hy do people think and act politically in the manner they do? Despite the foundational nature of this question, answers are unfortu- nately incomplete and unnecessarily tentative, largely because political scientists do not take seriously the possibility of nonenvironmental influences. The sug- gestion that people could be born with political pre- dispositions strikes many as far-fetched, odd, even perverse. However, researchers in other disciplines—- notably behavioral genetics—-have uncovered a sub- stantial heritable component for many social attitudes and behaviors and it seems unlikely that political atti- tudes and behaviors are completely immune from such forces. In this article, we combine relevant findings in behavioral genetics with our own analysis of data on a large sample of twins to test the hypothesis that, con- trary to the assumptions embedded in political science research, political attitudes have genetic as well as en- vironmental causes. 1