQ 130.
Studies of early childhood tend to exaggerate the apparent effects of home life on intelligence, but measures taken in late adolescence show far less of an influence. Nonetheless, here is a study showing a genetic influence on intelligence by 2 years of age.
Emergence of a Gene x socioeconomic status interaction on infant mental ability between 10 months and 2 years. Tucker-Drob EM, et al. Psychol Sci. 2011 Jan;22(1):125-33. doi: 10.1177/0956797610392926. Epub 2010 Dec 17.
Recent research in behavioral genetics has found evidence for a Gene × Environment interaction on cognitive ability: Individual differences in cognitive ability among children raised in socioeconomically advantaged homes are primarily due to genes, whereas environmental factors are more influential for children from disadvantaged homes. We investigated the developmental origins of this interaction in a sample of 750 pairs of twins measured on the Bayley Short Form test of infant mental ability, once at age 10 months and again at age 2 years. A Gene × Environment interaction was evident on the longitudinal change in mental ability over the study period (2001-2003). At age 10 months, genes accounted for negligible variation in mental ability across all levels of socioeconomic status (SES). However, genetic influences emerged over the course of development, with larger genetic influences emerging for infants raised in higher-SES homes. At age 2 years, genes accounted for nearly 50% of the variation in mental ability of children raised in high-SES homes, but genes continued to account for negligible variation in mental ability of children raised in low-SES homes.
Finally, in middle to upper class 17 year olds Harden, Turkheimer and Loehlin (2007) have done a study on a larger sample of children (839 twin pairs).Behav Genet (2007) 37:273–283 DOI 10.1007/s10519-006-9113-4123
Genotype by Environment Interaction in Adolescents’ Cognitive Aptitude. K. Paige Harden Æ Eric Turkheimer Æ John C. Loehlin
Abstract In a replication of Turkheimer, Haley, Waldron, D’Onofrio, Gottesman II (2003, Socioeconomic status modifies heritability of IQ in young children.
Psychological Science, 14:623-628), we investigate genotype–environment (G · E) interaction in the cognitive aptitude of 839 twin pairs who completed the National Merit Scholastic Qualifying Test in 1962. Shared environmental influences were stronger for adolescents from poorer homes, while genetic influences were stronger for adolescents from more affluent homes. No significant differences were found
between parental income and parental education interaction effects. Results suggest that environmental differences between middle- to upper-class families influence the expression of genetic potential for intelligence, as has previously been suggested by
Bronfenbrenner and Ceci’s model (1994, Nature-nurture reconceptualized in developmental perspective: a bioecological model Psychological Review, 101:568-586
In terms of sample size this is a better reference than his 2003 paper.
Authors should not keep quoting Turkheimer (2003) as the final word on the subject of SES mitigating genetic effects on the of intelligence. Harden, Turkheimer and Loehlin (2007) is the better reference in terms of sample size, though the data collection happened 50 years ago.
To my mind the best study by far in terms of 1) sample size 2) contemporary data collection and 3) age sample (young adults being the “finished product” as far as establishing their own occupations and social status is concerned) is the Bates, Lewis, Weiss (2013) paper.
My thanks to my commenters and email correspondents. Can we try ensuring that authors include the Bates, Lewis and Weiss (2013) paper when they talk about the effect of socio-economic status? Also, can you remind them that SES is not a pure measure of social unfairness, but contains significant elements of intelligence and personality, which are themselves partially under genetic influence?