Studies of brothers and twins have shown that about 50% of the variance in educational achievement and 40% of the variance in occupational status reflects between-family variance. About half of the between-family variance for educational achievement and even more for occupational status is due to genetic effects and the remainder is due to sharing the same environment.
With data on 35 pairs of male twins reared apart and 56 pairs reared together we investigated the extent to which genetic variance in SES can be attributed to genetic variance for cognitive abilities.
For both educational achievement and occupational status there was statistically-significant genetic variance both in common with and independent of genetic variance for cognitive abilities.
Thus, there are genetic effects contributing to familial similarity for SES that are not the same as those of importance for cognitive abilities. Candidate traits that may account for this remaining genetic variance in SES are personality, interests, or talents not represented in standard cognitive tests.