Every culture with a gender binary imposes gender onto all aspects of human appearance and behavior. This is done in order to reify that binary and make it appear to be "natural," rather than arbitrary. As an example, let's look at handwriting. Most people can make a better than chance guess at someone's gender identity based on a handwriting sample. But why?
A couple of years ago, a reader asked me how she could make her handwriting more "feminine." I had a general sense that in the US, "feminine" handwriting is neater, more loopy, and in its most mockable form has little circles or hearts dotting each letter I. It turns out it's much more complicated than that, and the entire field of inquiry can easily devolve from legitimate forensics into quackery like "graphology" and "evolutionary psychology." The section has become the most popular part on my how-to site for transgender people, as there is considerable general-market interest in the topic, even in our age of texting and IMs. I include 30 handwriting samples (worth reading because they are cute aphorisms). Learn about gradient, structure, concavity, and maybe even make your chicken-scratch a little more legible, no matter what your gender.