For singles, apparently, the price of sex is going up, and not just for men who share Eliot Spitzer's taste. Ordinary people legally exchange valuable items for sex all the time, says Daniel Kruger, an evolutionary psychologist at the University of Michigan.

Kruger's claim had me flashing back a few days, when I let my fiance fill my gas tank. Was this generous and gentlemanly behavior or a sex-for-fuel exchange?

It might seem obvious to most regular people but to scientists who study such things, there's no clear line between gift-getting and prostitution.

In a paper published in this month's issue of the journal Evolutionary Psychology, Kruger interviewed 475 college students and found that 27 percent of the men and 14 percent of the women reported trying to trade something to get sex. "Sometimes it was money, sometimes it was funding voice lessons, and sometimes it was giving tickets to the Ohio State versus Michigan game," he said. "There's a black market for those tickets - they're quite sought after."

Conversely, about 5 percent of men and 9 percent of women reported offering sex in the hope of getting some kind of freebie.

It's not surprising that men are the ones more likely to give gifts or money or fuel, and women are more likely to receive such bonuses. That happens among penguins (nest-building rocks for sex), as well as monkeys and even insects (food for sex).

But now the economy is bad, how's a guy to get some action if he can't afford those coveted football tickets or even a tank of gas? "Some of them did laundry," Kruger said, or they cleaned up a woman's room.

What woman could possibly resist that? Men, prepare your vacuum cleaners.

- Faye Flam