Jost’s default model of sexual development influenced far more than the study of genitalia and sex-related anatomy. By the late 1950s, scientists had im­ported the idea into the study of behavior. They theorized that testosterone primed the male brain, readying it for sex-related activities such as mounting, intercourse, and territorial defense. The female brain developed gender in testosterone’s absence. The idea seemed to map perfectly onto Jost’s account of anatomical development. But behavior was a more slippery subject than anatomy. Despite the confusions presented by intersexuality—in humans or in animals—anatomical development remained a reasonably clear-cut way to measure hormonal effects. There were testes or ovaries, an epididymis or a fallopian tube, a scrotum or vaginal lips. But research on sexual behavior moved beyond questions of anatomy to questions of masculinity, femininity, homosexuality, bisexuality, and heterosexuality.