Many thanks to Eugene for inviting me to guest-blog about my new book Co-ed Combat (Sentinel (Penguin USA)). Today, I am going to provide an overview of the book, and subsequent entries will discuss specific issues in greater depth. I obviously cannot cover all of the issues raised in my book, but many that I don't address initially may come up in responses to comments.

Co-ed Combat starts from the premise that policies concerning sexual integration of combat forces should be measured first by their effects on military effectiveness. Other goals, such as expansion of women's opportunities, must give way to the extent that they impair combat effectiveness. Although the premise is contestable, it is a foundation upon which virtually all political discussions of the role of women in the military rests. Advocates of sexual integration of combat forces seldom argue that military effectiveness must be traded off against equal-opportunity concerns; instead, they contend that there is no tradeoff at all.

Under policies in place since early in the Clinton administration, women are permitted to serve on warships (other than submarines) and in combat aviation. They are still barred from "direct ground combat," however, including positions that "collocate" with (that is, operate side-by-side) ground-combat units. The Army seems to be violating the collocation rule routinely in Iraq, a practice that results in increased combat exposure for women, and some argue for completely scrapping the bar on women in ground combat.

I argue that those who believe there are no substantial tradeoffs involved in including women in combat roles are wrong. Inclusion of women in those roles results in a segment of the force that is physically weaker, more prone to injury (both physical and psychological), less physically aggressive, able to withstand less pain, less willing to take physical risks, less motivated to kill, less likely to be available to deploy when ordered to (partly, but not exclusively because of pregnancy), more expensive to recruit, and less likely to remain in the service even for the length of their initial contracts. Officers and NCOs must reassign physical tasks (or do them themselves) because women cannot get them done fast enough, if at all.

The fact that women, in general, are less effective warriors is only part of the problem. The more fundamental problem comes from the mixing of men and women in combat forces, which creates a variety of problems for reasons rooted in our evolutionary history. Women frequently are placed in units with men who do not trust the women with their lives and who do not bond with women the way that they do with other men.

The groups into which women are introduced become less disciplined and more subject to conflict related to sexual jealousy and sexual frustration, and men receive less rigorous training because of women's presence. Officers and NCOs must divert attention from their central missions to cope with the "drama" that sexual integration brings. Men, who traditionally have been drawn to the military because of its appeal to their masculinity, now find that the military tries to cure them of it to make the environment more comfortable for women.

Against these impairments of the military's ability to wage war, what are the benefits to the military of full combat integration? One possible benefit is an increase in the recruiting pool. Contrary to rhetoric, however, the pool is not "doubled" in any meaningful sense. Sexual integration of the military generally has increased the pool by only fifteen to twenty percent. Expansion of the potential pool of combat volunteers (in the ground forces, at any rate) would probably be more on the order of one percent at most.

If it is not numbers that women bring, then it must be something unique to women, but it is not obvious that women qua women would bring much in the way of specific benefits to the combat forces. In short, no one argues that eliminating the combat exclusion would unleash the whirlwind on America's enemies.

I should emphasize that my arguments are not an indictment of military women, although I do not believe that many women are suited to combat, especially, but not only, ground combat. But, in researching my book, I was struck by the high regard that most military men I spoke with have for military women outside the combat context -- even though most of these men opposed women's participation in combat. One can simultaneously appreciate military women's service to their country and also believe that all-male combat forces are more effective than mixed-sex ones.

The argument that full integration would be effective rests on a number of assumptions, including:

• That the high-tech nature of modern warfare means that the sexes no longer differ much in combat-relevant ways

• That as long as a woman possesses the individual physical and psychological attributes of an effective soldier, her inclusion in a combat unit would not impair its effectiveness

• That the primary obstacle to integration are men's "masculinist" attitudes, which can be overcome with adequate training and leadership.

All of these assumptions are flawed, in my opinion, and, as a result, the costs and difficulties of sexual integration of combat forces are often substantially underestimated.

I will discuss some of these issues in subsequent posts. My next post will talk about physical differences between the sexes and their continued importance in combat.

I appreciate the many thoughtful and enlightening comments to my earlier post. Many of the issues raised are matters I am writing about in this post or in subsequent ones.

Advocates of integration of women into combat forces often downplay the sex difference in physical capacity, correctly pointing out that some women are stronger than some men. In fact, however, there is little overlap between the sexes in terms of strength.

