Social scientists have debated whether belief in a biological basis for sexual orientation engenders more positive attitudes toward gay men and lesbians. Belief in the biological theory has often been observed to be correlated with pro-lesbian/gay attitudes, and this gives some “weak” support for the hypothesis. There is far less “strong” evidence that biological beliefs have caused a noteworthy shift in heterosexist attitudes, or that they hold any essential promise of so doing. One reason for this divergence between the weak and strong hypothesis is that beliefs about causality are influenced by attitudes and group identities. Consequently beliefs about a biological basis of sexual orientation have identity-expressive functions over and above their strictly logical causal implications about nature/nurture issues. Four other factors explain why the biological argument of the 1990s was an intuitively appealing as a pro-gay tool, although there is no strong evidence that it had a very substantive impact in making public opinion in the USA more pro-gay. These factors are that the biological argument (a) implied that sexuality is a discrete social category grounded in fundamental differences between people, (b) implied that sexual orientation categories are historically and culturally invariant, (c) implied that gender roles and stereotypes have a biological basis, and (d) framed homosexual development, not heterosexual development, as needing explanation. Understanding this literature is important and relevant for conceptualizing the relationship between biological attributions and social attitudes in domains beyond sexual orientations, such as in the more recent research on reducing transphobia and essentialist beliefs about gender.