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.