Women, on average, have only one-half to two-thirds the upper-body strength of men. The probability that a randomly selected man will have greater upper-body strength than a randomly selected woman is generally between 95 and 99 percent, depending upon the measure and the sample. Most of this difference is due to differences in the quantity of muscle tissue, a difference attributable primarily to sex hormones.

Although most discussion of physical sex differences focuses on strength, the sexes also differ on a host of other performance measures, such as running speed, aerobic and anaerobic capacity, endurance, and throwing speed and accuracy. These abilities are all potentially important in combat.

Some assert that these large physical differences can be overcome through training. In fact, however, training often increases the sex difference. Both sexes benefit from strength training, and in samples of out-of-shape individuals, women may initially gain more from training than men. Nonetheless, the overlap between the sexes decreases, because training not only increases the strength of both groups, it also decreases the variability within the groups. When males and females both start out in good physical condition, women gain less from further conditioning than men do, so the gap between the sexes actually increases.

Related to differences in strength and bone mass is the high rate of injuries, especially stress fractures, suffered by women in physical training. An extensive study of physical capacity by the British Ministry of Defence concluded that only about 0.1 percent of female recruits and 1 percent of trained female soldiers could satisfy the required physical standards for infantry and armor without sustaining substantially higher rates of injuries than men.

Much of the momentum for sexual integration of the combat arms rests on the assumption that the substantial sex differences in physical capacity, while real, are no longer significant, because battlefield prowess is now "a matter of brains, not of brawn." Thus, the lessons of primitive warfare -- or even that of any warfare prior to the late 20th century -- are thought to have little to teach us. This assumption is both misguided and dangerous.

Modern ground combat still requires substantial physical strength. Today's infantry soldier often carries between 75 and 100 pounds, and sometimes more. Just his rifle, ammunition, helmet, and body armor can weigh 60 pounds. Add to that food, water, night-vision goggles, various other electronic gear (and the batteries for it), and pretty soon the soldier is carrying a very heavy load -- indeed, heavier than that of the soldier of World War II.

After carrying this heavy load, soldiers often must dig in to hard ground for shelter, perhaps in 120-degree heat. If there is concern about chemical or biological agents, as at the outset of the Iraq war, soldiers may have to wear stifling protective gear, which imposes greater physiological stress on women than on men.

Then, of course, comes the infantryman's reason for existence -- engaging the enemy -- for which the soldier must have remaining energy reserves. Hand-to-hand combat (yes, it still happens) is the last resort of all war-fighters, as well as of those occupying support positions, whether signalmen, clerks, cooks, or truck drivers.

Hand-to-hand combat obviously requires physical strength, but it is far from the only fighting activity for which strength is essential. Many other activities do, as well, whether the lifting of heavy artillery shells or machine guns or carrying (or dragging) an injured comrade out of the line of fire.

Many combat-support positions also require physical strength. A study conducted in the 1980s found that all Army men in heavy-lifting Military Occupational Specialties (MOSs) were qualified for their jobs, but only about 15 percent of women were. The military has been reluctant to impose strength requirements widely, however, and even if realistic standards were set for particular jobs, adverse conditions often interfere with the neat system of MOSs. "It's not in my job description" is not a permissible response in a firefight.

Physically grueling tasks are not limited to ground combat. When a Navy EP-3E reconnaissance plane collided with a Chinese fighter plane over the South China Sea in April 2001, the muscular pilot had to "wrestle" the plane down to a safe landing on Hainan Island. He reported that it took "every ounce" of his strength to keep the plane in the air until he could land. Perhaps there are many men who would not have been able to meet that challenge, but it is unlikely that any female pilot could have.

Similarly, if a ship gets struck by a bomb, missile, or mine, all hands may have to turn to the tasks of damage control, such as fire fighting, flood limitation, and evacuation of the wounded. In 1988, after the guided-missile frigate USS Samuel B. Roberts struck an Iranian mine in the Persian Gulf, it came closer than any other U.S. ship since the Korean War to be sunk due to hostile action. Sailors of all specialties turned to fighting the resulting fire and flooding.

Because the captain of the Roberts was concerned that shells would "cook off," he ordered one of the magazines cleared of ammunition. A "bucket brigade" of fifty sailors -- twenty percent of the ship's crew -- passed the fifty-pound shells from man to man. Although the regular job duties of many of these sailors did not require heavy lifting, if the sailors had been unable to perform when necessary, the Roberts would almost certainly have sunk. Yet a Navy study found that almost all Navy women fail the physical standards for critical damage-control tasks, while virtually all men pass.

