Khalid Shaikh Mohammed had a bigger plan. The 35-year-old jihadist didn’t want to just knock down tall buildings. He wanted to kill all the men and then hold a press conference.

As the story is told in the 9/11 Commission Report, the Number Two man to Osama bin Laden — now in U.S. custody — has told interrogators that there was yet another aspect to the September 11 plot beyond targeting the Capitol, the Seattle Space Needle, and a few nuclear plants. Mohammed told bin Laden that he wanted to take control of one more hijacked plane, systematically slaughter all the men on board, and then hold a press conference in which he would tell the world how America was being punished for supporting Israel.

I don’t want to sound like a Freudian here, but there is a very crude sexual dynamic to all this. Understanding it will give us an idea of the threat we are up against – and why it seems so difficult to enlist almost half the country (specifically the female half of the country) to the task.

Killing all the men and sparing women of the enemy is as old as history. When Moses sent the Israelites again the Midianites, he commanded them:

Now therefore kill every male among the little ones and kill every woman that hath known man by lying with him.

But all the women children, that have not known a man by lying with him, keep alive for yourselves. (Numbers 31: 17-18)

The strategy has a simple biological basis. Women are the “scarce resource” in human reproduction. One man can impregnate a thousand women but a woman can only be impregnated, at any given time, by one man. Men reproduce by controlling women. Women reproduce by going with controlling men.

Human civilizations — the peaceful ones, at least — have controlled this inherently volatile dynamic through a grand social contract called “monogamy.” Monogamy is a purely artificial norm which says that each man may marry only one woman. It was unquestionably born in the long pre-history of humankind when people survived in small hunting-and-gathering tribes that never numbered more than 40-50. In such vulnerable groups, the cooperation of every individual was essential. If a dominant male started monopolizing females, then the lowest-status males would be left without mates. These disaffected individuals would either cause endless disturbance (as they do today) or would be forced to migrate, leaving the unit vulnerable to attacks by other males. The most efficient strategy was to limit each male to one wife, thereby guaranteeing that each male had a reasonable opportunity of finding a mate.

Somewhere along the line of history, however, portions of humanity have abandoned this compromise. Polygamy is common among herding people and among the “horticultural” cultures of the tropics, where women do the farming. Indeed, it is always under pressure in any society, since it dissatisfies two large cohorts — dominant males, who are denied access to multiple wives, and low-status females, who are denied access to the best males.

In The Economics of Justice, Richard Posner speculates that polygamy developed among herders because, for the first time in history, they had “fixed assets” — stocks of cattle and sheep that could be exchanged for another important asset, wives. Since herders are nomadic and “pastoral,” they did not have to defend territory and could afford to lose a few extra males.

In tropical Africa and Polynesia, on the other hand, polygamy evolved because women became more productive. As hunting became exhausted, men declined to adopt agriculture, which was “women’s work.” As a result, women became economically independent. Such women did not have to depend on men for income but could simply attach themselves to the most powerful men for protection. These men — whose wealth usually derived form politics and land — were often able to accumulate dozens of wives.

As a result, the “bachelor herd” of mammalian biology is reborn in these societies. A significant residue of unattached males with little hope of mating becomes a significant social constituent. For these unattached males, there are two potential strategies: challenge and overthrow the dominant males within the society, or turn outward and conquer other societies.

The former is the story of Islam. There has never been a time in Islamic history when a large cohort of males was not challenging the central authority. The Shi’ites, the Assassins, the Mamluks, the Wahabis, the Mujahedin — there is no end to them. (Even today, “Fundamentalists” are challenging the authority of every Moslem state, including those established by previous generations of Fundamentalists.)

The latter, on the other hand, has been the story of Western and Eastern history. Right through the 19th century, there was always a polygamous “Mongol Horde” beating at the door of western European and Eastern civilizations. John Sobieski, the King of Poland, saved Vienna in 1683. The Chinese built their wall.

Islam, which sanctions polygamy, is not the cause of this historical pattern, but it ratified what already existed. Osama bin Laden is simply the latest incarnation of Genghis Khan — the leader of a nomadic, polygamous culture that has set up its “base” (“Al Qaeda”) in the same barren portion of the world.

Until September 11, 2001, America had essentially moved outside this historic dynamic. The Mongol Hordes, the Huns, the Moguls, the Turks — all existed “over there” or somewhere in the pages of history. Now the world has grown small enough so they are at our doorstep.

Is there any solution to this? There is. These polygamous cultures will have to adopt the ways of the civilized world — agriculture, trade, work, democracy, and monogamy. (We have our own share of disaffected males, called “criminals,” who cause a great deal of mayhem themselves.) The world will never become completely civilized until every male individual is joined to the task of building instead of destroying.

One more thing. This underlying sexual dynamic explains — to my mind at least — why women seem to have such difficulty in grasping the idea Islam forms a direct threat to our civilization. My wife assures me, with complete sincerity, that Mohammad Atta never would have flown a jumbo jet into the World Trade Center if people had been “nice” to him while he was living in this country. This makes him, of course, a victim of discrimination, an object of American prejudice — and ultimately a man without his own history or religion.

Women are natural appeasers. When two bull moose are battling over a harem, the females simply wait until a winner emerges and them go with him. Contemporary human females seem to retain some of this generic passivity. They cannot fathom the life-or-death stakes these conflicts present to men. Deep in their hearts they know that — like Khalid Shaikh Mohammed — the victor will ultimately spare their lives. Women — and the male politicians who appeal to them — honestly believe that hostile outsiders bent on destroying our civilization can be appeased.

William Tucker is a columnist for the New York Sun. His book, Monogamy and Its Discontents, will be published next year.