Both terms sex and gender are used interchangeably but they differ in meaning. Sex is a biological term which refers to the division of species on the basis of their anatomy and reproductive functions. Sex can be determined by chromosome patterns of gene expression, namely an XX set for female and an XY set for male. Such genetic distinction is called chromosomal sex and both terms male and female are strictly biological. Sex is also based on the type of gonads: either testes or ovaries; the presence or absence of the sex hormones estrogen and testosterone; external genitalia, or secondary sex characteristics like facial hair in men and breasts in women.
In contrast, gender is a psychological term which describes the individual's sense and reaction to biological sex. It is determined by biological, psychological and sociological factors as a range of characteristics distinguishing between femininity and masculinity. Those are societal beliefs and stereotypes about gender role activities. Gender consists of three elements: gender role, gender identity and sexual orientation/preference. Gender role defines the appropriate ways in which individuals should behave according to their sex, therefore, all activities and behaviors are categorized as masculine or feminine by society. Gender identity is the individual's subjective and personal sense of their own sex. Sexual orientation and preference describes a person's romantic, emotional or sexual attraction to another person from the same or opposite sex.
The "nature versus nurture" debate extends to gender and development of gender identity. Scientists argue about whether gender is determined by biological factors or it is acquired as a result of individual experiences and environmental factors. Empiricists believe that gender is a product of environmental influences and is especially influenced by the way individuals are treated by their parents, caregivers, friends and relatives. According to the 1955 theory of the psychologist and sexologist John William Money (1921-2006) individuals are psychosexually neutral at birth and gender identity develops only after that as a consequence of the nurture people receive during early childhood. Nativists, on the contrary, believe that gender is a result of nature and develops before birth thanks to hormonal action.
During early prenatal development all embryos are female and both sexes have identical external genitalia. The formation of male sex organs happens under the influence of the androgen dihydrotestosterone. The presence of Y chromosome causes the production of testes determining factor (TDF) which prevents the further development of female sex organs. The exposure of the fetal brain to dihydrotestosterone leads to male-specific changes in the shape and size of several structures like corpus callosum and cerebellum. Scientists believe that under such prenatal hormone exposure the so-called "brain sex" is formed which is predisposed to a particular gender identity usually coinciding with the individual's biological sex. Therefore, specific hormone exposure in infancy affects adult human behavior.
For supporters of the nurture theory of psychosexual differentiation gender identity starts developing immediately after birth when gender socialization process begins. Newborns are assigned with their gender roles from the beginning since they are dressed in gender-specific clothing and colors and are allowed to play with gender-specific toys. According to research, parents treat baby boys and baby girls differently. Boys, for instance, are bounced more often and encouraged to be active and alert, while girls are usually rocked and cuddled.
Since shaping the child's gender identity starts immediately after birth it is difficult to determine whether masculine or feminine behavioral traits result from biological or environmental factors. However, there are several theories about the development of gender in children such as social learning theory, cognitive developmental theory and gender schema theory.
According to the social learning theory, children observe their parents, relatives and peers and copy their gender-appropriate behaviors. Family members emphasize gender-appropriate behaviors and condemn those that are not when children are at the most impressionable age.
The second theory, cognitive developmental theory, maintains that children's understanding of gender develops together with their intellectual abilities. Young children may not understand until certain age that gender is a stable characteristic. This theory suggests that the development of gender identity is age-dependent, while according to the social learning theory it is a continuous process which starts from the first interactions of the child with the people around it.
The gender schema theory, in turn, holds that the development of cognitive abilities to form schemes is crucial for understanding gender. Those schemes are influenced by information within the surrounding environment which constantly provides children with new information about gender role and they continually upgrade their already developed gender schema. Therefore, their understanding of gender is constantly renewing and improving.