If physical performance were all that mattered in combat, the military could employ sex-neutral physical standards to select those men and women with the requisite abilities. It has generally been unwilling to do so, however. (For example, an 18-year-old female is given more time to run two miles than a 41-year-old man.) Moreover, the number of women who could satisfy the physical standards is sufficiently low that the adjustments that would be needed to allow women to serve would dwarf any benefit derived from an infinitesimal increase in the recruiting pool.

In any event, physical capacity is only one part -- and not the most important one -- of combat effectiveness. The sexes also differ along a number of combat-relevant psychological dimensions, the subject of my next posts.

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.

Many of the issues raised in the comments were thoroughly explored in the give-and-take there, so I will respond only to a few comments.

Several comments dealt with the issue of endurance, and it is an important one. One commenter cited a study purporting to show a female advantage at ultra-long distances. I do not have a copy of the article cited (my university's electronic subscription to that journal begins in 1998), but the abstract indicates that men and women were matched for "56 km race time, age and training." But matching for speed and training (presumably meaning aerobic capacity) means that the most fit women would be compared against less fit men. That is, if you take the top woman in terms of speed and aerobic capacity, you would compare her not to the top man but rather to a man who had the same speed and aerobic capacity as the top woman, who would generally be quite a distance from the top man.

The difference between being able to perform a task and being able to perform it over and over again (someone mentioned repeated combat sorties) can be very important in combat. That is the reason that women in the Israeli Defense Force are not eligible for combat assignments in the Armored Corps even though they do serve as tank instructors. Experience showed that the women could not load shells over sustained periods of time, which they do not have to do as instructors but may have to do in combat.

Several commenters remarked on the story about strength and the EP-3E pilot's experience over the South China Sea. The point of that story was not that all military planes present equivalent strength demands but that strength demands can crop up when things go wrong, even if a job does not require strength when things go right (which is the same point made about the USS Samuel B. Roberts). Moreover, no matter how high-tech the aircraft, once you are shot down, you are essentially an under-armed infantryman whose obligations are to survive (and assist fellow crew members in doing so, perhaps by dragging them from the wreckage), evade pursuers, resist potential captors, and escape from captivity. The harrowing stories of many shot-down pilots suggest that physical fitness and strength can make the difference between freedom and captivity and between life and death.

The post concerning psychological sex differences drew very animated responses. Let me just say a little bit to clarify the point that I was making. When talking about physical sex differences, my point was that they are relatively easy to measure but there is not much overlap between the sexes. If that were the only issue raised by sexual integration, the sensible thing would be simply to provide sex-neutral tests of physical capacity and let the chips fall where they may -- which would result in only a small number of women being deemed eligible for combat (and probably pressure for "gender-norming," as well).

When talking about psychological sex differences related to combat, there is more overlap but these differences are also more difficult to measure. There are various psychological tests to measure such things as physical aggressiveness and risk-taking, and these tests routinely show substantial differences between men and women. Most authorities on combat behavior would agree that physical aggressiveness and willingness to take physical risks (not suicidal recklessness as some commenters suggested) are associated with combat effectiveness. If you look at settings in which differences in aggressiveness and risk-taking have real-life consequences -- that is, when people are actually risking their lives, whether for heroic or criminal purposes -- you see a much greater sex difference in actual behavior than is revealed by psychological testing.

Ideally, again, one would select just those individuals who have the mix of psychological traits that allows them to overcome fear in the face of mortal danger, to be willing to take the fight to the enemy if the mission demands it -- risking their lives in the process -- and to inflict lethal violence on the enemy when the situation calls for it. I'd be surprised to hear that anyone -- even the most ardent supporter of sexual integration -- believes that this description fits men and women equally.

That's where the lack of predictability comes in. It is a staple of the combat-behavior literature that it is often a surprise who turns out to be an effective fighter (and who doesn't). Because some people do very well in training but bomb out in actual combat, you can't count on training to weed out those who won't do well. If you believe that there is a substantially higher proportion of Xs in Group 1 than Group 2, but you cannot identify which ones are the Xs, it is rational to attempt to maximize the number of Xs by selecting from Group 1 rather than from Group 2.

One or more of the commenters made the valid point that women who want to serve in the combat arms are not going to be the "average woman." That is true, but the men who serve in the combat arms are also not "average men." There will be a selection bias operating in both groups, although no doubt the female combat volunteer would deviate more from the female average than the male combat volunteer would from the male average.

