According to a 1998 study released by two Canadian psychologists who analyzed population patterns, the European conquest of the Americas, the Nazi aggression in World War II, the Iraqi invasion of Kuwait, and the ethnic wars in Africa and the former Yugoslavia may have begun in part because of an unusual demographic feature common to all. Christian G. Mesquida and Neil I. Wiener found a correlation between societies with a high percentage of young, unmarried, violence-prone men and the occurrence of wars. The psychologists contend that societies in which young males comprise 35 percent to 55 percent of the population are more likely to begin a war than societies which have a lower percentage of young men.

Mesquida and Wiener contend that a surplus of young males creates a shortage of land, food, and marriageable women. Although the men may not realize their true motivation for going...