'Dirty' secret behind passionate kiss revealed

"Ummmms" and "aaahhhs" are not the only co-partners of kissing, for a passionate lip-lock unleashes a chain of chemical changes that really turn your head, claims a new study.

As Valentine's Day approaches, Wendy Hill, professor of psychology at Lafayette College, Pennsylvania, has taken the opportunity to shed light on the smooch.

In her study, Hill found that a meeting of lips can spark a complex chemical surge into the brain that makes a lover feel excited, happy, or relaxed.

Also, it is being speculated that the hormone release may be triggered directly by an exchange of sexually stimulating pheromones in the saliva.

"This study shows kissing is much more complex and causes hormonal changes and things we never thought occurred," The Times quoted her as saying.

"We tend to think more about who we are kissing and how it feels, yet there are a lot of other things happening," she said.

To reach the conclusion, the research team looked at the impact of kissing on levels of two hormones, oxytocin and cortisol, in 15 male-female couples before and after holding hands and before and after kissing.

Oxytocin is known to be involved in social bonding, so the researchers predicted that its levels would rise, while cortisol, a stress hormone, would fall. The results showed that cortisol levels fell in both sexes, but while oxytocin levels rose in men they fell in women.

Detailed results will be published at the American Association for the Advancement of Science's annual conference in Chicago this week.