Gender of rearing
Gender of rearing refers to the gender presentation with which a child is raised. In nearly all cases a child is assigned gender in infancy based on external genitalia. Parents follow this with gender-specific clothes, toys, activities etc, and socially-determined opportunities, restrictions, and expectations exist in every culture. In adulthood, in most cases, a person's gender identity is identical to their gender of rearing.
From the 1960s to the 1990s academics and the feminist movement posited that gender of rearing was the prime factor that deternine gender identity, and psychological sex differences. In recent years, though, research in genetics and other fields of biology suggests a return to the school of thought that holds 'nature' as more influential than 'nurture' here.
Gender of rearing becomes of interest when a child is born intersex or an adult identifies as transgender. Intersex, which refers to a condition where an infant has indeterminate external sex organs, is nearly always 'fixed' by surgery shortly after birth. The child is reared in the gender corresponding to its external genitalia, and this is where the gender of rearing or 'nurture' argument has been weakened in the last two decades. Many adults born interesex have reported that the gender they were assigned and molded to at birth was not where they felt a sense of belonging. Transgender people, born with a defined set of genitalia, identify with a different gender from that of their rearing. Whether there are genetic or biological factors at play is a vital area of research, and could add interesting data to the assessment of how important gender of rearing is.