Body & Behavior: Why Do Our Bodies Do That?

Ask why the body does something and you normally get a purpose. Skin is bare so that we can sweat. We kiss to judge a mate. We blush because we are embarrassed. Each sounds like an explanation, and each is a claim that can be checked — and the failures are more informative than the successes.

This pillar is the hub for the Body & Behavior cluster, and it makes one argument across the cluster's questions. A body is a set of compromises made under selection, and behaviour is what those compromises do when they meet an environment: naked skin with dense sweat glands is one, a nerve tuned to being stroked at walking pace is another, and a face that reddens when you would rather it did not is the same problem from another angle.

The evidence is uneven, which is the point. Anatomy can be measured across species, so claims about skin and nerves can be comparative and quantitative. Behaviour leaves no fossils, so claims about kissing or blushing rest on surveys and short-term experiments, not on the fossil record.

The skin is a compromise, not a costume

Hairless is the wrong word for human skin. Counting hair follicles and eccrine sweat glands in five skin regions of three primates — macaque, chimpanzee, human — shows that the change was not a single loss. Human hair cover is greatly attenuated, but human follicle density is chimpanzee-like and significantly lower than a macaque's. Eccrine gland density, meanwhile, is on average tenfold higher in humans than in either of the other two, which are strikingly similar. The authors conclude that reduced hair density came first, in the common ancestor of humans and apes, and that expanded eccrine glands plus the loss of fur cover happened only in our lineage (Kamberov et al. 2018).

That order matters: "Why are humans hairless?" is partly a question about apes. "Why do humans sweat like no other primate?" is the specifically human question, and a comparative survey of 15 eccrine traits across 35 primate species suggests an answer with a climate in it. Most trait variation fitted a model in which natural selection does the work, and two traits tracked climate: the glycogen content that powers sweat production and sodium reabsorption was higher in warm, low-rainfall habitats, and capillarisation was higher where it is hot. The authors read this as selection for sweating capacity in primates that have eccrine glands on the body surface and live in hot, dry places, and propose that it was part of the initial increase in hominin thermoregulatory capacity (Best & Kamilar 2018).

Colour has a cleaner quantitative story, and it is about UV rather than race. Reflectance measured across indigenous populations correlates strongly with absolute latitude and with ultraviolet radiation, and the correlation is strongest near 545 nanometres, close to the absorption maximum of oxyhaemoglobin — melanin acting on UV effects in the dermis, not merely on the surface. The cline fits a compromise between photoprotection, which favours dark skin where UV is intense, and vitamin D synthesis, which favours light skin where UV is scarce (Jablonski & Chaplin 2000).

What none of this supplies is a date: skin and hair do not fossilise, so the sequence is reconstructed from living tissue and the timing is not measured at all.

Trait	Kind of evidence	Best-supported claim	Still disputed
Reduced hair cover	Comparative anatomy, 3 primates	Reduced in the common ancestor of humans and apes; human follicle density is chimpanzee-like	When, and why humans alone lost the rest
Eccrine sweat glands	Comparative anatomy, 35 species	Tenfold higher density in humans than in chimpanzees or macaques; capacity tracks hot, dry habitats	Whether glands or fur loss came first
Skin colour	Reflectance across populations	Clines track UV as a photoprotection/vitamin D compromise	Dates, and the specific selective agents
C-tactile afferents	Microneurography; patients lacking myelinated afferents	Tuned to 1-10 cm/s and project to insular cortex	What the signal is for, as opposed to what it does
Facial blush	Patients with unilateral sympathetic lesions	Active sympathetic vasodilatation via the cervical outflow	Why the face, and why visible

Gentle touch has its own nerve

The skin is not one sense organ. Myelinated afferents carry the fast, discriminative signals: where, how hard, what shape, which edge. A second population, unmyelinated and slow, appears to carry something else. In patients who had lost their myelinated tactile afferents but kept the small unmyelinated ones, soft touch still activated the insular cortex — a region tied to the body's internal state rather than to the spatial map of the skin (Olausson et al. 2002).

The tuning of those fibres is the striking part. Recording from single C-tactile afferents while brushing the skin at different speeds, Löken and colleagues found that the unmyelinated fibres responded most vigorously at intermediate velocities of 1 to 10 centimetres per second, while myelinated afferents kept increasing their firing with speed. Healthy subjects rated exactly those intermediate velocities as most pleasant. The authors concluded that C-tactile afferents form a privileged peripheral pathway for pleasant tactile stimulation, likely to signal affiliative social body contact (Löken et al. 2009).

