"A history of sex", The Economist, 2004 September 23.

By examining the DNA of living people, [Dr Michael Hammer at the University of Arizona in Tucson] and his colleagues have found....a lot of variability in the mitochondrial DNA (a type of DNA which follows the female line), and much less in the Y-chromosome DNA (which follows the male line). The most plausible explanation for this is that a few men in each generation contribute the bulk of the Y-chromosomes to the next.
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This news will not surprise biologists. Although a moment's thought shows the old canard that males are actually, on average, more promiscuous than females cannot be true (since every reproductive act involves one of each) biologists have known for a long time that in most species males want to be more promiscuous than females.

One result which did surprise the researchers was that men's genes tend to travel farther than women's. Some 70% of the world's modern cultures practice patrilocality—in which a woman moves from her native village to her husband's village when she marries. Until now, it has widely been assumed that this practice would result in women's genes migrating farther afield than men's. So, not only are fewer men than women procreating, but they are travelling farther to sow their oats. Clearly, the tall, dark stranger from another place has been an attractive proposition to women for quite some time.