One of the problems with spreading my blog entries out over the course of a week is that it is tempting to conclude that each posting is claimed to present a sufficient reason -- standing alone -- to exclude women from combat. What I tried to do in my book, and what I hope to do with the posts on this blog taken as a whole, is to identify a number of difficulties presented by integration of women into combat roles. The ultimate question is whether the combined effect of these difficulties would predictably lead to a less-effective military.

With Eugene's assistance, I have attempted to link all of these related posts together. We'll see if I have been successful.

My last post discussed some average psychological differences between the sexes. This post will discuss potential effects of these and other differences.

The "free rider" problem is potentially huge in combat. It is entirely rational from a selfish perspective for a soldier to keep his head down out of the belief that any minuscule effect that his battlefield actions might have on the outcome of the battle is far outweighed by the potential loss of his life. What really needs to be explained is not the fact that there are free riders but rather that there are so few of them. The explanation lies, in part, in the fears and motivations of soldiers.

Men going into combat for the first time face an array of fears. Among them, of course, are the fear of being killed or seriously injured. Some commentators have also identified a fear of killing, although it's not clear that "fear" is the proper word to describe the reluctance to kill often exhibited by western soldiers.

These fears -- fear of death, fear of injury, and fear (or at least reluctance) to kill -- are negatively motivating, and they are likely to affect men and women differently. Women's greater fear of death and injury and greater aversion to physical risks are likely to affect their combat performance negatively. Just being in a war zone is considerably more stressful to women than to men. A study of male and female support troops in the Gulf War, none of whom had seen combat, found that women reported significantly more psychological stress than men, especially stress in anticipation of combat.

Empathy also has negative effects, as it not only engenders a reluctance to kill but is also associated with greater guilt for having killed. Some reports coming back from Iraq suggest that women are suffering higher levels, and a more severe form, of PTSD than men are.

Perhaps surprisingly, fear of injury and of death are not the greatest fears that soldiers face going into battle (especially for the first time). A recurrent feature of military memoirs is that fear of cowardice dwarfed the fear of injury and death. As Samuel Stouffer's study of the American soldier in World War II found, showing cowardice in battle brought not just censure for cowardice itself; even more powerfully, "to fail to measure up as a soldier in courage and endurance was to risk the charge of not being a man." The fear of not measuring up as a man is highly motivating, but it is not one that motivates women.

In thinking about the relative suitability of the sexes for combat, one might think about real-life situations in which individuals have a choice whether to engage in activities that involve a combination of relevant traits -- such as fear, risk-taking, and physical aggressiveness. For example, who are the people who foil robberies, chase down purse snatchers and carjackers, and rescue others from criminal assaults? The answer is that these people are overwhelmingly men.

Combat Courage and "Defining Bravery Down"

If one of the primary motivators of men in combat is the need to prove their manhood and the need to maintain the respect of their comrades, one must ask what motivates women in combat? Surely, it is not the need to prove their manhood (or their womanhood, for that matter). As for respect of their comrades, women can maintain it at a much lower level of courage than men can, a fact that has substantial implications for the level of combat courage that one might expect to see from both women and men.

Aristotle wrote that "a man would be thought a coward if he had no more courage than a courageous woman." That may seem a gratuitously chauvinistic comment, but it captures an important truth. How often does one even hear of a woman referred to as a coward? The dictionary defines "cowardice" as "disgraceful fear or timidity." We do not decline to label women cowards because women do not display fear or timidity. Instead, we do so because we do not find women's fear or timidity disgraceful in settings in which we would see disgrace in men.

Because of the strong norms of equality in combat groups, if lesser courage is expected -- or at least accepted -- from women, that fact is likely to define bravery down for all. In his book "Hero or Coward," German military analyst Elmar Dinter observed:

We should never forget that the average soldier would really like to run away from the fighting. The group prevents him from doing this. If group morality allows for an "honourable" means of flight, it will be accepted gratefully.

Exactly this dynamic may have been in play in 2004 when a mixed-sex platoon of reservists refused a direct order to drive a fuel convoy, although the Army's reticence about the incident compels one to rely on (perhaps unfair) speculation. The reservists argued that it was a "suicide mission" because their trucks were not armored. News reports did not indicate who the ringleaders of the mutiny were, although it came to light when a female specialist left a message on her mother's voice mail asking her to "raise pure hell."

One could easily see how the sex composition of the group could have contributed to the incident. Expressions of unwillingness by female soldiers would give cover to the men to go along. By supporting the women who did not want to take on the mission (if that is what happened), the male soldiers could convert in their minds a cowardly refusal to take on a dangerous mission into a brave -- even honorable -- willingness to accept discipline to "protect" the women. The mission may be aborted, but honor within the group would be preserved.

