The topic of sex differences, or the lack thereof, is a perennially controversial issue. Sex differences in and are the best known: Few people are surprised by findings that men are more likely to commit a violent , or that women are generally more discriminating about jumping into bed with someone they hardly know (e. ., Daly & Wilson, 1983; 1988; Kenrick et al., 1990).

Despite the fact that those differences are large, there are nevertheless big arguments about exactly what one can conclude from the findings. There is a common misperception that contributes to those arguments, but before I address that misperception, let me talk about a slightly less controversial sex difference.

Source: Original drawing by Douglas T. Kenrick, used with permission

"Tend and befriend" versus "fight or flight" patterns

In a now-classic paper, published in 2000, UCLA’s Shelley Taylor and her colleagues reviewed evidence that the famous “fight or flight” response was more characteristic of men’s reactions to than of women’s.

Taylor proposed instead that women’s responses to stress were more likely to involve what she and her colleagues dubbed a “tend and befriend” reaction. Tending refers to behaviors that protect the female’s self and her offspring; befriending refers to behaviors that reinforce her .

My favorite example of this sex difference comes from a classic film by the distinguished anthropologist Irven DeVore. The film is not about humans, but about baboons.

When a predator arrives on the scene, the males in the baboon troop stand together and face the predator. The males have especially large canine teeth, and a group of males is capable of defending the troop against a leopard and most other dangerous predators, except a group of lions. The females, on the other hand, tend to huddle together in social groups, often surrounded by their offspring.

Whereas the male-typical fight and flight responses have been linked to hormones such as adrenaline and , Taylor and her colleagues reviewed evidence suggesting that females' tending and befriending responses might be linked to the / system, and to hormones such as , , and endogenous . Their argument was that different evolutionary pressures on males and females selected for these two different predominant responses to threat.

Developmental psychologist David Geary and anthropologist Mark Flinn, both then at the University of Missouri, published a response to Taylor and colleagues’ paper. Geary and Flinn agreed that it was conceptually useful to distinguish “fight or flight” from “tend and befriend” systems.

At the same time, they argued that those two systems are certainly not mutually exclusive to one sex versus the other. They reviewed theory and evidence from evolutionary biology and other fields that human males are also likely to have been selected to tend to their offspring and befriend one another (in the interest of fighting with outgroup members, for instance). They further argued that there have been selection pressures on females to compete with one another as well.

Taylor and her colleagues responded to Geary and Flinn’s analysis by pointing out that some of the underlying systems were, in fact, sex-differentiated. Nevertheless, the exchange illustrates an important point: It would be incorrect to presume that every woman will respond to stress in a different way from every man.

The sexes are similar in the way they respond to stress in some ways and different in others. Both sexes may fight, run, tend, or befriend, depending on the circumstances, albeit with different probabilities, and different triggers for the different reactions in men versus women.

Pink, blue, and gray: The dividing lines between the sexes are almost always blurry

The exchange regarding the “tend and befriend” system is also instructive about the broader question of sex differences. This was a relatively benign and reasonable debate, perhaps because there is nothing too politically controversial about saying that females are generally more cooperative and nurturant than males (as I say this, I realize it’s easy to turn almost any statement about sex differences into something offensive).

But much more controversy has surrounded the claims about sex differences in aggression and sexuality, particularly when researchers have pointed out that those differences appear to be based on biological factors that hold universally across human cultures, and even across a wide range of other animal species (e.g., Daly & Wilson, 1988).

One factor that contributes to those controversies is the perceived political implications of any inferences drawn from findings of universal or biologically based sex differences. For example, some believe that to admit that evolutionary pressures might have selected for more intra-sexual in males is to suggest that it is therefore natural to encourage males but not females to take positions of in modern organizations and political groups.

That kind of reasoning is an example of the naturalistic fallacy, which is the mistaken notion that because something is “natural,” it is therefore “good.” It is easy to see the fallacy in this line of reasoning by observing that “natural” processes include cancer and parasitism in wasps (which prey on caterpillars by paralyzing them and then laying eggs inside the paralyzed bodies).

But another factor contributing to controversies about sex differences is this: although there are some biologically based differences in male and female morphology that are almost completely non-continuous (breasts and testicles, for example), there are very few behaviors that cannot be commonly found in both sexes. Even for something such as extreme physical violence, where males commit something on the order of 90 percent of homicides worldwide, there is nevertheless the 10 percent of homicides committed by females. And although men are generally more likely to favor unrestricted sexual behavior, there are some females who are more unrestricted than the average male, and some males who are more restricted than the average female.

Some take the existence of overlapping distributions as evidence against the involvement of biological factors, but that is a logical mistake. Some women are physically taller than the average man, and vice-versa, but that does not negate the argument that there are biological factors contributing to an average difference in height.

So, it’s important to resist the temptation to hear about a finding suggesting a psychological difference between females and males, and then jump to the conclusion that that statement refers to every single male and every single female.

Some related discussions

I discuss some broader psychological factors that contribute to the misinterpretation of scientific findings in "The Science of Antiscientific Thinking."

One of the largest and most reliable sex differences is the discrepancy in the preference for younger versus older partners (see "Why Are Older Hollywood Actors Paired With Such Young Women?" and also "When Statistics Are Seriously Sexy"). Again, there are exceptions even to that very large sex difference, but the exceptions help us understand the fuller story (see "The Mind as a Coloring Book II").

Another reliable sex difference involves the relative desirability of hearing a partner first tell you “I love you” before versus after first having sex, but once again, there are theoretically sensible within-sex differences in that phenomenon as well (see "Never Tell a Woman You Love Her, Unless…").