This article situates Mexican sexology, and how it engaged homosexuality and gender nonconformity, within more familiar nation-building projects in Mexico following the Revolution (1910–20). It argues that much like with understandings of race, Mexican sexologists, influenced by neo-Lamarckism and ‘Latin' eugenics, viewed sexuality as caused largely by social and environmental factors, rather than simply as a congenital characteristic. Such experts advocated for social solutions for what they saw as the ‘state of danger’ that homosexuality represented, targeting their interventions at youths, (...) who were seen as pliable, future citizens, rather than adults, who were largely seen as irredeemable and best isolated from the national body. The article explores discursive, ideological, and methodological threads in Mexican sexology from the 19th to the mid-20th century, the field’s professionalization and transnational connections, case studies of youths in which the preferred solutions involved promoting family stability and coherence, and adult cases ranging from prison studies to the case of Marta Olmos, recipient of Mexico’s first widely known sex reassignment. Overall, it demonstrates important intersections between sexology and nation-building projects related to criminology, anthropology, and eugenics, and how the attempted management of homosexuality and gender nonconformity sheds light on Mexican development more broadly. (shrink)