And yet I've held onto more than my share of losing hands in the game of love. Can science tell us anything about why we stay involved with people we should jettison?

A surprising source of insight came to me from The Mind of the Market, a book by Michael Shermer. He describes all sorts of ways we make irrational choices - choices that don't serve our best interests or those of anyone else.

"We hang onto losing stocks, unprofitable investments, failing business, and unsuccessful relationships," he wrote. And why?

The human brain has an affinity for the status quo, says Shermer, who came through Philadelphia last month on a book tour. Though he has a background in psychology, he's better known as founder of Skeptic magazine and author of such popular books as Why People Believe Weird Things.

In his latest work, he details all kinds of evidence that we overvalue what we already have. It's something psychologists call the endowment effect, he says. In one recent experiment, researchers gave some students a $6 coffee mug and others nothing. The students without the mugs wouldn't pay more than $2.75 to acquire one, but those with the mugs wouldn't accept less than $5.25 to part with them.

In another example he cites in the book, two options for car insurance were offered to people in this region in the early 1990s - one more expensive than the other.

In Pennsylvania, the state made the cheaper but less-comprehensive plan the default, and so, not surprisingly, most people stuck with it. But in New Jersey, the default was more expensive, and it became the more popular one.

Worse yet is the so-called sunk-cost effect - ruinous to your stock portfolio, your love life and your poker game. To illustrate, Shermer asked me to imagine that I needed $20,000, for, say, tuition payments. I owned $40,000 worth of Apple stock, which had doubled from an original $20,000 investment. I also owned $20,000 in Ford stock, which had shrunk from an initial $40,000 investment. Which stock should I sell?

Shermer said he, like most people, would be tempted to sell the winning Apple stock, somehow hoping he might still recoup his Ford losses. But that's irrational. We should dump the loser and keep the winner, and yet we tend to feel invested in whatever has already cost us money - or time.

He agreed that this principle applies in the realm of love. "People hang in there a lot longer than they should," he said. "Rationally, we should just compute the odds of succeeding from this point on."

But love is more difficult than investing. "When we're in love, our hormones take over, and we ignore the signs we would normally pay attention to," said Sabitha Pillai-Friedman, a Philadelphia-area sex-and-relationship counselor. "The excitement of dating affects our judgment."

That does wear off, she says, "but by then, we're emotionally attached." And if the relationship is already unhappy, it only gets worse from there.

In other words, love can ruin your life. Sort of. "Love is what you describe it," she says. There's no agreed-upon definition. Some people think you can leave a person you love if he makes you miserable; others say they must not love someone if they choose to leave.

If love is all you need, however, half of all couples wouldn't be getting divorced after promising eternal love. That's not the worst of it, said Pillai-Friedman. "There are also quite a huge number of marriages that are chronically unhappy, and those aren't included in the 50 percent."

Incompatible core values can be a good reason to leave, as can a partner who consistently makes you feel bad about yourself.

All this helped me make an important break. I'd been thinking about leaving this column, "Carnal Knowledge," for some time.

I've discovered many fascinating things about the evolution of sex and the sexes, about the whys and hows of the mating game. I got the opportunity to write a book called The Score: How the Quest for Sex Shaped the Modern Male, which will appear in bookstores this spring.

I admire those who've made it their life's work to break down the taboos and write or talk openly about sex. But my primary aspirations go in a different direction.

Many years ago, I dreamed of becoming a science journalist. I worked hard to see my dreams come true. The sex column was a midcareer adventure that I assumed I'd keep for a couple of years. It's been almost three. After this column, I return to my true love.