Why Do We Kiss?
Kissing is not one behaviour with one purpose. The strongest evidence supports two jobs: assessing a partner and maintaining a bond, with stress relief as a plausible side effect. It is common but far from universal, and it is older than our species, because the ancestor of the great apes probably kissed too.
Kissing is common, but not universal
The claim that more than 90 percent of cultures kiss is an assumption, not a measurement. Jankowiak, Volsche and Garcia coded the romantic-sexual kiss across 168 cultures and found evidence of it in 77, or 46 percent. Reporting also rose with social complexity: the more stratified the society, the more likely the kiss turned up.
That 46 percent is a floor rather than a verdict. Where an ethnographer never mentioned kissing, the culture was scored negative, and a missing note is not proof of absence. Read it as widespread and culturally variable.
Two functions keep winning: assessing a partner and keeping one
One of the largest surveys of what kissing is for asked 902 people aged 18 to 63 how much kissing mattered in different mating situations. Wlodarski and Dunbar found support for mate assessment: women, people who rated their own mate value highly and people with a more unrestricted sociosexual orientation put more weight on kissing, and more often said a first kiss had changed their attraction to someone. They also found support for attachment: kissing mattered more in long-term relationships, especially to women, and kissing frequency tracked relationship satisfaction. The idea that kissing mainly raises arousal got very little support.
A survey of 1,041 students found the same asymmetry. Women treated kissing as mate assessment more than men did, and men were far more willing to have sex with a bad kisser, a mean of 2.23 against 1.24 on a zero-to-four scale. All of this is questionnaire data: it records beliefs about kissing, not effects.
The experiment: more kissing, less stress
Experiments that manipulate kissing directly are rare. In the best-known one, Floyd and colleagues took blood samples and self-reports from 52 adults in marital or cohabiting relationships, then randomly assigned them to increase how often they kissed or to change nothing. After six weeks the group told to kiss more reported lower perceived stress and higher relationship satisfaction, and their total serum cholesterol had fallen relative to controls.
It is a genuine controlled experiment, and it is small: 52 people, one trial, and a manipulation that relied on participants doing as asked. Stress reduction is the weakest supported function and the one most often repeated as settled fact.
Your head has a preferred side
Kissing also carries a motor signature. Guentuerkuen watched 124 couples kissing in public in the United States, Germany and Turkey and recorded which way each pair turned their heads. Eighty couples, or 64.5 percent, turned right, against 44 who turned left, close to the two-to-one ratio expected if individuals have a right-turning bias. The likely source is the head-turning preference seen in fetuses and newborns, which never quite disappears.
The study is observational: one observer, public places only, one kiss per couple, and no way to read an individual's bias from a pair.
The behaviour predates our species
In 2026, Brindle, Talbot and West treated kissing as an evolutionary trait. They defined it as non-agonistic oral-oral contact with some lip movement and no food transfer, coded which Afro-Eurasian primates have been observed doing it, and ran a Bayesian phylogenetic reconstruction. The result placed kissing in the common ancestor of the great apes, between 21.5 and 16.9 million years ago, and implied that Neanderthals probably kissed too.
The comparative data reframe the human version. In two large captive chimpanzee groups, mouth-to-mouth contact accounted for 9.5 percent of consolation behaviour after a fight, behind grooming at 28.1 percent, embracing at 26.8 percent and gentle touching at 19.2 percent. Reconciliation and consolation, documented in chimpanzees since de Waal and van Roosmalen in 1979, are the likely ancestral job. Romantic kissing is the human elaboration, not the original function.
