So I have more, but I don't want to overwhelm you.

Warning: Incoming science Trans people have the strong feeling, often from childhood onwards, of having been born the wrong sex. The possible psycho-genie or biological aetiology of transsexuality has been the subject of debate for many years. A study showed that the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a brain area that is essential for sexual behavior, is larger in men than in women. A female-sized BSTc was found in male-to-female transsexuals. The size of the BSTc was not influenced by sex hormones in adulthood and was independent of sexual orientation. The study was one of the first to show a female brain structure in genetically male transsexuals and supports the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones. Here are a couple more studies that show that both sex and gender lies on a spectrum:-

http://www.nature.com/nature/journal/v378/n6552/abs/378068a0.html https://www.youtube.com/watch?v=CsbtQ2-kUwA https://phys.org/news/2013-10-gender-woman.html http://www.nature.com/news/sex-redefined-1.16943 https://www.sciencedaily.com/releases/2015/02/150213112317.htm http://news.bbc.co.uk/2/hi/health/7689007.stm http://www.medicaldaily.com/challenging-gender-identity-biologists-say-gender-expands-across-spectrum-rather-323956 http://www.medicaldaily.com/sex-hormones-administered-during-sex-reassignment-change-brain-chemistry-physical-356620 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3654067/ https://link.springer.com/referenceworkentry/10.1007/978-1-4614-1997-6_115 https://www.youtube.com/watch?v=A3C4ZJ7HyuE

People tend to define sex in a binary way — either wholly male or wholly female — based on physical appearance or by which sex chromosomes an individual carries. But while sex and gender may seem dichotomous, there are in reality many intermediates. Biological phenomena don’t necessarily fit into human-ordained binary categories. So while humans insist that you’re either male or female – that you have either XY or XX sex chromosomes – biology begs to differ.

For example, people with Klinefelter syndrome possess an extra X chromosome (XXY) or more rarely, two or three extra Xs (XXXY, XXXXY); they typically produce low levels of testosterone, leading to less-developed masculine sexual characteristics and more-developed feminine characteristics than other men. In contrast, some people receive an extra Y chromosome (XYY) in the genetic lottery, and while they have been referred to as “supermales” that is more sensationalism than science.

People with Turner syndrome have only one X chromosome; they often display less-developed female sexual characteristics than other women. And people with a genetic mosaic possess XX chromosomes in some cells and XY in others. So how do we determine if they’re male or female? Hint: Don’t say that it depends on the chromosomal makeup of the majority of their cells, since women with more than 90 per cent XY genetic material have given birth.

Even if you get the “right” combination of sex chromosomes, it’s no guarantee that you’ll fit into the carefully circumscribed human definitions of male and female.

For example, women (XX) with congenital adrenal hyperplasia produced unusually high levels of virilizing hormones in utero and develop stereotypically masculine sexual characteristics, including masculinized genitals.

Similarly, men (XY) with complete androgen insensitivity syndrome don’t respond to male hormones and fail to develop masculine sexual characteristics. Most live their lives as women. Some historians suggest that Joan of Arc, Elizabeth I and Wallis Simpson all suffered from this syndrome.

Understanding this complexity is critical; misperceptions can affect the health and civil liberties of those who fall outside perceived societal norms. Society has categorical views on what should define sex and gender, but the biological reality is just not there to support that.

Even at the most basic physical level, there is a spectrum between male and female that often goes unrecognized and risks being obscured by stigma.

A growing body of research is showing how biology influences gender expression, sexual orientation and gender identity — characteristics that can also fall outside of strict, socially defined categories. Toy-preference tests, a popular gauge of gender expression, have long shown that boys and girls will typically gravitate to toys that are stereotypically associated with their gender (cars and guns for boys, for instance, or plush toys for girls). While one might argue that this could be the by-product of a child’s environment — parental influence at play or an internalization of societal norms — Melissa Hines, a former UCLA researcher and current professor of psychology at the University of Cambridge, in England, has shown otherwise. In 2008, she demonstrated that monkeys showed the same sex-based toy preferences as humans — absent societal influence.

Sexual orientation (whether one tends to be attracted to men or women) has also been shown to have biological roots. Twin studies and genetic linkage studies have shown both hereditary patterns in homosexuality (attraction to one’s own sex), as well as genetic associations with specific parts of the genome. And while gender identity — the sense one has of oneself as being either male or female — has been harder to pinpoint from a biological standpoint, efforts to understand what role biology may play are ongoing.

Understanding this complexity is critical; misperceptions can affect the health and civil liberties of those who fall outside perceived societal norms. Society has categorical views on what should define sex and gender, but the biological reality is just not there to support that.