With the twin method, we tested the theory of general trust. Among data collected on 1,040 twins, data from 328 pairs of identical twins and 103 pairs of same-sex fraternal twins were analyzed to examine genetic and environmental influences on the general trust level. A univariate genetic analysis showed that genetic factors and nonshared environmental factors accounted for 36% and 64% of the phenotypic variance of general trust respectively, while shared environmental factors did not contribute to it. This supports the argument that the general trust level is set responding to individual social circumstances, not by familial circumstances. A substantial degree of correlation between general trust and extraversion suggested that high trusters were more extraverted, which could support the emancipation theory of trust. Finally, a multivariate genetic analysis demonstrated that there is no genetic factor contributing specifically to general trust. All the genetic factors were shared by general trust and other personality traits, i.e., hostility, positive emotions, warmth, and altruism. The results suggested that heritability of general trust observed with the univariate analysis was a "reactive heritability" from personality traits. The implication of the data from the evolutionary psychological viewpoint is discussed.