Children differ, as any parent of two or more knows. Some babies sleep through the night, others are always awake; some are calm, others are fussy; some walk at an early age, others after a long wait. Scientists have proved that genes are responsible for these early differences. But people assume that as children get older and spend more time under their parents’ influence, the effect of genes declines. They are wrong.
Identical twins tend to get more dissimilar looking as they age due to random wear and tear and a desire to assert one's individuality (e.g., the Kaczyńskis style their hair differently). But they often get more similar in behavior as they spend less time together. For example, say one identical twin is at the 92nd percentile in dominance / leadership while the other one is at the 90th percentile. Growing up together, the second twin will tend to see himself as having a subordinate personality, but when they stop spending all their time together, he will start to realize that other people tend to defer to him and expect him to lead. (Robert A. Heinlein's novel "Time for the Stars" provides a detailed example of this in action.)
For a century or more, we have understood that intelligence is largely inherited, though even today some mistakenly rail against the idea and say that nurture, not nature, is all. Now we know that much of our personality, too, is inherited and that many social attitudes have some degree of genetic basis, including our involvement in crime and some psychiatric illnesses. Some things do result entirely from environmental influences, such as whether you follow the Red Sox or the Yankees (though I suspect that Yankee fans have a genetic defect). But beyond routine tastes, almost everything has some genetic basis. And that includes politics. ...
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. ...The gene-driven ideological split that Alford and his colleagues found may, in fact, be an underestimate, because men and women tend to marry people with whom they agree on big issues—assortative mating, as social scientists call it. Assortative mating means that the children of parents who agree on issues will be more likely to share whatever genes influence those beliefs. Thus, even children who are not identical twins will have a larger genetic basis for their views than if their parents married someone with whom they disagreed. Since we measure heritability by subtracting the similarity among fraternal twins from the similarity among identical ones, this difference may neglect genetic influences that already exist on fraternal twins. And if it does, it means that we are underestimating genetic influences on attitudes.