The reason that kissing feels so pleasurable is that it sparks a surge of hormones in our brains, according to new research.

    Couples who share a passionate kiss this Valentine's Day will enjoy sensations of relaxation and excitement because of a complex series of chemical processes, as well as their love for their partners.

    The study showed that women need more than just a kiss to experience the same chemical high as men - with additional features such as a romantic atmosphere of dimmed lights and mood music also required.

    Wendy Hill, professor of psychology at Lafayette College, Pennsylvania began the research to find out why the mundane physical activity of rubbing lips can elicit such a gratifying emotional response.

    Her team tested the levels of two hormones, cortisol and oxytocin, in 15 couples before and after holding hands and kissing.

    They found that kissing reduced the levels of cortisol, a stress hormone, in both sexes. But levels of oxytocin, a hormone linked to social bonding that they expected to be boosted by kissing, only rose among the men.

    The scientists have since replicated the tests in more intimate settings, to see if the less-than-alluring environment of the university health centres where the original research was carried out hampered women's hormonal surge.

    The final results will be presented at the annual conference of the American Association for the Advancement of Science in Chicago this week.

    "This study shows kissing is much more complex and causes hormonal changes and things we never thought occurred," said Prof Hill.

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

    It is not clear how kissing provokes such hormonal reactions, but some scientists believe they are triggered by the exchange of pheromones – chemicals our bodies release to attract sexual partners – in the saliva.

    This interaction may also have health benefits. Helen Fisher of Rutgers University, New Jersey, said: "If you share your germs with somebody, you're boosting your internal defence system."

    This is not the first research to analyse the physical effects of kissing. In 2007 British scientists measured the brain and heart activity sparked by passionate kissing, but found it was less intense that the stimulation produced by eating chocolate.

    Romantic love has also been shown to have a close link to neurological activity, with scans showing that it has similar effect to cocaine on our brains. 

    根據新的研究,接吻讓人感到愉快的原因是它刺激大腦大量地分泌荷爾蒙.

    因為一系列復雜的化學過程和他們彼此之間的愛,情人節熱吻的情侶們可以感受到放松和興奮.

    研究表明女性要達到和男性同樣高的化學反應,一個吻是不夠的--還需要例如昏暗夜里一個浪漫的氣氛和曼妙音樂.

    Wendy Hill,賓夕法尼亞拉法葉大學心理學教授,開始研究為什么摩擦嘴唇這樣的身體活動可以引起愉悅的情感反應.

    她的團隊測試了15對情侶牽手和接吻后皮質醇和催產素兩種荷爾蒙的分泌水平.

    他們發現接吻在兩性中都會產生皮質醇--一種壓力荷爾蒙.但是催產素,這種和社會結合密不可分的荷爾蒙,只有男性在接吻時才會產生.

    科學家在更親密的環境中重復試驗,來驗證當初在大學健康中心測試得出的不浪漫的環境會阻礙女性荷爾蒙分泌的結論是否成立.

    最后結果將在本周芝加哥舉行的美國科學進步協會年會上亮相.

    Hill教授說,"這個研究表明接吻過程復雜得多,它改變了荷爾蒙,產生我們從來沒想過的物質."

    "然而在這些事情潛移默化發生的時候,我們更關心我們在和誰接吻,感覺如何."

    盡管接吻是如何引發荷爾蒙的變化還不清楚,但是一些科學家相信荷爾蒙的變化是唾液中交換的性激素--我們身體釋放出來吸引異性的化學物質--的觸發器.

    這個相互作用可能對健康也有好處.新澤西州Rutgers大學的Helen Fisher說,"如果你和他人分享你的細菌,你的體內防御系統就會加強."

    這不是接吻這種身體影響的第一個試驗.在2007年英國科學家測量了熱吻后的大腦和心臟活動,但是發現它比吃巧克力的刺激更輕松.

    觀察表明,浪漫的愛情和精神活動也有聯系,它和可卡因對我們大腦的影響類似.