Notice what is and is not established. The channel exists and is tuned, and the tuning matches reported pleasantness: that much is physiology, and it replicates. That it evolved for social bonding is an inference from the tuning and from where the signal lands. The velocity band also undercuts the loose claim that any touch calms: the same hand moving faster or slower is less pleasant, which looks like a channel with a preferred input rather than a general sense of contact.

What kissing is for, according to the evidence

The romantic kiss is the cleanest case of a behaviour that feels universal because it is universal nearby. In a sample of 168 cultures drawn from ethnographic databases and a survey of ethnographers, lip-to-lip romantic-sexual contact appeared in 46 per cent of them, and its frequency correlated with social complexity: the more stratified the society, the more kissing. The authors concluded that there is no evidence the romantic-sexual kiss is a human universal, or even a near universal (Jankowiak et al. 2015). A behaviour can be widespread, old in some lineages and still not universal.

Within cultures that do kiss, function has been probed by asking people about mating situations. An international questionnaire completed by 308 men and 594 women aged 18 to 63 supported a mate-assessment function: women, participants high in mate value and those with a more unrestricted sociosexual orientation rated kissing as more important, and said an initial kiss was more likely to change their attraction. Kissing was rated more important in long-term relationships, and kissing frequency tracked relationship satisfaction. The same data gave very little support to kissing's primary function being to raise arousal (Wlodarski & Dunbar 2013).

Experimental evidence is thinner. Fifty-two adults in marital or cohabiting relationships were randomly assigned either to increase romantic kissing for six weeks or to change nothing; the kissing group reported improvements in perceived stress and relationship satisfaction, and showed lower total serum cholesterol than controls (Floyd et al. 2009). The sample is small, the outcomes self-reported, the trial short: a real result, and a weak foundation for a story about why kissing evolved.

Clinging to warmth: attachment is a body system

In the 1950s the dominant account of infant attachment was that the mother is a conditioned reinforcer: the child loves whoever feeds it. Harlow's surrogate experiments made that account hard to hold. Infant rhesus monkeys reared with two artificial mothers — one of wire that dispensed milk, one of cloth that dispensed nothing — spent nearly all of each day on the cloth mother and under an hour and a half a day on the wire one, and ran to the cloth mother when they were frightened. Harlow's summary was that contact comfort is a variable of overwhelming importance in the development of affectional response, whereas lactation is of negligible importance (Harlow 1958).

Those experiments would not be approved today, and the deprivation they produced damaged the animals that survived it. What they established is narrow and useful: touch is a channel of its own, not a by-product of feeding.

Blushing is a signal you cannot fake

Blushing is one of the few visible signs a person cannot produce on request. The physiology was mapped in 23 patients with a unilateral lesion somewhere along the sympathetic pathway to the face: thermoregulatory flushing and sweating were impaired on the denervated side of the forehead in most patients wherever the lesion sat, but impairment of emotional vasodilatation — the blush — appeared only with peripheral lesions of the second or third neuron. The pattern suggested that sympathetic vasodilator fibres travel with the vasoconstrictor and sudomotor fibres to the face, and the authors concluded that the cervical sympathetic outflow is the main pathway for thermoregulatory flushing and emotional blushing (Drummond & Lance 1987).

Why signal at all? Keltner and Buswell argued that embarrassment is a distinct emotion rather than a mild form of shame, with a recognisable display — gaze aversion, a controlled smile, blushing — and an appeasement function that defuses a situation that could escalate (Keltner & Buswell 1997). The experimental support is mostly about observers. Across five studies, people who were more easily embarrassed reported more prosociality and behaved more generously, and observers rated embarrassed targets as more prosocial and less antisocial than targets showing a different emotion or none, were more willing to give them resources and more willing to affiliate with them (Feinberg et al. 2012). The blush carries part of that load: in two experiments with 66 and 62 participants, actors who blushed after a transgression or a mishap were judged more favourably than the same actors without a blush, on sympathy and trustworthiness among other dimensions (Dijk et al. 2009).

Embarrassment also needs a self that can be seen from outside. Chimpanzees exposed to a mirror learned to use it to inspect marks applied to their own faces, which is the standard operational test for self-recognition (Gallup 1970). The caveat is the usual one: rating studies measure judgements of photographs and vignettes, not the messy business of a real social repair.

Wanting is not liking

The simplest model of motivation — you do what feels good — fails on ordinary experience, and the failure has a name. Berridge and Robinson separate wanting, the incentive salience that makes a cue attractive and pulls behaviour toward it, from liking, the hedonic impact of consuming something. The two are not the same process, they use overlapping but different circuitry, and they can come apart: a cue can trigger intense wanting for something that is no longer liked (Berridge & Robinson 2016). This is why "reward" is a poor single word, and why the chocolate question in this cluster is really two questions: why the craving, and why the third piece tastes better than the sixth.

