It's only natural: Curious behaviors and what they say about you

It's your body, and you like to think you've got it under control. But underneath the calm exterior lurk unruly instincts and urges that are struggling to escape, putting you at risk of embarrassment or ridicule. These disreputable behaviors -- the likes of the fart, hiccup, itch and yawn -- are familiar to us all, yet they're also decidedly curious. Although they've been the source of folklore and puzzlement since antiquity, they've largely been overlooked by scientists.

After all, where is the scientific grandeur in such ignoble acts? I take a different view. Where others see forbidden areas, I find unexplored territory and new frontiers of research. So I've made a point of studying our curious behaviors. What I've found sheds new light on our body, our mind and our evolution as a social animal.

YAWNING

Whatever the purpose of a spontaneous yawn -- and this remains hotly contested -- the most extraordinary property of human yawning is its contagiousness. When we see someone yawn, our body is hijacked by a primal neurological process that's hard to resist. Imagine a yawning person with mouth stretched wide open, eyes squinting, taking a long inhalation followed by a shorter outward breath. Are you yawning yet?

Yawns are so catching that almost anything associated with them can stimulate more yawns, including seeing, hearing, reading about, or even thinking about yawning. My colleagues and I have found that silent videos of yawning people trigger contagious yawns in about 55 percent of observers within five minutes, and almost everyone reports being at least tempted to yawn.

Surprisingly, given that a gaping mouth is the most conspicuous element of yawning, videos that had the mouth edited out were just as effective at making viewers yawn. In fact, videos showing just a yawning mouth evoked no more yawns than one of a smiling face. That may be because an open mouth is not exclusively associated with yawning and could be doing something else such as singing or yelling. We respond to the overall configuration of the yawning face, including the squinting eyes.

From the evolutionary perspective, spontaneous yawns are ancient -- occurring in most vertebrates -- whereas contagious ones are relatively modern, being confined to social mammals including chimpanzees and perhaps dogs.

In humans, spontaneous yawning develops while we're still in the womb, but the contagious variety doesn't appear until a child is 4 or 5. This is also roughly when children start being able to attribute mental states to themselves and others, strengthening the idea that contagious yawning is linked with sociality. Although the neurobiology of this curious behavior is little understood, it is clear that when it occurs we become mindless beasts of the herd. As a yawn propagates through a group, it drives a ripple of physiological and emotional connection, transforming individuals into a superorganism.

ITCHING

Itching is an exquisite torment that earned a place in Dante's Inferno, but it has its virtues. The skin is our body's first line of defense against invasion and we're neurologically primed to maintain its integrity. So, when threatened by insect pests, toxic flora or other irritants, an itch guides us to the problem area and motivates us to scratch, in an attempt to dislodge the invader and quell the discomfort. Only the skin, not internal organs, gets itchy.

We also respond to tactile false alarms when we itch in response to skin conditions such as eczema, athlete's foot and psoriasis and, even more mysteriously, as a result of thyroid disease, diabetes and some neuropathologies. Itching is inhibited by pain, but while vigorous, tissue-damaging scratching can offer blessed relief in the short term, it can produce even more itching, locking us into a self-perpetuating itch-scratch cycle.

Like yawning, itching is contagious. You can "catch" an itch from observing someone scratching, attending a lecture about itching, or viewing slides of itch-producing pests such as lice. Even reading this may make you itchy.

Contagious itching makes evolutionary sense: your neighbor's pesky flea may jump from its host to you but won't get far if you're already scratching.

HICCUPPING

Hiccupping starts with a sudden inhalation produced by a downward jerk of the diaphragm and contraction of the muscles between the ribs, and ends almost immediately by glottal closing to produce the "hic" sound. Although of unknown purpose, this enigmatic act is one of the most common prenatal behaviors, suggesting a developmental role.

Hiccupping starts at around 8 weeks of gestation, peaking between 10 and 13 weeks, then declines through the remainder of life. For an unfortunate few, however, hiccups return with a vengeance in later life in the form of persistent bouts lasting 48 hours or longer. Men are nine times more likely to suffer this than women. The record for chronic hiccupping is held by Iowa farmer Charlie Osborne who hiccupped for over 67 years. Fortunately, hiccups usually stop during sleep.

