Before my first kiss, I scoured The Internet (Google Scholar + tracing blog posts back to their underlying studies - this was some time ago so I don’t still have the sources) for every valuable scrap of information available on the topic while trying to stray away from anything opinion-based. This is the result.
The bloody protrusion of skin and muscle on one’s visage, and a tactile sensory organ with an undeserved number of nerve endings, sends waves of data to the brain’s somatosensory cortex at a mere touch. With a skin layer depth of 3, compared to the rest of the body’s 16, they are fragile and translucent, and color peeks through from blooood filled capillaries in the mucus-filled membrane below the thin skin (the vermilion zone). The lips, labium-superius and inferius-oris, ideally a 1:1 ratio, form a border between the exterior skin of the face and the inside of the mouth. In a kiss, they are a barrier to breach or a target.
The kiss likely has origins in premastication, (shown through tongue use in modern french kissing), although Freud viewed kissing as a symbol of breast deprivation. The act of kissing from a medical and historical perspective is gross, yet the rewards of romantic kisses seem to outweigh any risks. Although upwards of 80 million bacteria (potentially 500 types of germs and viruses) can be shared with mouth-to-mouth, those may strengthen your immune system and are rarely dangerous, especially compared to the inherent danger in other typical human behaviors. Just don’t bite hard enough to be vampirous, blood-sharing is not safe.
In return for the shared saliva, participants receive a natural high consisting of oxytocin (contributing to attachment in a relationship), dopamine (creates pleasure), serotonin (a mood regulator), and, in addition, a kick of adrenaline through norepinephrine. Along with those hormones and neurotransmitters, it is possible that kissing increases proximity to pheromones (“carriers of excitement”), although the human ability to sense or be influenced by them is debated. That chemical cocktail provides both short- and long-term benefits, from temporary euphoria (although you do gain tolerance to dopamine over time) and stress relief (drops in cortisol and therefore stress decrease) to a closer, more satisfying relationship. Kissing can also apparently provide significant relief from hives and other mild allergic reactions. Are those benefits worth your mouth’s microbiota growing to slightly resemble that of your partner?
Continuing with a warning: it was found that approximately 60% of people have previously broken off a relationship due to a kiss, possibly due to genetic incompatibility. That is typically detected through the tasting of saliva with mucin lubricated tastebuds, along with olfaction of the sebum secreted by sebaceous and apocrine glands in the skin. Olfaction, or smelling of people, may have been the “original” way of kissing and is still common in Asia. In China, instead of kissing, they rub their noses against a cheek, and in southeast Asia, sniff kisses are common, to the point where “kiss me” in movies/media is replaced with the equivalent to “smell me”. Smell features highly in importance for the quality of a kiss and serves as a health assessment device through the chemosensory cues of detected phero-/hormo-nal information. Smell is also useful for determining the fertility of females since as estrogen increases, special cells are created in the mouth lining, breeding more bacteria which can cause worse/different breath. Also, fuller or redder lips are typically a sign of more estrogen. Back to genetics: similar major histocompatability complex (MHC) genes between humans may cause someone to instinctively may become unattracted to another person (though the opposite happens when on hormonal birth control), since if their genes smell too similar to their own, their children won’t have ideal genetic diversity, lowering a potential mutual child’s survival rate.
The process of kissing involves preparation and anticipation. Hygiene, especially of teeth and breath, is sought after. Environment plays a role, and seclusion leads to more intimacy. Before kissing, warning or consent is generally appreciated. When going in for contact, over two-thirds of people tilt their heads to the right. As the orbicularis oris puckers your lips, in combination with 34 other facial muscles (and 112 postural muscles are engaged), blood vessels dilate, sending more oxygen to the brain, decreasing blood pressure, and potentially improving headaches or cramps. Each participant’s lips redden, and their pupils dilate. Eyes may close, or remain open, taking peeks. 5 (out of 12 total) cranial nerves relay the sensory overload to your brain, where it is processed. Physical caressing often complements the kiss, during which 2-26 calories are burnt per minute. Then it ends. Exhilaration, perhaps suggestion of another round.
Males and females view the specifics of kissing differently. Males tend to prefer wet open-mouthed kisses while females are more conservative and enjoy less tongue use. The difference may be due to the transfer of testosterone from the male’s saliva, and as females are quite sensitive to the hormone, they get more aroused. Speaking of arousal, most females won’t willingly have sex with someone before kissing them and use kisses to gauge commitment, while males generally view them as a way to advance to sex. Males view fertility signals as quite important when deciding whether or not to kiss, along with face, skin, and body attractiveness. Females place more value on teeth than males do, and both genders find good grooming as well as lip softness and fullness important. Back to males: they are more passionate when kissing, and enjoy pleasure-noises from females, and they also initiate kissing as a conflict resolution far more often than females do. Assertiveness in kissing is also attractive, to both genders. Overall, kissing is viewed as most intimate out of non-sexual affectionate behaviors, and first kisses, in particular, have very high recall as people retain around 90% of memories and details, even as time goes by.
Discussion on Hacker News here