Habits are the other half of the same story. In a free-operant task, extensive training made participants' behaviour insensitive to a reduction in the outcome's value: they kept responding for a reward that had been devalued, the standard behavioural definition of a habit. Imaging across training sessions showed task-related cue sensitivity increasing in the right posterior putamen and globus pallidus as the behaviour became habitual (Tricomi et al. 2009). A behaviour can therefore migrate from being chosen for its result to being triggered by its cue, and the shift is visible in the striatum rather than merely metaphorical.

The first memory is usually a reconstruction

Almost everyone can name an early memory, and almost everyone's is older than the record supports. In the largest survey of its kind, 6,641 respondents gave their first memory and the age at which it happened. The mean age was 3.2 years, in line with decades of work, but 2,487 of them — 38.6 per cent — dated it to age two or younger, and 893 to age one or younger. Those early dates are the improbable ones, because verifiable memories rarely come from the preverbal period below about two years. The authors interpret such memories as reconstructions assembled from remembered fragments plus facts about one's own infancy, which for the person holding them simply are memories (Akhtar et al. 2018).

Rooms change behaviour, and the rats are not the whole story

Environment is the least glamorous variable and moves behaviour furthest. John Calhoun's crowded rodent pens became famous because they produced a "behavioural sink": with everything provided except space, populations boomed and then behaviour broke down (Calhoun 1962). The image travelled far beyond the apparatus, and a history of its reception argues that the popular version simplified the work and outran the message its author wanted to send (Ramsden & Adams 2009).

A result inside one apparatus becomes a law about cities only if the extrapolation is tested, and crowding is not one variable: density, resource distribution, social structure and the option to leave all change what happens.

The table below puts this pillar's behaviours side by side, with the strength of the evidence in a column of its own.

Behaviour	Proposed function	Evidence strength	Main dispute
Romantic kissing	Mate assessment; pair-bond maintenance	Moderate: survey, self-report, one small trial	Not universal (46 per cent of 168 cultures); functions inferred from attitudes
Slow stroking at 1-10 cm/s	Affiliative social contact	Strong on mechanism, weak on function	What the channel is for, not whether it exists
Clinging to a soft surrogate	Contact comfort rather than nutrition	Strong in infant rhesus monkeys; unrepeatable ethically	How far it generalises to human attachment
Blushing	Appeasement; signal of prosociality	Moderate: mechanism established, social effects from ratings	Whether rating studies survive contact with real interactions
Habit after overtraining	Cue-driven control of repeated action	Moderate: outcome devaluation plus imaging	Whether human habits are as encapsulated as rodent habits
Crowding	Social pathology	Weak outside the apparatus	The popular version outran the data

Common misunderstandings

Kissing is a human universal. It appears in 46 per cent of 168 sampled cultures, and more often in socially stratified ones (Jankowiak et al. 2015). The near-universal kissing figure that circulates in popular accounts is not what the cross-cultural data show.

Blushing is a malfunction. It is active vasodilatation on a sympathetic pathway (Drummond & Lance 1987), and it changes how other people treat you: blushing faces are judged more favourably after a transgression (Dijk et al. 2009), and embarrassment signals prosociality to observers (Feinberg et al. 2012). Uncomfortable is not the same as functionless.

Every trait is an adaptation with one job. Some traits are by-products of other traits or of the way a body is built, which is the argument Gould and Lewontin made against reading every feature as an engineered solution (Gould & Lewontin 1979). Even where a function is real, there can be several at once with different evidence behind them — as with kissing, where mate assessment and pair-bonding both have support and neither has enough.

Habits are a failure of character. Overtraining shifts control of a repeated action to cue-driven circuits in the posterior putamen (Tricomi et al. 2009), and cue-triggered wanting dissociates from liking (Berridge & Robinson 2016). A habit is a different control system, not a weaker version of a decision.

Your first memory is a recording. Nearly 39 per cent of a large sample dated a first memory to age two or younger, which is earlier than verifiable episodic memory goes (Akhtar et al. 2018). What feels like playback is usually a reconstruction that has been rehearsed into familiarity.

Frequently asked questions

Is romantic kissing a universal human behaviour?

No. It appeared in 46 per cent of 168 sampled cultures and was more frequent in socially stratified societies, so it behaves like a cultural practice, not a human universal (Jankowiak et al. 2015). Where it does occur, the questionnaire evidence points more to mate assessment and pair-bond maintenance than to arousal (Wlodarski & Dunbar 2013).

Why do we blush?

Blushing is active vasodilatation of facial skin carried by the cervical sympathetic outflow, which is why it cannot be produced or stopped at will (Drummond & Lance 1987). Observers read it: blushing faces are judged more favourably and more likely to be forgiven after a transgression (Dijk et al. 2009), so it signals even though it feels like a malfunction.

