It is a truism that individuals don't fight wars; groups do. That's one reason that the Army's "Army of One" campaign was so controversial. A group can be more or less than the sum of its parts, and the way members of groups interact can be at least as important as the traits of individual group members.

The importance of cohesion to military performance has long been recognized by authorities on warfare. Formation of, and functioning in, large cohesive groups is easier for men than for women, and men are more accepting of hierarchy than women are. Also, injection of women into male groups can undermine the cohesion and cooperation that is necessary to the group's functioning. Sexual competition plays a large role, but it is not the only reason.

A number of studies have found that including women in military groups can adversely affect cohesion. Although in studies of troops in garrison women often are found to have a positive effect on cohesion, in field and deployment settings the effect tends to be negative, and the greater the danger, the greater the negative impact of women's presence. Psychologist Leora Rosen found, for example, that the presence of women deployed in Somalia, where risk was relatively high, had more negative effects than in Haiti, where the risk was quite low.

One of the principal reasons for women's adverse impact on cohesion is that men find it difficult to trust women in dangerous situations, no matter how much they might like and respect women generally. This is true of soldiers, police officers, and firefighters. Not only are men concerned that women will not be able to drag them out of danger if the need arises, but also that women will be insufficiently aggressive in the event of conflict.

Trust is the most important value to all kinds of groups -- even ones not facing danger. Danger enhances the importance of trust, so that it is particularly important to cohesive combat groups.

The decision to trust is "fast and shallow." That is, we don't generally agonize about whether to trust somebody -- instead, it comes to us fairly quickly based upon rules of thumb that we are largely unaware of. In that respect, it is much like sexual attraction: basically, you feel it or you don't. Trust can wax or wane depending upon experience, but it is very difficult to overcome an initial lack of trust. Reasoned arguments about why you should trust somebody who strikes you intuitively as untrustworthy are not likely to be very effective.

To explain why men's reluctance to trust women may be intractable, it is useful to analogize to men's preferences in selecting mates. Psychologists have shown consistent patterns of male mate preferences, with men tending to place a premium on youth and beauty, which have been indicators of fertility over evolutionary time. This preference is found cross-culturally and is stable over time. Men having such a preference would have been at a reproductive advantage over men who found grey hair and wrinkles the ultimate turn-on.

Mating is not the only kind of association that would have had substantial fitness consequences over evolutionary time. Men's choice of comrades for warfare and hunting would also have been highly consequential in our ancestral environment because of the danger involved and the dependence of individuals on their comrades. If so, then one would expect that men today might possess innate preferences for certain kinds of comrades for dangerous enterprises, and these would be comrades who display the traits that would have been markers of effective fighters in our past.

Certainly, we see such preferences acted out. Even in childhood, the farther boys roam from home, the stronger their preference for same-sex comrades. Men prefer friends who are physical risk-takers, and, as men face danger, their preference for all-male groups increases. Men tend to pick up on cues that would have been associated with combat effectiveness in the past (and in the present as well) -- courage, strength, dominance, leadership -- in short, masculinity.

A study of Korean War soldiers found that "masculinity" and "leadership" were the two most important traits of soldiers who were judged to be effective fighters. A related study found that men who had been rated as being effective fighters were independently rated by other soldiers as desirable combat comrades after a week's exposure to each other, even though the men's combat histories were not disclosed.

If men are innately predisposed toward trusting certain kinds of comrades, it may be extremely difficult -- if not impossible -- for women to trigger that trust in men. This effect would prevail even if warfare actually had changed so much that the traditional warrior virtues are no longer relevant.

In making gut-level decisions, the human mind tends to be attentive to the kinds of information available to us in our ancestral environment. So, good grades at a military academy or high scores on a personality test would be unlikely to engender trust even if they were in fact correlated with combat performance, in the same way that a woman's appearance will be more important to the strength of a man's sexual attraction to her than a certificate of fertility from a medical specialist. Intuitive judgments are not easy to change with reasoned argument.

Thus, there is reason to believe that some impediments to effective sexual integration are, in a sense, "hard-wired" into us. If so, the resistance of combat troops to sexual integration is not something that they are going to "grow out of."

A recent survey at the Air Force Academy supports this view. A full 40 percent of cadets, both male and female, expressed the view that women will never be completely accepted in the military because of the physical and psychological differences between the sexes. Twenty percent of male cadets said that women shouldn't even be at the Academy, which has been sexually integrated for over three decades. And, it might be noted, the Air Force is the service with the highest proportion of women.

Some respond to this line of argument by contending that a tendency of men not to trust women is "men's problem," not women's. The issue is not, however, whose "fault" it is (and it is not clear that the concept of fault is even relevant here). Instead, the point is that this lack of trust -- whatever its source -- poses a risk to the effectiveness of military units. Thus, the lack of trust is a problem for both men and women, as well as for the military (and the nation) as a whole.

In addition to its effects on unit cohesion, sexual integration creates a number of manpower challenges, among them being the effects of pregnancy and motherhood, which are the subjects of my next post.