I must admit that I find Facebook’s selection scheme for gender identity:
However, I think it’s probably a good launching point to talk about both gender identity and biological sex.
I’m just going to kick things off with an explanation of biological sex. If you are squeamish about talking about genitals and hormones in a biomedical fashion, TURN BACK!
Biological sex sounds easy but isn’t just a simple thing.
Gonads (ovaries, testicles), external and internal genitalia (vulva, clitoris, vagina, penis, prostate) are two parts of physical sex. Gonads start life partially as part of the middle layer of the 3 early layers in the embryo (the mesoderm) with a few cells from the 2nd of 3 primitive kidney-like structures (the mesonephros). The primitive gonads can develop into either ovaries or testicles, or, very rarely, organs that have a mixture of both, or some undifferentiated components (e.g. ovotestis; true hermaphroditism). Life requires at least one X chromosome. People with one sex chromosome only (XO, Turner’s Syndrome) develop as infertile females essentially; the “default” state is to develop as a female and the SRY coding area on the Y chromosome is what causes development of testes and male genitalia.
Similarly to how ovaries and testicles are the female and male counterparts of each other, the penis and the clitoris are counterparts, the scrotal skin and labia majora (extenal labia) are counterparts. There’s some equivalent to prostatic tissue that exists within the vagina, the Skene’s gland which may or may not be the “G-spot”. In male foetal development, the primitive vagina and labia fuse to form the perineum, which is why there is a ridge down the middle of the perineum between penis and anus.
Development of the genitalia is also driven by hormonal conditions of the foetus. In the male foetus, testosterone and the related hormone 5-alpha dihydrotestosterone (DHT) promote the development of male genitalia while Müllerian inhibiting substance causes regression of the Müllerian ducts that would otherwise grow into fallopian tubes, uterus, cervix and upper vagina. The foetus itself creates sex hormones from its developing gonads as well as some contribution from the maternal blood supply. Males and females at any stage of development including adulthood will always create both oestrogens and androgens but in varying quantities and ratios. There are also genetic conditions where SRY is missing or where androgens don’t work or there are increased androgens or any number of things. So you can have “XY females” (XY gonadal dysgenesis- the immature gonads are highly likely to become cancerous later in life and are usually removed) and what may appear to be “XX males” (most commonly, congential adrenal hyperplasia where the adrenal glands overproduce hormones).
Then of course, there is the fact that reproductive system congential anomalies are by far and large the most common chromosomal or physical congenital anomaly. In fact it’s something like 1:100, and mainly in males. Some of it is stuff like micropenis or hypospadias or clitoromegaly. Some of it is more complex anomalies like cloacal abnormalities where like in reptiles, the female urogenital tract and anus end all in one hole. In the more complex disorders it’s more common to have other physical anomalies or syndromes. One of the reasons for the high- and increasing- rate of reproductive system anomalies (also found in all other species on earth) is due to increased pollutants containing “oestrogen-like compounds”. Most of it is industrial waste. There’s some hypothesised contribution from the urine of women on the pill but honestly I think that is bogus.
In addition, chromosomal sex has options other than XO, XX and XY. Basically, X chromosome encodes many genes necessary for life. There is almost complete deactivation of any extra X chromosomes so that, for example, females won’t just die from having 2 sets of those same genes and overexpressing them. As a result, anyone who has more than 1 X chromosome is referred to as a “mosaic”- some of their cells express one X, some express the other, and … if you have multiple X some express those other ones. You can have any number of extra X’s. XXY, XXX are not all that uncommon. XXY is Klinefelter’s syndrome which results in tall, thin, infertile and somewhat feminised males. XXX causes a phenotype similar to XX but with lower fertility. XYY also exists and is associated at times with slightly heightened aggression and sometimes mild intellectual disability. The more sex chromosomes you have the less fertile you are and the more likely to have intellectual impairment. In addition XO, XXY, XYY etc are associated with physical anomalies (like webbed neck and aortic problems in XO) that are not related to sexual differentiation or gender.
Sexual development clearly isn’t over the moment you’re born. Boys and girls have pretty much identical hormonal states until puberty. It’s only at puberty that they start expressing high enough levels of sex hormones (oestrogens and androgens) that you really get major differences. The hypothalamus sends signals to the pituitary gland which signals the gonads to GET INTO ACTION! And this is what causes “secondary sexual characteristics” like breasts, pubic & armpit hair, penis enlargement - the Tanner stages of puberty are based on evaluating these. So you get changes at this time as well and can get disorders and variations at this point too.
As adults there are still other things which will affect hormone levels. PCOS in females causes extra androgen levels. Obesity can cause PCOS. Obesity can also cause feminisation in males sometimes. Liver failure can cause reduced conversion of oestrogen to testosterone in males too. People sometimes take medications that can induce hormonal changes too- sometimes for sex reassignment, but more commonly for contraception, to treat or prevent cancers, because of menopause or other things.
Hormonal state can sometimes affect gender orientation too. But for reasons we don’t fully understand, sometimes if someone has their sex reassigned as a baby and hormone treatment given, they have the gender identity of their chromosomal or original biological sex, and sometimes they have the opposite. This can lead to disastrous consequences.
And I haven’t even talked really about fertility or how gametes (sperm and eggs) form!
So biological sex is very complicated. It’s not simple at all. There are many different states that could be considered “intersex” (hermaphroditism is a medical word and is not at all the preferred term for people who are intersex). Intersex people have various gender identities and it is not always obvious what it would be just by guessing!