Is optimism heritable? We gave the Attributional Style Questionnaire (ASQ), a measure of optimism, to 115 monozygotic twin pairs (MZ) and 27 dizygotic twin pairs (DZ). The intraclass correlations of the ASQ scores were 0.48 for MZ twins (P < 0.0001) and 0 for DZ twins. Though the sample size of DZ twins is small, these results suggest that there may be a substantial genetic effect on optimism. We speculate, however, that the mechanism for the transmission of this, and other complex personality traits, may be highly indirect.