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.