Before answering this, we must note that it is very hard to know what the origins of human behaviors are. The “nature versus nurture” debate is one that seems destined to continue indefinitely. We simply cannot know for sure whether certain behaviors are due more to biology or to one’s upbringing.
Secondly, we must distinguish between gender identity and gendered behaviors. Gender identity can be defined as our sense that we are male or female. This is surely determined more by biological sex than by one’s environment. Gendered behaviors, on the other hand, are simply areas in which men and women, on average, behave differently. Here, it is much harder to determine whether nature or nurture is the cause.
Gender identity is surely caused mostly by biology. People who are born male almost always identify themselves as men. The exceptions, transgendered people, are relatively rare. If their condition were a result of environment, there would surely be more such people.
By contrast, gendered behaviors are harder to attribute. Many gendered behaviors seem to be rooted in environment. For example, the differing propensity to wear makeup is surely something that is caused by the environment in which men and women are raised. But it is harder to tell with things like aggressive behavior. Are men more aggressive because they have been raised that way, or because they have more testosterone? The answer is not clear.
Thus, it seems that gender identity is caused more by biology while gendered behaviors may be caused either by nature or by nurture.