The scientific study of kissing is called ‘philematology’ (philos in ancient Greek = earthly love). During a kiss, couples exchange 9 mg of water, 0.7 mg of protein, 0.18 mg of organic compounds, 0.71 mg of fats, and 0.45 mg of sodium chloride, along with 10 million to 1 billion bacteria according to one estimate. Kisses use as little as two muscles, burning only 2 to 3 calories, while passionate kissing involves up to 34 facial muscles along with 112 postural muscles and burns around 26 calories per minute.
The original theory was that primate mothers chewed their food for their babies. However, as evolution continued the kiss began to be used to pass on information regarding biological compatibility of a mate via pheromone chemical signals as well as promote social bonding and expressing love, with the ultimate goal of procreation.
With the kiss partners are able to get close enough to each other to assess essential characteristics about each other, none of which are consciously processed. Although the vomeronasal organs – which are responsible for pheromone detection and brain function in animals – are thought to be vestigial and inactive in humans. Research indicates we do communicate with chemicals.
One study found that when women were asked to smell t-shirts of different men and choose their favourite, the choice was not made randomly but was based on the man whose major histo-compatibility complex (MHC) – a series of genes involved in the males immune system – was different from their own. The importance of this is that different MHCs mean less immune overlap which indicates more healthy offspring.
While men are not selective in terms of kissing, women are very choosy. This is because on an evolutionary level women were looking for a mate to raise their offspring with, and kissing could be an unconscious but accurate way for women to assess the immune compatibility of a mate, before she invests too much time and energy in him.
While males will have sex with women without kissing them beforehand as well as have sex with a woman who is not a good kisser, most women will never have sex without kissing first. Men tend to initiate French kissing and research suggests this is because saliva contains testosterone and this increases the sex drive of their mate. Furthermore, men are able to sense a woman’s level of estrogen which is a predictor of her fertility .
“There is evidence that saliva has testosterone in it,” said Rutgers University anthropologist Helen Fisher, “and testosterone increases sex drive. And there is evidence that men like sloppier kisses with more open mouth. That suggests they are unconsciously trying to transfer testosterone to stimulate sex drive in women.”
Hormone levels change after kissing. Specifically cortisol (stress) levels decreased in men and women after kissing, and the longer a couple is together the lower their stress hormones get. Interestingly oxytocin levels increase in men, however women’s levels decreased. One theory is that women need more than a kiss to stimulate attachment and bonding. Kissing raises testosterone which increases sex drive, and also increases dopamine promoting romantic love, and oxytocin (men only) which promotes bonding.
Other benefits includes a modest increase in blood pressure and heart rate which helps our cardiac health, increased saliva produced during active kissing which helps to prevent tooth decay, and men who kiss their wives in the morning live five years longer on average and also make more money.