I remember, as a kid of probably 13, having a conversation with my father in which he was talking about the social differences between men and women (mostly by talking about the things that women do, and not actually addressing anything that men do), and he kept claiming that every difference (each of which was a stereotype rather than a true difference) was due to "fundamental differences" between men and women. It probably could have been guessed at that point that I was destined to study anthropology because I immediately pointed out that A) he was basing his claims on stereotypes and we both knew of many women who didn't behave in the way he was describing, and B) most of the attributes that he was describing as being fundamentally feminine were at least as likely to be cultural as biological, and therefore were fundamentally feminine only insofar as the culture of late-20th century North America said that they were feminine*.
My father just fixed me with a stare, and said "you have a thing or two to learn about women, and your life is going to be rough until you do."
Here we are, 21 years later, and, well, I still say that he was basing his claims on stereotypes and confusing culture with biology. To be fair, though, what he was doing wasn't all that different from what most people do. And, in fact, he was pointing to what he believed were real differences, but wasn't putting value judgements on them - women weren't worse or lesser, just different in his opinion.
But value judgements or no, this type of reasoning is still faulty. In order to get at why, though, we have to take this apart a bit.
First, let's talk about three types of differences, what I would call categorical differences, statistical differences, and cultural differences (no doubt people who categorize things for a living would have a different way of labelling these):
Categorical Differences: Categorical differences are things that draw sharp distinctions between groups. If you do not have trait X, you do not belong in group Y. For example, women as a rule do not have testicles. And if you have a uterus, that's a pretty good indication that you are not a man**.
Statistical Differences: Statistical differences can be summed up as "people in category A are more likely to have a given trait than people in category B." It is fair to say that, on the average, men are taller than women. On the average. There are women who are quite tall - my older sister is 6' even - and men who are quite short - Kaylia has a friend who is only 5' tall). A statistical difference is a difference in the average distribution of an attribute (such as height), not a hard-and-fast difference that separates groups.
Cultural Differences: These are differences that are based entirely on learned behavior that are culturally determined. So, for example, in North America a skirt is considered feminine clothing, but in other parts of the world, it may be considered unisex or even masculine (think of the kilt).
Where this gets messy is that there is constant feedback between these types of differences. For example, the fact that women can become pregnant and nurse a child has resulted in cultural constructs that revolve around this fact - constructs that may be altered when technology (another cultural adaptation) allows the basic rules to change (consider, for example, that there are tools that allow milk to be stored for future use, meaning that nursing women don't have to be with infants to feed them). Likewise, the fact that men are, on average, larger and physically stronger than women has resulted in a set of power dynamics in our culturally-defined gender roles that can be altered by a woman who is larger/stronger than most men, who is armed, or who is trained to fight.
The basic point is this - there are a number of different sources of gender differences, some of them are categorical differences based in the nature of our bodies, others are statistical differences based on the nature of our bodies, and others are cultural differences that may (or in some cases may not) be influenced by physical differences, but even when they are influenced by physical differences are as much arbitrary products of the idiosyncratic histories of our cultures as they are an adaptation to physical realities.
Many people have tried to claim cultural differences as categorical differences, or assumed culturally-based assumptions to be not only true but due to categorical differences (such as the notions that women are less interested in sex than men, or that women inherently attach more emotional meaning to sex than men do, because sex supposedly serves completely different purposes for both - a claim that fails in light of research on the subject). This is faulty thinking, no matter how many books such as The Rules or Men are from Mars, Women are from Venus take advantage of this sort of fallacious thought to generate sales.
Likewise, there is a tendency to treat statistical differences as categorical differences. I don't know how many times I have heard people use the results of studies showing slight cognitive differences between men and women as justification for sexism in the workplace or in training programs. Even if it were absolutely the case that men are generally better at spatial reasoning, to use one frequently-cited example, it's still a statistical difference, and there are many women who excel at this and many men who perform poorly. Very often, when you look into the actual data for the alleged gaps between the genders, you'll find that there is general overlap, and the differences between genders are statistically noticeable but negligible for all practical purposes.
And all of this feeds into culture, with both real differences and perceived differences being exaggerated, shaped, and mutilated into cultural practices that run the gamut from quaint to disturbing. In some cases, the very real biological origin of the practice may be clear, even if modern practices or technology make it irrelevant or even counter-productive. In other cases, burrowing through the reasoning finds no basis beyond sourceless stereotype for the cultural practice.
And this is where my basic frustration with gender differences comes from. Most of them are exactly that - gender (that is, culturally-defined sex roles) differences, and not sex (biologically-defined) differences. Even when sex differences are at work, they are filtered through our culture in ways that make it difficult at best to determine what is actually due to categorical differences between men and women, what is due to statistical differences between men and women, and what is simply the result of cultural/historical oddities. Yes, men and women are different in very definite fundamental ways, our biology shows that, but how these differences play out in our basic cognitive and emotional faculties, rather than due to training and acculturation, is very much open to question. We are making strides in better understanding this, but it is still an open debate.
Which brings me back to the conversation with my father. In the end, he informed me that men and women were different, that his observations of the difference were accurate, that it wasn't culture that defined our gender roles, but biology. When I said that I thought he was over-simplifying things, he said that, when I was older, I'd understand and that I'd see he was right.
I'm now older, and one hopes wiser. I still disagree.
*That was the gist of it, though I don't remember the exact words. Hey, it was 21 years ago, whatcha' gonna' do?
**Let's not get into gender re-assignment surgery, as it's another can of worms altogether.