OK, we all know that men and women do not always see eye to eye. We can have different , desires, ideas and actions … sometimes. Other times, we are very much in synch. If you stop and think a bit about biology, it turns out that men and women are a lot more similar than most of us realize. In this post, I am going to suggest that sometimes focusing on the similarities (or better put, the "overlaps") between males and females can help us towards a better understanding of where behavioral differences actually come from.

First, let's acknowledge the core differences in biology between males and females. These are evolutionarily, and practically, important and they do matter. Females have babies (gestate and give birth) and lactate, and males do not. Males are, on average, about 10 to 15 percent larger than females and tend to have greater upper body strength. Males’ brains grow for a bit longer and are a bit larger than females. But remember, as long as it is a healthy human brain (anywhere between 1,000 and 2,000 cubic centimeters) size does not relate to function. There are also some skeletal differences between men and women due to childbirth (wider pelvis) and male size/musculature (more rugged developments on male bones). Most of you reading this already know these differences … but do you know about the similarities?

Our hormones are the same. They function the same ways and we all have the same hormones … there are no “male” or “female” hormones. There is some important variation in hormone levels and patterns, and there are some differences in how the hormones interact with male and female bodies. On average, men tend to have higher resting levels of some androgens (like ), and females may have higher levels of certain reproductive hormones like Follicle Stimulating Hormone or Estradiol at certain times in their menstrual cycles. However, these same reproductive hormones also work in men and are involved in the process of sperm production. There is substantial overlap in the process and patterns of our entire system.

Our brains are the same. Aside from the slight size differences and the possibility of some differences in an area called the straight gyrus, there are no reliably and repeatedly demonstrated morphological brain differences between the sexes. Now, this is not to say that there is not a great deal of variation in brains across our species or that in some cases adult males’ and females’ brains can react differently to stimulus; there is a lot of variation in neurological structure and probably some in function … but it is primarily across individuals, not sexes.

Genitals? Most people think that male and female genitals are about as different as can be: penis = male and vagina = female. But even this basic dichotomy is not really correct: the genitals emerge from the same mass of embryonic tissue. For the first six weeks of development, the tissue masses develop identically. At about six to seven weeks, depending on whether the fetus has or XY chromosomes (usually), the tissues start to differentiate. One part of the tissues begins to form the clitoris or penis and another forms the labia or scrotum. Another area begins to form into either the testes or the ovaries. This means that physiologically, male and female genitals are made of the same stuff and work in similar ways.

What about sexual behavior? In general, humans have a lot of sex, they have it in a variety of different ways, and most importantly, males and females both have complex sexual lives. Substantive recent overviews of sexual behavior show few major differences between males and females in sexual activity: Men and women have more or less the same amount of sex in the same kinds of ways across the lifespan (remember, it does take two to tango). But there are some important differences. For example, married women report lower interest in sex with their husbands the longer they’ve been with them, and younger men report higher frequencies of and interest in visual . But are these primarily biological differences, or is something else going on? We still have a lot to learn about … and as with many other areas, it looks like variation is highest between individuals, not between sexes.

There is no doubt that our evolutionary histories result in important differences between the sexes. But these same histories and biology also result in core similarities between the sexes that are equally as important in understanding our lives. Biological differences between males and females can relate to behavioral dissimilarities (such as in physical and aspects of reproduction), but the majority of our biological characteristics (like our brains) reveal that males and females are much more similar than they are different.

So why do we almost always try to explain behavior by implicating biological (evolutionary) differences between the sexes? Could it be that our perceptions of what is “natural” for the sexes is biased? Why don’t we try to start some of our inquiries into human nature with a level playing field? Let’s not assume that there is a relevant sex difference until one actually emerges from the data.

Individual variation in our species is really important and the fact that the sexes overlap as much, if not more, than they differ should tell us something about how to ask questions about human nature. Misrepresentation of human biology and evolutionary patterns in males and females by focusing only on the differences while ignoring the overlaps facilitates a myopic view that inhibits good science.

Review the references for some good readings on this topic.