My estimates of the percentage of IQ variance family accounts for will prove sufficient to accomplish their purpose. I aim at supplementing kinship studies (with fresh estimates of family effects by performance level and subtest), not at replacing them. This chapter will show that my estimates are comparable with the twin studies, taking the twin estimates as a given. It will also show that my estimates are in accord with adoption studies, with particular reference to identifying the age at which family effects cease.

Twin studies

We can use my results for the percentage of variance due to family effects to partition cognitive variance into its three main components. These are: genes (including environment matched to genes), family, and current environment uncorrelated with genes. Fortunately, kinship studies show that “uncorrelated with genes” environment (or chance environment or uncommon environment) is steady between ages 6 and adulthood, which is something we would expect from a set of “random” factors. If we add that percentage on to the family percentage and deduct the sum from 100 percent, we get my own estimate of the influence of genes.

Dutch kinship study

Using Stanford-Binet (2001) Vocabulary, Table 9 performs that service. The Dutch values are from Holland (McGue et al., 1993) and their estimates are typical of kinship studies.

Their estimate of 18 percent accounted for by chance (“uncommon environment”) is a bit lower than the usual 20 percent (Haworth et al., 2010; McGue et al., 1993), but it is close enough. The values in bold show that we have achieved our objective of a good match with kinship data: the six comparisons show on average a difference of only 7.45 percent; the match is almost perfect at age 18.

The termination of family effects

However, there is one result of the twin studies we have yet to confirm: that family effects disappear entirely sometime during adulthood.