Yesterday's post dealt with physical differences between the sexes. It produced a lot of good comments that I will try to get to today or tomorrow. This morning's post will look at a number of psychological differences. Although measurable sex differences in most psychological traits related to combat tend to be smaller than physical differences -- so there is more overlap between the sexes -- they are still substantial.

RISK PREFERENCE: From toddlerhood on, males have a greater preference than females for risk -- especially, but not only, physical risk -- a fact reflected in the substantially higher rate of accidental death among boys worldwide and the roughly twelve-fold sex difference in workplace deaths among adults in the U.S. A meta-analysis of 150 risk-taking studies covering subjects of all ages concluded that "males took risks even when it was clear that it was a bad idea," while females "seemed to be disinclined to take risks even in fairly innocuous situations or when it was a good idea."

FEAR LEVELS: Risk-taking and fear are intimately related, and females from infancy experience greater fear than males. Sex differences in fear and risk-perception have two components. Women are more likely to perceive risk in a situation than men are, and even when the sexes perceive the same level of risk, women have higher levels of fear.

Psychologist Anne Campbell has argued that sex differences in fear and risk-taking are a consequence of differences in selective pressures acting on the two sexes over evolutionary time. Women have stood to gain less than men from risk-taking, which among men is often related to reproductive competition. Moreover, women have more to lose in terms of reproductive fitness than men, because in primitive societies the death of the mother is a greater blow to the odds of a child's survival than the death of a father. Indeed, the death of the mother often amounts to a death sentence for her children. Thus, Campbell argues, women's minds have evolved to rate the costs of physical danger higher than men's do.

PHYSICAL AGGRESSION AND DOMINANCE: As with risk-taking, sex differences in aggression and dominance appear early in development, being present from about two years of age. Among adults, the clearest evidence for sex differences comes from criminal activity, with men being incarcerated for violent offenses at a rate more than ten times that of women.

Men not only engage in more physical forms of attack, they also have more positive attitudes about aggression. They are more inclined to view it as an acceptable way of achieving one's ends, and they experience less guilt and anxiety about having acted aggressively than women do.

NURTURANCE AND EMPATHY: Women score higher on most measures of empathy, which, to paraphrase a former president of the United States, consists of the ability to feel someone else's pain. This greater empathy may be responsible for the heightened guilt and anxiety that women feel about acting aggressively.

The sexes also differ in the circumstances that attenuate empathy, as demonstrated by a recent study examining empathic responses with functional magnetic resonance imaging (fMRI) of the brain. Subjects watched two players playing a game, some players playing fairly and some unfairly. Players were then given electrical shocks.

When a player who had played fairly was shocked, both male and female subjects showed activation of brain areas that respond both to one's own pain and to observation of pain in others. When an unfair player was shocked, however, the empathic response of male -- but not female -- subjects was substantially reduced. Areas of the brain associated with reward processing, on the other hand, showed enhanced activation in men, but not women, when the unfair player was shocked. These findings suggest that men's empathy may be more easily "switched off" and that they may derive greater psychic satisfaction from inflicting harm on those perceived as deserving it.

PAIN TOLERANCE: Although it is commonly asserted that women have a higher tolerance for pain than men -- a belief apparently resting on women's endurance of painful childbirth -- a large body of data refutes that argument. Instead, women generally withstand pain less well than men. A major review of pain studies found differences of over one-half a standard deviation for both pain threshold (the level at which a stimulus is perceived as painful) and pain tolerance (the level at which pain is no longer bearable).

Like sex differences in strength, these psychological sex differences -- all of which are mediated by sex hormones -- are individual differences that do not hold true for all members of their respective sexes. Moreover, the individually measured psychological differences are smaller than the physical differences described yesterday. Some women possess more physical courage and willingness to kill than some men.

Because of the overlap between the sexes, arguably combat personnel should be selected on the basis of these traits rather than using sex as a proxy. However, unlike strength, which can be easily and cheaply screened for, future courage under fire cannot be readily measured. A consistent theme in the combat-behavior literature is that one never knows who is going to be an effective soldier until the shooting starts, and the identity of the good fighters often turns out to be a surprise.

Because individualized predictions of combat performance are not a practical way to select personnel, at least on a wholesale basis, proxies such as sex are more necessary when it comes to predicting whether one has what it takes psychologically to be effective in combat than they are for strength.

My next post -- I hope later today -- will consider the effects of these psychological sex differences, as well as some others, on combat motivation.