This study charted the development of gendered personality qualities and activity interests from age 7 to age 19 in 364 first- and secondborn siblings from 185 White, middle/working-class families, assessed links between time in gendered social contexts (with mother, father, female peers, and male peers) and gender development, and tested whether changes in testosterone moderated links between time use and gender development. Multilevel models documented that patterns of change varied across dimensions of gender and by sex and birth order and that time in gendered social contexts was generally linked to development of more stereotypical qualities. Associations between time with mother and expressivity and time with father and instrumentality were stronger for youth with slower increases in testosterone.