Scientists are increasingly taking a rather clinical look at romance; in fact, some are studying the science of kissing for clues about how relationships work or don't work. Here's Clayton Sandell.

For as long as we've been doing it, scientists have only recently begun to analyze what happens when lips meet lips. Researchers say when we kiss, our brains are on overdrive, giving us all kinds of subconscious clues about someone, like their health or fertility.

I mean, you can see them, smell them, taste them, touch them, feel them. You're not only getting a lot of information about who they are, but you're giving a lot of information about who you are.

And if it's bad information, that all-important first kiss can stop a courtship cold.

- I had one relationship where, you know, the guy, he was not a good kisser at all – it didn't go very far.

Men tend to have more testosterone in their saliva, which may help to speed up the – mating process. This may be one of the reasons that men like uhm, sloppier kisses, wetter kisses, more open-mouth kisses, with more tongue action.

Okay! Too much information.

- I think wet kisses are really gross, so I think that's a turn-off.

New studies into what turns us on shows how kissing triggers mood changes in our brains. Researchers Wendy Hill and Evan Lebovitz at Lafayette College in Pennsylvania had couples take part in a kissing experiment. Nice work if you can get it. Preliminary results found that stress levels actually dropped after kissing and hormones that make us swoon, like oxytocin, went up.

Oxytocin is associated with deep feelings of attachment, sort of a 'cosmic union' with somebody.

So if you believe the science, kissing boils down to brain waves and biology. And all the flowers, candy, and cards don't really matter. Or do they?

Women want cards and letters! Men want kisses…'cause it leads to sex. And you don't need a scientist to tell you that.

For the World News Webcast, Clayton Sandell, ABC News, Washington.