Kissing chemistry

Unlocking the secrets of the lip-lock

CHICAGO – With all due respect to the old song, a kiss is not just a kiss.

Scientists say romantic kissing affects hormones involved in stress and attachment, and may help people determine whether they’ve found “the one.” Researchers discussed the science of smooching at a press conference February 13 at the annual meeting of the American Association for the Advancement of Science.

More than 90 percent of human societies practice kissing, says Helen Fisher, an anthropologist at Rutgers University in New Brunswick, N.J. Chimpanzees kiss too. And even those who don’t kiss still have a lot of facial contact with others. This leads Fisher to believe that kissing probably offers some evolutionary advantage.
Men tend to prefer wetter, open-mouth kisses with lots of tongue action, Fisher notes. This style of kissing may allow men to transfer more testosterone to their female partners to put the ladies in the mood. The open-mouth kiss may also help the guys figure out where a woman is in her menstrual cycle. “This really is a powerful assessment tool,” Fisher says. “A first kiss can kill a relationship.”

Couples that get past the first kiss aren’t done with kissing chemistry, though. Wendy Hill of Lafayette College in Easton, Penn., and her students brought 15 heterosexual couples into the student health center for a kissing experiment. Each of the volunteers drooled into a cup before the experiment began so the researchers could measure levels of the stress hormone cortisol in the saliva samples. Researchers also took blood samples to measure levels of oxytocin, a hormone associated with bonding.

Couples were asked to lock lips for about 15 minutes or to hold hands and chat for the same amount of time. Then the researchers collected another round of saliva and another blood sample.

At the beginning of the experiment, women naturally had higher levels of oxytocin than men, and women who took birth control pills had higher levels of the hormone than women who did not. After kissing, men’s levels of oxytocin increased, but women’s levels of the hormone dropped, Hill says. The result was completely unexpected and, as of yet, is unexplained. Hand-holders showed a similar pattern in oxytocin levels — men’s levels increased, and women’s levels dropped — but to a lesser extent than in the kissers.

Stress hormone levels in both male and female volunteers dropped when they spent time kissing or holding hands with their honeys. Kissing reduced stress more than hand-holding. And the longer a couple had been in a relationship, the more cortisol levels dropped, Hill says.

Hill and her colleagues speculated that the unexpected drop in women’s oxytocin levels might have been affected by the clinical environment. So the researchers are now altering the experiment to see if a more romantic setting would influence women’s hormone levels. In this scenario, kissers and hand-holders sit on a couch surrounded by electric candles, flowers and light jazz to help set the mood. Lesbian couples are also being tested in the new experiment. So far, the researchers have data only on cortisol levels. Again, the volunteers’ cortisol levels dropped after kissing or holding hands with a partner. The scientists have not yet measured levels of oxytocin.

The team is also measuring another stress hormone and other hormones involved in romance to see how these are affected by kissing.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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