|There are two common ways of reaching this conclusion. One is to compare adopted children’s traits with those of their biological parents, on the one hand, and with those of their adoptive parents, on the other. If a closer correlation exists with the biological parents’ traits, then we say that the trait is to that degree inherited.|
The other method is to compare identical twins’ similarity, with respect to some trait, with the similarity of fraternal twins, or even of two ordinary siblings. Identical twins are genetic duplicates, while fraternal twins share only about half their genes and are no more genetically alike than ordinary siblings are. If identical twins are more alike than fraternal twins, therefore, we conclude that the trait under consideration is to some degree inherited.
There's nothing controversial (at least scientifically) with these studies -- twin studies are used all the time to study these sorts of problems (and, to my level of satisfaction, have proven that homosexuality, at least in men, has a strong genetic component).
Interestingly, Wilson mentions a study that looks at the genetic basis of why people do or don't vote, which is something I've written on in the past:
|Genes also influence how frequently we vote. Voting has always puzzled scholars: How is it rational to wait in line on a cold November afternoon when there is almost no chance that your ballot will make any difference? Apparently, people who vote often feel a strong sense of civic duty or like to express themselves. But who are these people? James Fowler, Laura Baker, and Christopher Dawes studied political participation in Los Angeles by comparing voting among identical and fraternal twins. Their conclusion: among registered voters, genetic factors explain about 60 percent of the difference between those who vote and those who do not.|