First, some definitions (from the first article by Ghosh and Walker), first of gender identity and gender role:
Gender identity is defined as a personal conception of oneself as male or female (or rarely, both or neither). It is intimately related to the idea of gender role, which is defined as the outward manifestations of personality that reflect the gender identity. Simply put, gender identity is a self-label; gender role is a label given by society because of behavior and appearance. For example, if a person thinks of himself as a male and identifies himself as such, then his gender identity is male. However, his gender role is male only if he demonstrates typically male characteristics in behavior, dress, and/or mannerisms.
So gender is more of a social/psychological construct than an absolute biological outcome. Biology does determine the sex of an individual, which is often confused with the term gender:
The 2 terms sex and gender are often used interchangeably in the vernacular. However, [...] these words are not synonymous. [...] Sex, from the Latin word sexus, is defined by the gonads, or potential gonads, either phenotypically or genotypically. It is generally assigned at birth by external genital appearance.
If an intersex condition is present, one sex is often chosen with the intention of making social interactions and rearing simpler. Gender comes from the Latin word genus, meaning kind or race. It is defined by one's own identification as male, female, or intersex; gender may also be based on legal status, social interactions, public persona, personal experiences, and psychologic setting. Stating that sex is biologically determined, whereas gender is culturally determined, is fair.
So obviously, sex is determined by basic biology - XX for females, XY for males. The Y chromosome is the key; it encodes a gene named SRY, or testes determining factor (TDF). During the first 8 weeks of gestation, all fetuses are female. Around the eighth week, TDF is expressed, which leads to the development of testes. The testes begin producing testosterone, which is then converted to dihydotestosterone, the key hormone in virilization (making the fetus male). In the absence of the Y chromosome,the fetus will continue along the female developmental pathway. So that explains sex, what about gender identity?
Unfortunately, much of our understanding of gender identity stems from studying gender identity disorder patients, in which the process of gender identity development is believed to have misfired. This may be a useful strategy, but it is also like trying to figure out how a car runs by visiting a junkyard.
It is thought that the intrauterine hormone exposure sets a gender bias early in fetal development, which explains why female sex most often aligns with female gender identity, and male sex with male gender identity. There are other prenatal factors:
The environment in which a baby is reared with respect to gender begins to take shape prior to birth. Prenatal ultrasonography now allows the sex of a fetus to be determined quite accurately. In families who receive this knowledge of the child's biological sex, parental planning and reactions are often tailored by it. Names, items of clothing and toys, even specific thoughts about the baby, may be different depending on the anticipated sex. Thus, a preformed idea of the child's preferences is in place even before the child is delivered.
So a mother thinking about her child as a "boy" or "girl" and buying blue or pink drapes for the nursery has an effect? Personally, I am not sure about that, it seems like a stretch.
After birth, the parents are instrumental in reinforcing gender roles: little girls are hugged and cuddled with more, little boys wrestle and play with trucks. (According to Ghosh and Walker, this is more evident with fathers than mothers.) Gender identity may come from and be reinforced by these parentally-installed gender roles, and by 3-4 years old identity may be firmly set. Interestingly, gender-variant behavior seems to be better tolerated in females ("tomboys") than in males ("sissy-boys"), at least in Western society.
Looking at some biological bases of gender identity, there are a few chromosomal abnormalities and other mutations that can lead to sex or gender issues. In Turner's syndrome, the individual has only one X chromosome (XO genotype). These individuals are anatomically female, but are often infertile. Although the gender identity is usually reported as female, the infertility may lead to questions about gender role. Klinefelter's syndrome results from an extra X chromosome (XXY). Because of the presence of the Y chromosome, the fetus will generally develop as male. However, during puberty blood testosterone levels are lower than normal (XY) males, which leads to appearance of secondary female sexual characteristics, such as gynecomastia. These can lead to psychological problems and questions about gender identity.
Non-chromosomal causes include congenital adrenal hyperplasia (CAH) and androgen insensitivity syndrome. In CAH, a genetically female (XX) fetus is exposed to high levels of the hormome cortisol while in the uterus. This leads to enlarged external female genitalia, which sometimes are mistaken for male genitalia at birth. If the diagnosis is not made until puberty, as was often the case, these children were often raised as males, which can lead to gender identity problems when the female sex is revealed.
So is there a biological basis for gender? Since gender is a social (for gender roles) or psychological (for gender identity) construct, biology cannot be the only determinant. However, as outlined above for the disorders, there may be biological factors that influence gender. It seems to me that how a child is raised, the parental influence on gender roles, is the most important factor.
