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Sex differences are fairly controversial. According to an evolutionary perspective, we should expect sex differences in contexts where men and women have faced very different pressures.
One of the biggest areas of sex differences is, well, sex itself. Men and women, on average, have very different sexual psychologies. Men want to have more sexual partners, need to know someone for less time, watch more pornography and are more likely to have sexual fetishes than women. These are some of the largest sex differences known in psychology. Here is a summary from fellow Psychology Today blogger David Schmitt's 2011 paper.
Reproductive Rewards
The reproductive rewards for sex are much greater for men than for women. Here I'm going to focus purely on reproductive rewards, even though sex, especially in our species, has many other functions.
One of the major reasons for these sex differences between men and women is the genetic advantages of sex relative to the investment they put in. Men have smaller gametes than women, which means that sperm are far smaller than eggs. Sperm are the smallest cells in the human male body, while egg cells are the biggest cells in the female's body. Plus, If a man had sex with 100 women in a year, we would expect that he could theoretically have 100 children, although 50 or 60 would be much more likely. But if a woman had sex with 100 men in a year she could still only have one baby.
Considering the disparate investment is an interesting exercise here- what is the minimum investment men and women have to make in order to have an adult human with half of their genes?
We know women have to carry a baby for 9 months. But over our evolutionary history, a mother would have had to breastfeed for 3 years, on average, before the baby would be weaned. What's the minimum amount of investment a man could put in to have a baby? Let's be generous and say the minimum amount is an hour. This is called "minimum obligatory parental investment" and in human men and women, the asymmetry is bigger than in most other species.
Another way to consider this asymmetry is by putting a dollar value on it- let's say you had to put in a minimum down payment in order to have an adult with half of your genes. A man would have to invest about a dollar, whereas a woman would have to invest $32,000. Women also have many fewer opportunities to have children, since each one takes years of investment, so there is greater selection pressure on women to choose genes carefully. So, you can see why men might be psychologically disposed to have many more casual sex partners—he minimum investment they have to make is much smaller relative to women.
Reproductive Risks
But, there is another significant asymmetry that's usually not addressed in discussions about skepticism in sex difference- the asymmetry of sexually transmitted diseases.
And this starts with the genitals.
Men, if you haven’t noticed, have external genitals. The penis, among other things, dries up quicker because it’s out in the air. Bacteria and viruses tend to do better in warm wet environments. Women's genitals (and after years of teaching human sexuality I have no better way to put this) are more like a pocket. And this pocket is not only a much more hospitable environment for pathogens, but also pathogens can access the inside of a woman's body much more easily through the vagina. That’s why women are more likely to catch sexual diseases from men than the other way around and they suffer more from those diseases.
Women have a greater area of mucous membranes and experience more tissue damage during intercourse making them more prone to sexually transmitted infections like human immunodeficiency virus (HIV) human papilloma virus (HPV) and herpes.
Chlamydia is a good example to use here because millions of people have it at any one time. Compared to men, women are more than 3 times as likely to get chlamydia. This means a chlamydia+ man is much more likely to pass the disease to a woman than a chlamydia+ woman is likely to pass it to a man. Chlamydia is quite easy to treat now that there is wide access to antibiotics. But when women have untreated chlamydia 40% of them will get something called "pelvic inflammatory disease."
Pelvic inflammatory disease is uniquely possible for women because pathogens can travel out of the fallopian tubes and into the body cavity. See below a pretty good piece of evidence against intelligent design.
In terms of getting one'sgenes into the next generation, one of the worst things that can happen to anyone evolutionarily is sterility. Around 8% of women who get pelvic inflammatory disease will end up unable to have children. Compared to women, men can also have reduced fertility due to sexually transmitted infections, but almost never end up infertile.
Women, compared to men are also unique in that they can pass disease on to their babies when they're pregnant or nursing. Babies born to mothers with chlamydia are at risk for pneumonia and eye infections which can result in blindness. Mothers can pass HIV on to offspring during childbirth or while nursing. So, compared to men, women are more likely to catch diseases sexually, these diseases have a worse impact on them and they can also hurt their babies.
From an evolutionary perspective, a selection pressure has to be around for some amount of time before we can expect it to have an influence on human psychology. It's unclear how long these diseases have been around. But, risks to health can have a very strong impact on a species in very little time. Consider that a species of cricket in which males sing for females, the males went almost completely silent in just 20 generations because of a parasite.
The physical differences that cause this asymmetry in disease risk and severity between males and females go back millions of years. So, it is clear that the greater risk for females has been around for a long, long time.
A high rate of sterility and other problems in women would be a very strong selection pressure, and these risks have been around long enough to shape women's psychology to be significantly different than that of men. Unlike differences in parental investment, the difference in disease risk impacts women at all phases of the life cycle, including after menopause. These differences are probably also a major reason that women are much more sexually sensitive to disgust than are men.
References
Buss, D. M., & Schmitt, D. P. (2011). Evolutionary psychology and feminism. Sex Roles, 64(9-10), 768.
Fleischman, D. S. (2014). Women’s disgust adaptations. In Evolutionary perspectives on human sexual psychology and behavior (pp. 277-296). Springer, New York, NY.
Madkan, V. K., Giancola, A. A., Sra, K. K., & Tyring, S. K. (2006). Sex differences in the transmission, prevention, and disease manifestations of sexually transmitted diseases. Archives of dermatology, 142(3), 365-370.
