IMAGINE that the world consists of 20 men and 20 women, all of them heterosexual and in search of a mate. Since the numbers are even, everyone can find a partner. But what happens if you take away one man? You might not think this would make much difference. You would be wrong, argues Tim Harford, a British economist, in a book called “The Logic of Life”. With 20 women pursuing 19 men, one woman faces the prospect of spinsterhood. So she ups her game. Perhaps she dresses more seductively. Perhaps she makes an extra effort to be obliging. Somehow or other, she “steals” a man from one of her fellow women. That newly single woman then ups her game, too, to steal a man from someone else. A chain reaction ensues. Before long, every woman has to try harder, and every man can relax a little.
Real life is more complicated, of course, but this simple model illustrates an important truth. In the marriage market, numbers matter. And among African-Americans, the disparity is much worse than in Mr Harford’s imaginary example. Between the ages of 20 and 29, one black man in nine is behind bars. For black women of the same age, the figure is about one in 150. For obvious reasons, convicts are excluded from the dating pool. And many women also steer clear of ex-cons, which makes a big difference when one young black man in three can expect to be locked up at some point.