Six mechanisms, ranked by evidence
| Mechanism | Claim | Best evidence | Strength | Main dispute |
|---|---|---|---|---|
| Mate assessment | A first kiss tests a partner | Wlodarski & Dunbar 2013; Hughes et al. 2007 | Moderate | Attitudes, not outcomes |
| Pair bonding | Frequency tracks satisfaction | Wlodarski & Dunbar 2013; Floyd et al. 2009 | Moderate | Causal direction unclear |
| Stress relief | Kissing lowers stress | Floyd et al. 2009 | Weak to moderate | 52 participants, self-reported adherence |
| Arousal | Kissing initiates sex | Not supported in 2013 data | Weak | Contradicted by attitude data |
| Microbes | Kissing shares oral bacteria | Kort et al. 2014 | Moderate transfer, weak benefit | No health benefit shown |
| Deep history | Inherited from ape ancestor | Brindle et al. 2026 | Moderate | Sparse data outside large apes |
The 80-million-bacteria claim is real but oversold
The claim that one kiss transfers 80 million bacteria is a measurement, not a guess. Kort and colleagues studied 21 couples, had one partner drink a probiotic yoghurt with marker bacteria, and counted how many markers reached the other mouth during a ten-second intimate kiss. The average transfer was 80 million. Couples who kissed more often also had more similar salivary microbiota, and the resemblance faded with time since the last kiss.
What gets dropped is the rest of the result. Most transferred bacteria were transient, and similarity between partners' tongue-surface communities did not clearly track kissing; the authors attributed it to a shared lifestyle, environment or genetic factors. Kissing reshapes the saliva you carry for a while. It does not merge two mouths into one ecosystem.
Frequently asked questions
Why does kissing feel good?
No experiment in this literature has isolated a kiss-specific reward system, so a confident answer would be speculation. The indirect evidence is consistent: a first kiss can change how attracted you are to someone, couples who kiss more often report more satisfaction, and six weeks of kissing more went with lower perceived stress in a controlled trial.
Is romantic kissing a universal human behaviour?
No. It appeared in 46 percent of 168 sampled cultures and was more common in socially complex societies. Treat it as a cultural practice with deep biological roots, not something every human does.
Do other animals kiss?
Chimpanzees use mouth-to-mouth contact mainly to reconcile and console after conflict, and bonobos and orangutans have been observed doing it too. The 2026 reconstruction infers the behaviour was already in the great-ape ancestor: the raw act is older than us, the romantic script is ours.
Sources
- Jankowiak, W. R., Volsche, S. L., & Garcia, J. R. (2015). Is the romantic-sexual kiss a near human universal? American Anthropologist 117(3), 535-539. https://doi.org/10.1111/aman.12286
- Wlodarski, R., & Dunbar, R. I. M. (2013). Examining the possible functions of kissing in romantic relationships. Archives of Sexual Behavior 42(8), 1415-1423. https://doi.org/10.1007/s10508-013-0190-1
- Hughes, S. M., Harrison, M. A., & Gallup, G. G. (2007). Sex differences in romantic kissing among college students. Evolutionary Psychology 5(3), 612-631. https://doi.org/10.1177/147470490700500310
- Floyd, K., Boren, J. P., Hannawa, A. F., Hesse, C., McEwan, B., & Veksler, A. E. (2009). Kissing in marital and cohabiting relationships. Western Journal of Communication 73(2), 113-133. https://doi.org/10.1080/10570310902856071
- Guentuerkuen, O. (2003). Human behaviour: adult persistence of head-turning asymmetry. Nature 421(6924), 711. https://doi.org/10.1038/421711a
- Brindle, M., Talbot, C. F., & West, S. (2026). A comparative approach to the evolution of kissing. Evolution and Human Behavior 47(1), 106788. https://doi.org/10.1016/j.evolhumbehav.2025.106788
- Romero, T., Castellanos, M. A., & de Waal, F. B. M. (2010). Consolation as possible expression of sympathetic concern among chimpanzees. PNAS 107(27), 12110-12115. https://doi.org/10.1073/pnas.1006991107
- de Waal, F. B. M., & van Roosmalen, A. (1979). Reconciliation and consolation among chimpanzees. Behavioral Ecology and Sociobiology 5(1), 55-66. https://doi.org/10.1007/BF00302695
- Kort, R., Caspers, M., van de Graaf, A., van Egmond, W., Keijser, B., & Roeselers, G. (2014). Shaping the oral microbiota through intimate kissing. Microbiome 2, 41. https://doi.org/10.1186/2049-2618-2-41
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