Why does a hug calm you down?

Skin has a separate slow channel for affiliative touch. Unmyelinated C-tactile afferents fire most strongly when skin is stroked at 1 to 10 centimetres per second, the speed people rate as most pleasant, and they project to the insular cortex rather than to the spatial map of the body (Löken et al. 2009; Olausson et al. 2002). Infants in the surrogate experiments likewise clung to a soft mother that gave no milk (Harlow 1958).

Why can't I remember being a baby?

Almost nobody has a verifiable episodic memory from before about two years of age. In the largest survey of first memories, 38.6 per cent of 6,641 respondents dated theirs to age two or younger, which researchers treat as reconstructions rather than recordings (Akhtar et al. 2018).

Sources

1. Harlow, H. F. (1958). The nature of love. American Psychologist 13(12), 673-685. https://doi.org/10.1037/h0047884
2. Kamberov, Y. G., et al. (2018). Comparative evidence for the independent evolution of hair and sweat gland traits in primates. Journal of Human Evolution 125, 99-105. https://doi.org/10.1016/j.jhevol.2018.10.008
3. Best, A., & Kamilar, J. M. (2018). The evolution of eccrine sweat glands in human and nonhuman primates. Journal of Human Evolution 117, 33-43. https://doi.org/10.1016/j.jhevol.2017.12.003
4. Jablonski, N. G., & Chaplin, G. (2000). The evolution of human skin coloration. Journal of Human Evolution 39(1), 57-106. https://doi.org/10.1006/jhev.2000.0403
5. Olausson, H., et al. (2002). Unmyelinated tactile afferents signal touch and project to insular cortex. Nature Neuroscience 5(9), 900-904. https://doi.org/10.1038/nn896
6. Löken, L. S., et al. (2009). Coding of pleasant touch by unmyelinated afferents in humans. Nature Neuroscience 12(5), 547-548. https://doi.org/10.1038/nn.2312
7. 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
8. 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
9. Floyd, K., et al. (2009). Kissing in marital and cohabiting relationships: effects on blood lipids, stress, and relationship satisfaction. Western Journal of Communication 73(2), 113-133. https://doi.org/10.1080/10570310902856071
10. Gallup, G. G. (1970). Chimpanzees: self-recognition. Science 167(3914), 86-87. https://doi.org/10.1126/science.167.3914.86
11. Keltner, D., & Buswell, B. N. (1997). Embarrassment: its distinct form and appeasement functions. Psychological Bulletin 122(3), 250-270. https://doi.org/10.1037/0033-2909.122.3.250
12. Feinberg, M., Willer, R., & Keltner, D. (2012). Flustered and faithful: embarrassment as a signal of prosociality. Journal of Personality and Social Psychology 102(1), 81-97. https://doi.org/10.1037/a0025403
13. Drummond, P. D., & Lance, J. W. (1987). Facial flushing and sweating mediated by the sympathetic nervous system. Brain 110(3), 793-803. https://doi.org/10.1093/brain/110.3.793
14. Dijk, C., de Jong, P. J., & Peters, M. L. (2009). The remedial value of blushing in the context of transgressions and mishaps. Emotion 9(2), 287-291. https://doi.org/10.1037/a0015081
15. Berridge, K. C., & Robinson, T. E. (2016). Liking, wanting, and the incentive-sensitization theory of addiction. American Psychologist 71(8), 670-679. https://doi.org/10.1037/amp0000059
16. Tricomi, E., Balleine, B. W., & O'Doherty, J. P. (2009). A specific role for posterior dorsolateral striatum in human habit learning. European Journal of Neuroscience 29(11), 2225-2232. https://doi.org/10.1111/j.1460-9568.2009.06796.x
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18. Calhoun, J. B. (1962). Population density and social pathology. Scientific American 206(2), 139-148. https://doi.org/10.1038/scientificamerican0262-139
19. Ramsden, E., & Adams, J. (2009). Escaping the laboratory: the rodent experiments of John B. Calhoun and their cultural influence. Journal of Social History 42(3), 761-797. https://doi.org/10.1353/jsh/42.3.761
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Explore this topic

Three cluster articles are already published:

• Why Can't We Stop Eating Chocolate?
• Why You Can't Remember Being a Baby — Infantile Amnesia Explained
• The Calhoun Effect

Two more are written and queued for publication, so they are listed by title only:

• planned: Why Do We Kiss?
• planned: Why Do Humans Get Embarrassed?

Posts carrying this pillar's label will collect at Body & Behavior; the label page stays empty until one of them publishes.