This mismatch signals the emergence of a phenomenon studied more commonly in the animal kingdom than in the human one—the “operational sex ratio,” the scientific term describing what happens when one sex outnumbers the other. In human populations, gender balances can tilt following world wars or times of migration (think California Gold Rush), resulting in a shortage of men or women of marriageable age. Currently, the most blatant outbreak of the operational sex ratio is playing out in China, where sex screening or, worse, infanticide has led to an estimated 32 million more males under the age of 20 than females…

A more worrisome issue arises when men take advantage of their relative scarcity by making life miserable for would-be girlfriends. Why settle down when you are a guy and the supply of eligible women appears to be unlimited? The female students hate such a situation, which is one reason admissions offices end up accepting male applicants who are less academically qualified than their female counterparts. Their goal is to avoid the dreaded 60/40 gender imbalance on campus that everyone agrees is a threshold not to be crossed. Those gender preferences, which colleges rarely discuss, have become common among private, four-year colleges (and recently caught the attention of the U.S. Commission on Civil Rights, which has launched a probe into admissions discrimination against women).

All this leaves single women such as Rachel Downtain facing unwelcome choices. “Going the sperm-bank method is definitely not my first choice, but I am not willing to give up my dream of having a child just because I can’t find Mr. Right. I am having to realize that my fairy tale dream may just be inverted a bit . . . I may have the child before finding Mr. Right.”