One way to address questions about the origins and adaptive significance of personality dimensions is by comparing the personality structures of closely related species that differ in their socioecological circumstances. For the present study, we compared the personalities of captive golden-handed tamarins (Saguinus midas; N = 28), cotton-top tamarins (Saguinus oedipus; N = 20), and common marmosets (Callithrix jacchus; N = 17). All 3 species are New World monkeys of the family Callitrichidae. They thus share reproductive and behavioral characteristics but differ some in terms of their diet, habitat, and social organization. We expected that personality structures of closely related tamarin species would overlap more, both in terms of number of dimensions and their content, than either would overlap with the personality structure of common marmosets. We assessed personality using behavioral observations and compared the personality structures by means of cross-species correlations and fuzzy set analyses. Principal component analyses identified components that we labeled Agreeableness, Assertiveness, and Extraversion in golden-handed tamarins and common marmosets and components labeled Confidence and Extraversion in cotton-top tamarins. The greater personality similarities of the two phylogenetically more distant species suggest that differences in social organization, and in both habitat diversity and complexity, contributed to the evolution of personality. However, we also found that behaviors clustered in similar ways in the two tamarin species, suggesting that phylogenetic relatedness and genus-specific socioecological characteristics, such as the degree of reproductive competition, shaped personality structure in this primate family. (PsycInfo Database Record (c) 2020 APA, all rights reserved).