There's an old joke in the gay community: What does a lesbian bring on the second date? A U-Haul. What does a gay guy bring? A friend.

Gay women, according to the stereotype, are all too ready to move from fiery romance to feathering the nest. And what follows is Lesbian Bed Death, a warm, cuddly, committed but asexual relationship.

Of course, this is a stereotype. Throngs of old-married lesbians have sizzling sex lives, while scads of gay men enjoy monogamous relationships that Ward Cleaver would envy. Still, there is some neurochemical truth to this joke.

Certainly, gay men and lesbian women love and bond the same way as straight people do: During sex and orgasm, their brains produce spurts of oxytocin and exciting rushes of dopamine. Oxytocin is the neurochemical of generosity, trust and social memory. The interaction of oxytocin and dopamine in the brain's reward system ties the pleasure of sex to that particular partner, creating the bond we call love.

In gay love, however, the lovers' systems are more alike than those of a heterosexual pair. Most important, estrogen enhances the bonding effects of oxytocin, while testosterone mutes them. While the effects of orgasm's oxytocin surge may last for hours in a woman's body, they may clear from a man's body in just a half hour.

A new study led by Kristin Kramer of the University of Memphis shows that manipulating oxytocin at birth can make changes in the central nervous system that only show up later in life.

There's growing concern that the jolt of pitocin routinely used in U.S. hospital births could have unforeseen consequences. This study provides more ammunition. At the same time, it did not show that high doses of oxytocin interfere with social behavior later.

Why do some people quickly link up with mates who love them good and strong, while others gravitate to people who hurt them, dump them or withhold love?

It's all in the neurochemistry. I've come up with a metaphor that helps explain this painful syndrome.

When you're a little kid, you get used to your mom's spaghetti sauce; it's the one that tastes right, the one against which all other spaghetti sauces will be judged. (Please substitute latkes, baba ganoush, banh xeo or whatever; and for mom, use dad, another primary caregiver, or Boston Market.) When you leave home and get more experience, your tastes may broaden. But when you're a kid, it's the ONLY real spaghetti sauce.

Your parents take you out to a dinner at a fancy Italian bistro, and that spaghetti just sucks. You sleep over at a friend's, whose dad simmers organic heritage tomatoes with oregano fresh from the garden? Ick!

Well, every family lives in a neurochemical stew that's just as redolent as that sauce. It's made up of cortisol, for stress; oxytocin, for intimacy; adrenaline, for fear; serotonin, for calm -- and probably a few other things we haven't yet figured out. And the taste of the family chemistry feels normal. It maybe doesn't feel good, but it feels normal.

It's a different kind of home cooking: the neurochemical kind. So, if you grew up simmering in cortisol and adrenaline, and you meet someone whose chemistry is oxytocin-rich, it feels bland. Wimpy. Boring or smothering. Without that seasoning of stress, the blend isn't right.