Identifying and quantifying sex differences and similarities has been a central research question and fascinated scientists for centuries. A large body of work has been accumulated on this topic; however, conclusions are often drawn as if they are applicable across cultures even though studies have predominantly relied on Western samples. This chapter reviews cross-cultural literature on several early childhood sex differences in domains of development that have caught attention in the literature recently: gender-typed play, gender identity, and gender expression. We also offer an overview of possible influences on sex differences, including evolutionary, biodevelopmental (genetics, sex hormones, and immune factors), and sociocultural mechanisms (socialization and macro-cultural factors). Given that a cross-cultural perspective has often been lacking in this literature, this chapter reviews research on early gender development in males and females from Western populations as well as the non-Western populations wherever possible to highlight important cultural (in)consistencies.

Androphilia refers to sexual attraction and arousal to adult males, whereas gynephilia refers to sexual attraction and arousal to adult females. Male androphilia is considered one of the outstanding paradoxes of evolutionary biology because its very existence flouts our expectations concerning what constitutes an evolutionarily viable trait (Bailey & Zuk, 2009). In humans, male androphilia is heritable, as evinced by twin studies (Alanko et al., 2010; Bailey et al., 2000; Kendler et al., 2000; Långström et al., 2010), as well as research in the area of molecular genetics (Hamer et al., 1993; Mustanski et al.,2005; Sanders et al., 2015). Despite the heritability of this trait, androphilic males reproduce at far lower rates when compared to gynephilic males, if they reproduce at all, which, very often, they do not (e.g., Bell & Weinberg, 1978; King et al., 2005; Saghir & Robins, 1973; Schwartz et al., 2010).