Gender is something people become aware of at an early age. As soon as children can form complex ideas about the world, they begin to assign themselves and others a gender, and these ideas are frequently not congruent with the expectations many others have. Like all things biological, gender is messy.
For the sake of next few paragraphs, I use gender to mean a self image and a pattern of behavior that aligns itself with norms that are recognized as patterns exhibited more frequently by males, females, neither or both. It will be different for each society in its manifestations.
Despite traditional societal forces pressuring humans to conform to a standard, nearly one percent of adolescents in the US currently identify as transgender. Many also do not identify with any gender. They are non-gendered, or multi-gendered. While the largest proportion identify with a particular gender, strict definitions fail to describe most humans. Like all things biological, gender is messy.
Gender is determined internally and externally, biologically and socially. Internally, our bodies define gender within our brains, but it is written into every cell in some way. Like all gendered animals, we arrive at gendered behaviors through complex interactions between genes, epigenetic phenomena, signals from other cells, and cues from our environment. These influences include the hormonal currents bathing our cells, the way we perceive the world, and the way the world imposes expectations on us.
Non-human animals and humans have learned and inherited gendered behaviors. Externally, gender is taught and enforced by family, friends, and institutions that give order and structure to social interactions. This is as true for birds as it is for humans. Check out birds of paradise or bower birds for fantastic examples of both learned and inherited gender behavior. However, like all things biological, gender is messy.
Biologically, people are not born into the wrong body, but they may certainly be born into the wrong society: one that does not allow them to express their gender.
To try to change another person’s chosen gender through coercion is a form of abuse. Even though external forces influence our development, gender and sexuality cannot be externally forced on someone. The only authority on one’s own internal drive is the individual. It is hard for me and most others I have encountered to conceive that something can be both a choice and not a choice at the same time. That is because I lack imagination and ignore obvious truths about the world.
Some of the particulars of gendered behavior can be influenced by social pressure (the colors we wear, the games we play, the way we express our feelings, the jobs we take), but the ultimate determination of and definition of gender within any individual is their own right. Our institutions would serve themselves well to admit that like all things biological, gender is messy.
Some of the most obvious evidence of this comes from relatively rare genetic differences. For example, there are many people who are born XY (male), but do not develop external male genitalia normally. They have cells that don’t respond normally to testosterone and are frequently raised as female. Sports organizations have particular difficulty including these individuals. Some may choose to live as males. Others may adopt the assigned female gender. Others may reject either assignment. This is only one of many possible ways our biological development may not align with societal expectations of gender. These are not exceptions that prove the rule. They are just more examples of how inherently messy gender is.
There are a plethora of human attributes that are assigned to male or female gender, for example that men lack empathy and women are more cooperative due to the biological presets their gender bestows upon them. There may be some trends in certain directions, but popular beliefs are largely speculation based on a logical distortion called confirmation bias (what happens when we only pay attention to or measure things that confirm our beliefs, ignore or suppress things that don’t, and do not perform the requisite and often impossible experiments we need to tell the difference).
I cannot tell you with confidence what aspects of human behavior have a large biologically derived gender component. How to determine this seems easy only if one never questions how we arrived at gendered things. There may be ways to find out. The first could be to look for gender attributes that appear to be universal among different cultures. Yet, these are not definitive. The advent of global empires, communications, and development of global culture makes it nearly impossible to see how disparate societies developed in the past, relatively uninfluenced by each other. Even if we had this knowledge, society cannot speak for the individual. The majority does not define the minority.
It is hard to rely on historical accounts either. Most were written from an encultured male perspective, so there is a high probability of bias. Those who did not fit in lived at the margins of society and were ignored by history. People tend only to point out facts that agree with their positions and ignore or distort the rest.
We cannot randomize orphans to different gender treatments because it is not ethical or practical. Instead, we can be patient (not easy for humans) and look at aspects of society that change over time when social pressures are released. If there are no genetic/biological differences or only minor differences between the genders, then most differences will dissolve over many generations. However, our opinions about gender are rarely patient. The forces that gender us are entrenched.
What all this means is that understanding the weight of biological vs. sociological pressures on an individual’s gender will be argued for many hundreds or thousands of years. If released from the most oppressive social pressures, then the essential elements of gender will become more apparent over time. But, like all things biological, gender is messy.