A "hiccup generator" in the brainstem choreographs the widely distributed neurological and muscular components of a hiccup when it receives certain cues. These causes can range from distension of the stomach and irritation of the esophagus to various thoracic and nervous disorders. Remedies are even more diverse. In his Symposium, Plato listed breath holding, gargling and sneezing. Other purported cures include eating sugar, drinking water upside down, being frightened and putting your fingers in your ears.

In the course of my research, I've discovered another. The audio recorder cure simply entails my standing expectantly, microphone in hand, next to the hiccupper. It's particularly effective on children and shows the power of social inhibition over an ancient, instinctive act.

VOMITING

If you ingest a toxic substance, your body uses an effective and violent response to try to eject it: vomiting. However, you're also prone to retch at the mere sight, smell or sound of someone else doing it. Why? I became fascinated by this phenomenon as a child on a particularly nauseating family road trip when my cousin Karen was sick in the car, causing the other passengers to vomit.

Decades later, I got a chance to investigate contagious (or hysterical) vomiting. I found that girls of middle-school and high-school age are especially prone. Bouts usually occur during a group event that provokes anxiety. They tend to involve reports of vague smells such as vehicle exhaust fumes or sewer gas, or odd tasting or smelling food or drink. Symptoms are likely to be vague and the illness will resolve quickly and have no adverse effects.

Although contagious vomiting seems like a prime example of a bodily malfunction, in evolutionary terms, it's adaptive, permitting everybody in a group to benefit vicariously from the reaction of the person who takes the first taste of something toxic. Messy false alarms are a small price to pay for a potentially life-saving gut reaction. Indeed, some Central and South American peoples intentionally induce communal vomiting by drinking the ritual emetic ayahuasca, in their quest for purification and bonding. Cheers!

TICKLING

Tickling is exceptional in its philosophical, neurological, psychological and practical significance -- impressive credentials for a behavior often relegated to a footnote.

Everything starts with the observation that we cannot tickle ourselves. This is fortunate, otherwise we would go through life in a giant chain reaction of goosiness, confused about whether we touched something or it touched us. The neurological process that inhibits our response to self-touching also computes our discrimination of self and other. Who would have thought that the lowly tickle could offer a solution to the ancient and thorny philosophical problem of personhood?

This amazing insight even has a practical application. If computer scientists could create an algorithm to differentiate touching from being touched, they would increase the fine motor control of robots and be on the way to producing a machine with personhood.

The fact that we can't tickle ourselves makes tickling inherently social. It's an important means of tactile communication and bonding, and, I would argue, the basis of a baby's earliest preverbal conversations with carers. Although self-professed tickle-haters abound, my surveys indicate that we usually tickle and are tickled by friends, family and lovers, with the motive of showing affection and getting attention.

The capacity for mutual tickling enables the neurologically programmed choreography of tickle battles, physical play and sex play. The labored breathing this produces is the origin of laughter, with the ancestral "pant-pant" -- still produced by chimpanzees when tickled -- evolving into the modern human "ha-ha." I will further suggest that feigned tickle -- the basis of the "I'm going to get you" game -- is the most ancient joke.

FARTING

No investigation of our quirky bodily behaviors would be complete without considering flatulence. This uncouth act has attracted interest from scholars and the general public alike since antiquity. A growing appreciation of the importance of our gut microfauna has brought farting to the attention of gastroenterologists. My interest in the subject is more esoteric: Given the rich variety of sounds entailed, I wondered why we speak through our mouth rather than our butt.

This is not as frivolous as it first seems, given that no part of the human body evolved specifically for speech. We speak through the same orifice through which we breathe, eat, drink and vomit, and the vocal cords are two flaps of tissue that act as a seal to keep food and drink out of the airway when we swallow. So why did evolution not take the alternative option of using the abdomen and lower bowel as bellows -- some people do have such control -- and the anal sphincter as the vibrating seal?

Well, a major weakness of this idea is that while the oral vocal tract has the mouth, tongue, teeth and throat to shape sounds, the anus lacks such features. That hasn't stopped herring using farts to communicate, but the fish are an exception, and even then it causes problems. Flatulent herring attract the attention of hungry killer whales that home in on the sound of their breaking wind. Alas, buttspeak turns out to be a weak contender in the speech evolution sweepstakes.

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Robert R. Provine is a neuroscientist and Professor of Psychology at the University of Maryland-Baltimore County. His book "Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond".