Evidence that political attitudes and behavior are in part biologically and even genetically instantiated is much discussed in political science of late. Yet the classic twin design, a primary source of evidence on this matter, has been criticized for being biased toward finding genetic influence. In this article, we employ a new data source to test empirically the alternative, exclusively environmental, explanations for ideological similarities between twins. We find little support for these explanations and argue that even if we treat them as wholly correct, they provide reasons for political science to pay more rather than less attention to the biological basis of attitudes and behaviors. Our analysis suggests that the mainstream socialization paradigm for explaining attitudes and behaviors is not necessarily incorrect but is substantively incomplete.