The survey was conducted in 1992, when the disease was much less treatable than it is today. Francis first looked to see if there was a positive correlation between having a friend with AIDS and expressing a preference for homosexual sex. As he expected, there was. "After all, people pick their friends," he says, "and homosexuals are more likely to have other homosexuals as friends."

But you don't get to pick your family. So Francis next looked for a correlation between having a relative with AIDS and expressing a homosexual preference. This time, for men, the correlation was negative. This didn't seem to make sense. Many scientists believe that a person's sexual orientation is determined before birth, a function of genetic fate. If anything, people in the same family should be more likely to share the same orientation. "Then I realized, Oh, my God, they were scared of AIDS," Francis says.

Francis zeroed in on this subset of about 150 survey respondents who had a relative with AIDS. Because the survey compiled these respondents' sexual histories as well as their current answers about sex, it allowed Francis to measure, albeit crudely, how their lives may have changed as a result of having seen up close the costly horrors of AIDS.

Here's what he found: Not a single man in the survey who had a relative with AIDS said he had had sex with a man in the previous five years; not a single man in that group declared himself to be attracted to men or to consider himself homosexual. Women in that group also shunned sex with men. For them, rates of recent sex with women and of declaring homosexual identity and attraction were more than twice as high as those who did not have a relative with AIDS.

Because the sample size was so small -- simple chance suggests that no more than a handful of men in a group that size would be attracted to men -- it is hard to reach definitive conclusions from the survey data. (Obviously, not every single man changes his sexual behavior or identity when a relative contracts AIDS.) But taken as a whole, the numbers in Francis's study suggest that there may be a causal effect here -- that having a relative with AIDS may change not just sexual behavior but also self-reported identity and desire.

In other words, sexual preference, while perhaps largely predetermined, may also be subject to the forces more typically associated with economics than biology. If this turns out to be true, it would change the way that everyone -- scientists, politicians, theologians -- thinks about sexuality. But it probably won't much change the way economists think. To them, it has always been clear: whether we like it or not, everything has its price.

THE WAY WE LIVE NOW: 12-11-05: FREAKONOMICS Stephen J. Dubner and Steven D. Levitt are the authors of "Freakonomics: A Rogue Economist Explores the Hidden Side of Everything." More information on the academic research behind this column is at www.freakonomics.com.