They go to college, or they get a job:
Whom you marry depends on where you live, but also on how old you are and what race you are. Most people marry people of the same race, of a similar age and from the same area. 96 percent of married black women have black husbands, and over 96 percent of married white women have white husbands. [...]
There are two million men in US prisons and just 100,000 women; and the men in prison are spread unevenly across age, race and geography. Huge numbers of young black men are in prison, and that is bound to pose a problem for the young black women they might otherwise have married.[...]
In New Mexico, for example, 30 percent of young black men, aged 20-35, are in prison (or, less commonly, in a secure mental institution). That is an extreme case, but there are 32 states with more than one in ten young black men in prison, and ten states where one in six young black men are behind bars. That is a serious business for young black women.
According to economists Kerwin Kofi Charles and Ming Ching Luoh, where a large number of a particular racial group is in prison, women of the same age and race in that state do not enjoy the gains from marriage, or a stable relationship, that women in a more equitable situation do.
Charles and Luoh show that young black women facing a shortage of men try to increase their attractiveness as marriage prospects. The more men are in prison, the more likely women are to get themselves a job, and the more likely they are to go to college. College-educated people are much more likely to marry other college-educated people, so an education doesn't just make you smart, it wins you a smart husband or wife.
Improving their bargaining position in the marriage market is, of course, not the only likely reason for these decisions. Since the high incarceration rates of young black men mean young black women are less likely to marry, a college degree and a job look like a rational investment for a single girl who can't rely on a partner as a source of income. What's more, the likelihood of young black women not marrying is greatly exacerbated by another trend: it appears that young black men who are not in prison typically take advantage of their strong bargaining position by not bothering to marry at all.
The better-educated guys stay out of jail, and they are smart enough to realize that with the competition locked up, they don't have to get married to enjoy themselves. [And] even though it's mostly uneducated men that end up in prison, Charles and Luoh show that the negotiating position of women is so weakened that they end up more likely, not less, to "marry down"—that is, to marry men who are less educated than they are.
This is from the I-would-be-very-surprised-if-it's-not-simply-excellent 'Logic of Life' by Tim Harford, FT columnist, author of The Undercover Economist and economics writer extraordinaire. The paper (free access) is here.
You can (/should!) order Tim's book at amazon.co.uk if you live in the UK, or at amazon.com if you are in the US (and beyond, I guess). Buying the book by clicking on the amazon.co.uk link will put something less than one pound in my pocket via the amazon associates programme. The contribution would be greatly appreciated; the proportion of my demographic that is in jail is scandalously low, so I need the money desperately.
Buying through the .com link credits Messrs Cowen and Tabarrok of Marginal Revolution with a similar amount.