You probably wouldn’t go up to a stranger in a nightclub, tap them on the shoulder and say “hey, mind if I borrow your toothbrush?” because that would be objectively weird and disgusting.
So, how come when we fancy someone we’re more than happy to lick the insides of their mouth - AKA kissing with tongues - even though the idea is sickening when you describe it literally.
It all comes down to our lingering evolutionary urge to sniff out a genetically compatible mate, although nowadays a person's genetics isn't the only thing that draws us to them (otherwise Tinder and hook-ups wouldn't exist).
Still, people love a good snog in almost every culture in the world, and those which don't get up close by sniffing or smelling instead, explains Dr Sarah Johns, an expert in human reproduction and evolutionary psychology at the University of Kent.
“Kissing - like touching and smelling - is an emotion-driven act that allows us to identify the most compatible and ‘evolutionary advantageous’ partner," adds Fulvio Fulvio D'Acquisto, professor of immunology at Roehampton University.
“Humans don't have strong olfactory skills and kissing allows you to smell and taste a person and see if you have different immune responses as we tend to feel more attracted to someone with a different immune response,” explains Dr Johns. That makes a couple more likely to produce a child better equipped to fight infectious disease, which was pretty important before modern medicine and the advent of vaccines and antibiotics.
"The major histocompatibility complex is detectable in body odour, so by kissing and tasting someone it gives the opportunity to assess how similar or different that individual is to you biochemically," she adds.
"From an immunological perspective, this has the advantage of ‘favouring’ the reproduction of individuals that carry very little risk of incompatibility - and hence reducing the risk of miscarriages - ultimately supporting the survival of the species,” says Professor D'Acquisto.
To allow us to explore another person’s genes before making babies with them, the body suspends the feeling of disgust and sparks the urge to kiss.
“Research shows that sexual arousal lessen feelings of disgust,” says Dr Johns. One such study involved showing people pornography and asking them to do unsavoury tasks like moving tissues into the bin or drinking from a glass of water with a fly floating in it.
“Arousal seems to inhibit a lot of disgust response which makes sense because you share fluids and there is a risk of infection," she explains.
Kissing also comes with the added benefit of exchanging commensal pathogens, according to Professor D’Aquisto.
“These bugs constitute our microbiota: our unique mixture of ‘friendly bacteria’ that define us as unique immunological individual.
“Once again, the enriching of one’s own microbiota through kissing might serve the purpose of sharing, testing and increasing the ability of the kisser to face the threat of harmful bugs and hence favour the propagation of the species.”
And that is why we love kissing with tongues even though it’s gross.
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