Even if women actually were as courageous as men, they are not expected to be. That lower expectation of their courage -- irrespective of their actual levels of courage -- would almost inevitably result in reduced combat performance.

My next post will discuss the effect of sexual integration on cohesion and trust.

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.

I've discussed so far a variety of differences between men and women that affect their relative aptitude for combat roles. Another distinction between men and women that has significant effects on military readiness is that only women can become pregnant.

Approximately ten percent of military women are pregnant at any one time. During the Gulf War, pregnancy was the leading cause of women's being shipped back early to the United States. When the destroyer tender USS Acadia returned from an eight-month deployment during the Gulf War, thirty-six of the 360 women on board had been transferred off the ship because of pregnancy. The Acadia was the ship most prominently called "the Love Boat," but it is just one of many that have had that label attached to them.

A comprehensive study for the Navy of female shipboard personnel found an overall pregnancy rate of 19 percent per year. The highest pregnancy rate (27 percent) was on submarine tenders, the class of ships with the largest percentage of women.

With the unprecedented use of female personnel in Iraq and Afghanistan, one would think that the services would like to know what their losses are from pregnancy. According to a spokesman for Central Command, however, "We're definitely not tracking it." A Pentagon spokeswoman said that the Army does release information on how many women choose to leave the service because of pregnancy but not information on those who leave the war theater, implying that the information is tracked, simply not released. Only "general numbers" are released, she said, "to protect the rights of women, soldiers and the organization," although it is not clear how anyone's "rights" would be infringed by release of statistical information about pregnancy losses.

When it comes time to deploy, women fail to do so at three to four times the rate for men, the difference being largely due to pregnancy. Once a soldier is confirmed to be pregnant she becomes 'non-deployable' and will remain so for up to a year. After deployment, many women must be sent back home because of pregnancy.

A Navy study found that a quarter of women (compared with a tenth of men) were lost from ships for unplanned reasons. Large numbers of military pregnancies that are carried to term are unplanned (over 60 percent of those among junior enlisted personnel).

Pregnancy in the later stages means total absence of the woman -- who may or may not be replaced -- but even in the earlier stages it results in substantial limitations on a woman's ability to contribute to her unit. One Army MOS in which there are many women is "fueler." Fuelers are responsible for fueling vehicles and are critical to their units. Unfortunately, however, female fuelers are medically restricted from working in that job because of chemical exposure from the date their pregnancy is diagnosed. As the Army was preparing for Operation Iraqi Freedom, it had to impose a cap on the number of deployed women who could be allocated to that MOS, and it had to move men from other specialties into the fueler job, creating shortages elsewhere.

Women cannot serve at sea after their twentieth week of pregnancy, and even before that they must be removed from ships unless they are within six hours of a facility "capable of evaluating and stabilizing obstetric emergencies." After giving birth, mothers are excused from sea duty for a year.

Women's ability to avoid deployment by becoming pregnant is a constant source of resentment among men. Intentionally injuring oneself to avoid deployment is a court-martial offense; intentionally becoming pregnant to avoid deployment brings no penalty at all, nor does becoming pregnant to avoid deployment, missing the deployment, and then aborting the pregnancy -- a pattern that creates even intensified resentment. This latter phenomenon is almost certainly something that the military does not track, so it is hard to know how widespread it is, but while I was researching my book, several people (all Navy officers) spontaneously mentioned it to me.

Single parenthood is also a much greater problem among women than men. Although in raw numbers there are more single fathers than single mothers (because of the overwhelming disproportion of men in the military), the proportion of women who are single parents is much higher.

Comparison of the numbers of single mothers and fathers is meaningful only if "single parenthood" means the same thing for mothers and fathers, whereas it clearly does not. A Navy survey that inquired into the nature of custody arrangements found that 76 percent of single mothers had sole custody of the child, whereas only 16 percent of men did. While only 8 percent of single mothers had "joint custody (less than half the time)," 63 percent of fathers did. These are very different parental patterns, and they have substantially different effects on deployability -- differences that are obscured by simply labeling the involved personnel "single parents."

The military recognizes the incompatibility of single parenthood and military service. Army regulations, for example, bar single parents from enlisting, stating that "the Army's mission and unit readiness are not consistent with being a sole parent." The problem comes about when individuals already in the service become single parents. Single parents are required to file "Family Care Plans," identifying someone who will be able to take over parental responsibilities in the event of deployment, but if that arrangement falls through — or if the requirement is not complied with — then there can be a significant problem.

