Broadly speaking, I’ve noticed three different ways people can think about psychological gender differences: they can say there is no difference between men and women on a particular trait, they can say that there are two overlapping bell curves, or they can say that there is a fundamental, essential difference between men and women.

(To be clear, this is different from saying the difference is genetic or environmental. Whatever the cause of a psychological difference– genetic, environmental, or both– a person may think it resembles overlapping bell curves or that it is a fundamental and essential difference.)

First, some people think men and women are the same on a particular trait: for example, they might argue that men’s emotions are just as strong as women’s, that women are just as interested in sex as men are, or that men talk just as much as women do. They might think people are mistaken about the alleged difference: for example, the blog Language Log has written many posts arguing that men talk just as much as women do.

Sometimes, however, people think that men and women behave differently, but not because of an underlying psychological difference. They might believe that men and women face different incentives. For example, they might think that women are less likely than men to be interested in casual sex, not because they like casual sex less in the abstract, but because women are more likely than men to be shamed for having casual sex and less likely than men to have orgasms during casual sex. They might believe that women take more maternity leave than men do paternity leave because women have to recover from the physically difficult ordeal of pregnancy and childbirth and because pumping breastmilk is very inconvenient. Other times, people believe that different behavior is a product of discrimination. For example, women might be less likely to work in a particular field because hiring managers assume that they are incompetent.

Second, some people believe that there’s a psychological difference between men and women on the population level, but that many men have the more female-typical version of the trait and many women have the more male-typical version of the trait. This is easiest to see in a picture:

(Note that, in many cases, a person may believe the means are closer together than they are in this picture.)

An obvious example of a physical overlapping-bell-curves trait is height. Men are generally taller than women, but there are still lots of short men and tall women, and it’s not that surprising if any particular woman is taller than any particular man. Similarly, a person might believe that sex drive is an overlapping-bell-curves trait: men typically have higher libidos than women, but many men with low libidos and women with high libidos exist, and it’s not that surprising to find a heterosexual couple in which the woman has a higher libido than the man.

Third, some people believe in fundamental and essential differences between men and women. I’m going to do the worst job describing this one, because I basically don’t believe in fundamental and essential differences between men and women, but I’ll do my best.

The easiest way to observe this belief in the wild is to go to a bookstore’s relationship self-help section, where you will encounter books like Love and Respect: The Love She Most Desires, The Respect He Desperately Needs, Men Are From Mars and Women Are From Venus, and Act Like A Lady Think Like A Man. (Methodology: I looked on Amazon’s relationships bestseller list for books about gender.) As part of their fundamental premise, these books assume that no men primarily desire love, that no women want to retreat into their caves and refuse to talk to anyone when they’re upset, and that no men are willing to date women who don’t want sex that much.

But these beliefs affect more than relationship advice. I was recently reading Truth Overruled: The Future of Marriage and Religious Freedom, which made the following argument against gay marriage (edited for space):

The complementarity that defines marriage as the union of a man and a woman is crucial as well for the raising of children. There is no such thing as “parenting.” There is mothering, and there is fathering, and children do best with both… [University of Virginia sociologist] Wilcox finds that “most fathers and mothers possess sex-specific talents related to parenting, and societies should organize parenting and work roles to take advantage of the way in which these talents tend to be distributed in sex-specific ways.” These differences are not the result of gender roles or sex stereotypes. They are a matter of what comes naturally to moms and dads, what moms and dads enjoy doing with their children… [Rutgers University sociologist] Popenoe concludes: “We should disavow the notion that “mommies can make good daddies,” just as we should disavow the popular notion . . . that “daddies can make good mommies.” . . . The two sexes are different to the core, and each is necessary—culturally and biologically—for the optimal development of a human being.” What are the distinctive gifts of mothers and fathers? Wilcox reports, “Among the many distinctive talents that mothers bring to the parenting enterprise, three stand out: their capacity to breastfeed, their ability to understand infants and children, and their ability to offer nurture and comfort to their children.” And fathers, Wilcox writes, “excel when it comes to discipline, play, and challenging their children to embrace life’s challenges.” As Popenoe explains: “The complementarity of male and female parenting styles is striking and of enormous importance to a child’s overall development. . . . [F]athers express more concern for the child’s long-term development, while mothers focus on the child’s immediate well-being (which, of course, in its own way has everything to do with a child’s long-term well-being.) . . . [T]he disciplinary approach of fathers tends to be “firm” while that of mothers tends to be “responsive.” While mothers provide an important flexibility and sympathy in their discipline, fathers provide ultimate predictability and consistency. Both dimensions are critical for an efficient, balanced, and humane childrearing regime.’

This argument makes little sense from a non-essentialist point of view. From the point of view that psychological gender differences don’t exist, of course, it is nonsensical. With the exception of the capacity to breastfeed (which people with XY chromosomes do not have absent copious medical intervention), they argue, there is no reason that a parent of any gender can’t have any of those capacities. Who says fathers can’t nurture and mothers can’t discipline?

From an overlapping-bell-curves perspective, it is also silly. There are many more heterosexuals than there are gay people, and heterosexuals are more likely than gay people to have children. If even ten percent of women are as firm as the average man, then there are many more children who have a father and an unusually disciplinarian mother than there are children who have lesbian parents. Therefore, it makes sense to prioritize the unusual people getting married issue, perhaps by raising awareness that if you are a man who cares a lot about your child’s immediate well-being you should make sure to have children with a woman who prioritizes the future. (Alternately, since they’re willing to forbid gay marriage, perhaps they should require heterosexuals to take some sort of personality test to get married.)

But if men and women are basically different, those arguments sound like nitpicking. Sure, there are probably some playful women somewhere (they might argue), just like there are some people who only have one hand and some people who have eleven toes, but no one says “human beings have two hands except for certain people who have experienced tragic accidents.” At their core, men and women are basically different, and it makes sense to make policy based on that.

(Before the feminists and men’s rights activists of my blog howl, I’d like to point out that this is how the most striking physical sex differences actually work: there are in fact no cisgender men who can get pregnant and no cisgender women who can produce sperm. From a purely theoretical perspective, it’s not that odd to postulate that some psychological sex differences are equally striking.)