Age-structured homosexuality, such as that found in ancient Greece, also appears in some modern cultures in which adolescent boys go through a developmental period in which they are isolated with older males and perform fellatio on a regular basis. Role-specialized homosexuality contrasts sharply with our own cultural creation: the modern gay movement. If a scientist uses the same-sex attraction framework for homosexuality and considers that gay men and women have differently organized brains than straight men and women, then they have to postulate two different models, one for men and one for women. Scientists who think that biological variation causes or strongly contributes to human homosexuality go for the strong categories. If identical twins are both homosexual more often than are fraternal twins, researchers usually conclude that they have measured a genetic contribution to the development of homosexuality. In the 1990s a number of twin studies appeared that seemed to point toward a strong genetic component to homosexuality.