During the Gulf War, a number of military women with young children were transferred back to the United States because of the stress of being away from their children. Because of the longer deployments involved in the current conflicts, one doubts that this is a lesser problem today. Reliable data are not available (and perhaps do not exist), however, as the military has an obviously strong interest in not widely advertising the possibility of the return home for parents who miss their children.

My next post will be my last, and I will provide a few closing thoughts.

In my last post, I said that my next post would be my concluding one. However, I thought that it might be worthwhile separately addressing a few recurring points from the comments.

THE RACE ANALOGY: Several commenters suggested that the arguments made in my posts are illegitimate if they would be unacceptable if "race" were substituted for "sex." Acceptance of that argument would lead to the conclusion that the Women's National Basketball Association, sex-segregated bathrooms, and women's colleges are examples of apartheid.

The fact is that race and sex are different as categories. Although both of them have underlying biological bases, racial segregation in the military had nothing to do with the biology of race and everything to do with the social meaning placed upon race. Despite arguments to the contrary, however, sex is not just a social construct, and sex differences relevant to military service exist irrespective of what we think about them.

THE INDIVIDUAL-TESTING ARGUMENT: Several comments suggested that everyone should be given the same tests and any individual who can satisfy them -- irrespective of sex -- should be able to join. While that approach could work with strength, as I pointed out in one of my early posts, it works less well for psychological attributes. Moreover, some of the concerns about women in combat arise from the mere fact of women's "femaleness" as opposed to any particular individual traits.

Few people seem to be incensed by the military's use of age restrictions. If you are over 27 and try to join the Marines or Air Force, or if you are over 35 and try to join the Navy, or if you are over 42 and try to join the Army, you are extremely unlikely to be successful. Yet the same individualistic arguments can be made about people who are over the age limit as can be made about sex. After all, some people who are too old for enlistment no doubt would be more valuable to the military than some younger people whom the military would be happy to have. Of course, this could just mean that the age restrictions are ill-advised, too.

CITATION TO AUTHORITY: A number of people expressed regret (or more) that I did not cite authority for assertions in these posts. When I started writing my entries, I had to make a judgment about whether to cite to the relevant literature. I decided, for better or worse, not to, for a variety of reasons. First, assuming that I did not provide authority for every assertion, there was the difficulty in drawing the line between assertions for which I would provide authority and those for which I would not. Second, not all (or perhaps even most) readers of blogs expect or want to read heavily sourced, academic style writings, and I assumed that people seriously interested in the underlying research would go to my book (and, of course, I hoped that they would buy it for themselves, as well as for everyone on their Christmas card list).

Perhaps the most fundamental reason for not citing to the relevant literature is that a one-sentence assertion in a blog post might be summarizing several pages of my book, which in turn might be citing numerous authorities. To give one example, in my post on cohesion and trust I stated: "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." I was criticized for "bald assertion" in making this point, and, of course, the assertion was "bald," if that means that I cited no authority for it. However, that one sentence summarized about five pages from my book that contained nineteen footnotes that cited to over twenty separate sources, most from the psychological literature. That does not mean that my inferences and conclusions are correct, of course, but it does mean that I didn't make them up out of whole cloth.

I would like to thank Eugene again for inviting me to guest-blog and the readers who have provided thoughtful comments.

In these posts, I have touched upon some of the problems created by sexual integration of combat forces. There are many others that I cover in my book, including the tendency of men to protect women; the double standards often applied to women, which probably result, at least in part, from this same male protective impulse; the disruptive effect of sexual relationships and sexual attraction on group cohesion; and issues relating to female prisoners of war.

A common response to the issues that I raise here and in my book is that, yes, these are problems but they can be worked around; the military will "manage" these challenges with more leadership, more training, and more discipline. One could as easily say, however, that during the Battle of the Bulge GIs "managed" one of the coldest winters on record even with the challenge of inadequate clothing. But clearly both the soldiers and the war effort would have been better off if they had not been compelled to manage that particular challenge.

In deciding on rules about combat integration, the ultimate question can't be how to maximize women's opportunities. Instead, it has to be how to maximize the military's power to defeat the enemy. Clausewitz wrote that "everything in war is very simple, but the simplest thing is difficult." Mixing the sexes together in an integrated combat force adds substantially to what he described as the "friction" of war. The combat environment is difficult enough; we do no one any favors by making it even more so.