Zucker and Blanchard wrote that studies have consistently supported the "fraternal birth order effect" that homosexual transsexuals have more brothers than sisters and are born later in birth order. In contrast, in their study using an Asian sample they found that the transsexual men had significantly more sisters than controls. Zucker attributes this to the preference for males in Korean society causing parents to stop having children once they have a boy. Therefore, all male children are less likely to have older brothers and hence no fraternal birth order effect is observed.