William Tucker suggests that societies that practice polygamy will always produce a violent fringe of men too poor or too powerless to have even a chance of marriage (or any kind of stable relationship with women):
Polygamy? What does that have to do with anything? Am I suggesting that because some minor sheik outside Baghdad takes two wives, two young Muslim brothers in Massachusetts feel compelled to blow up the Boston Marathon?
Well, yes. In any human society there are approximately the same number of men and women. Under monogamy, which limits each man to one wife, everyone gets a fair chance to marry. When powerful and successful men are allowed to take more than one wife, however, as they are in a polygamous society, this creates a pool of unsuccessful men at the bottom of society who are constantly in conflict with the system.
The history of Islam has been one continuous story of rebel groups off in the desert and deciding that the religion being practiced by the authorities and their harems back in the cities is not the “true Islam.” They come crashing back upon the palaces, overthrowing the leaders (no Ottoman Sultan ever died of natural causes) and establishing a new regime that is just like the old one, where powerful are allowed to take multiple wives.
The fruits of polygamy are visible all over the Middle East. Because women are always in short supply, families can charge a “bride price” to any man who wants to marry their daughter. Because daughters are now worth money, they must be veiled and sequestered so they don’t run off with some callow youth. Older men desperate for wives push down into younger and younger cohorts of the population. Marriages between 35-year-old men and 13-year-old girls become common. [...]
But the main product of polygamy is a population of angry young men who are ripe recruits for terrorism. The Koran supposedly limits a man to four wives but in countries where there are vast disparities of wealth this is routinely violated. Osama bin Laden’s father, a successful Saudi businessman, had 22 wives and 54 children. The unbalance between unmarried men and the available women in Saudi Arabia is the highest in the world.