With Valentine’s Day around the corner, love is in the air for many.
The sight of smooching couples may warm your heart, or turn your stomach.
Sharing saliva may seem an odd way to express your affection for your other half - particularly when one kiss can transfer 80 million bacteria.
Why do we kiss?
In the UK, kissing is considered a normal way to show your partner you care.
Many may be surprised to learn, however, more than half of cultures worldwide do not engage in “lip-to-lip contact”.
After looking at 168 cultures, scientists from the University of Nevada in Las Vegas found just 46% displayed “the romantic-sexual kiss”, debunking the theory it is “human universal”.
The team found the more “socially complex the culture”, the “higher frequency of romantic-sexual kissing”.
They noted communities in Sub-Saharan Africa and New Guinea, as well as Amazonian foragers, did not appear to engage.
As recently as 2014, the indigenous Mehinaku people of Brazil reportedly found kissing “gross”.
Kissing may therefore have evolved alongside oral hygiene or the rise of “elite social classes” that celebrate “emotional displays”.
While the Nevada research suggests it is a relatively recent method of affection, Hindu Vedic Sanskrit written more than 3,500 years ago reportedly mentions kissing, suggesting it has long been “acceptable” in certain communities.
To uncover why we kiss, scientists looked to animals.
The BBC reported chimps kiss after conflict as a form of reconciliation, which is not considered romantic.
Bonobos get affectionate more often, even using tongues.
The great apes also apparently have sex as their version of a handshake, suggesting their kisses too are not particularly heartfelt.
No other animals are thought to share saliva.
This is said to come down to their acute sense of smell, with everything from boars to spiders “sniffing” out pheromones as a sign of fertility.
With a human’s sense of smell not as sharp, scientists from the University of Oxford found many turn to kissing to assess a potential “mate”.
Kissing may simply be a socially acceptable way of getting close enough to detect someone’s pheromones.
It could also “bolster” bonding between a romantic pair, even if just by “conforming to the imagined sexual script”.
Another theory is kissing is a sign of commitment, with people showing their partners they are willing to catch infectious conditions like flu to be close to them.
Is kissing good for us?
With many infectious pathogens lurking in saliva, kissing a sickly lover undoubtably puts you at risk.
Puckering up with a healthy person, however, could do you the world of good.
Scientists from the Netherlands Organisation for Applied Scientific Research looked at 21 couples, with one member of each pair being given a probiotic yoghurt drink.
The couple then enjoyed an “intimate kiss”, with bacteria counts on their tongues measured before and after.
The scientists “identified the probiotic Lactobacillus and Bifidobacterium bacteria” moved over to “most kiss receivers”.
During a 10 second kiss, the “total bacteria transfer” was averaged at 80 million bacteria.
High levels of “good bacteria” have been shown to boost our immune system, helping fight off disease.
The benefits do not stop there.
Japanese medic Dr Hajime Kimata won a scientific Nobel Prize for his research on how smooching may ward off allergic reactions.
He looked at 30 people with eczema, 30 with allergic rhinitis - a runny nose caused by allergens - and 30 healthy volunteers.
The participants were exposed to their “triggers”, dust mites or Japanese cedar pollen.
Both before and after exposure, they were told to kiss “freely for 30 minutes with their lover or spouse alone in a room”.
This was found to significantly reduce the development of allergic wheals, red swollen marks on the skin.
It also lowered levels of inflammatory protein markers typically released during an allergic reaction.
Kissing is also known to help people “unwind” before sex, making it more comfortable for a woman and easier for both participants to climax.
As if that was not enough, kissing has been linked to reduced anxiety, lower blood pressure and delayed signs of ageing - think of it as a work out for your face.