Several aspects of the Gender Diagnosticity (GD) approach of Lippa (1995) to measuring the psychological trait of masculinity-femininity within sexes were explored in four samples ranging from 363 to 5,859 individuals, including Swedish and Australian adults, U.S. elderly, and Australian adolescents. Two ways of deriving GD scales yielded highly similar results. Moderate stability of individual differences was found across ages 12 to 16 among adolescents, but substantial shifts over age occurred in relationships with Eysenck scales. Considerable generality of GD scales was obtained across languages and populations. Substantial heritabilities (about 40%) and minimal effects of shared family environments suggest that within-sex masculinity-femininity behaves as a fairly typical personality trait. Cross-age continuity appeared mainly to reflect the influence of the genes.