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President Trump has a signature handshake. It hit the world stage at the United Nations meeting last year when he grabbed Emmanuel Macron’s hand and appeared to aggressively pull the French president closer. Ever since, he’s shown a consistent tendency to loom into other people’s personal space, or pull them toward him.

This article is adapted from Graziano’s new book.

Everyone has a personal space, an instinctive protective zone. We’re always jostling to maintain our own space and to navigate around others’, and the honeycomb of abutting spaces forms the scaffold of our social world. Violating it as a means of social communication, a means of bullying, is common behavior. But we usually don’t do it in a calculated way. The rules of personal space run deep under the surface of consciousness. We act and react in an elaborate, animal dance, and only extreme examples—like the Trump handshake—catch our conscious attention.

The instinctive dance of personal space was first studied scientifically by a zoo curator, Heini Hediger, who directed the Zürich Zoo in the 1950s. Zoo animals tend to be comfortable only if their cages are properly shaped and sized to form a protective territory. But when studying animals in the wild, Hediger noticed a second kind of territory, a smaller, portable bubble of space attached to the body. He called it an escape distance, or a flight zone.

When a wildebeest sees a potentially dangerous animal—a lion, or perhaps Hediger with a tape measure walking around the veldt—it doesn’t simply run. The animal seems to make a geometric assessment. It remains calm until the threat enters an invisible protected zone, and then the wildebeest moves away and reinstates the flight zone. That escape distance is apparently consistent enough to measure it to the meter. I can imagine Hediger walking up to the same poor zebra over and over, interrupting its afternoon grazing, trying to get a reliable mark.

In general, the larger the animal, the larger the flight zone. According to Hediger, a wall lizard can be approached to within a few meters before it suddenly bolts. A crocodile has more like a 50-meter flight zone. Domesticated animals have small flight zones, often no more than a meter.

Hediger’s work on animals caught the attention of the American psychologist Edward Hall, who, in 1966, published the defining book on human personal space, The Hidden Dimension. Hall conceived of people as Hediger-like animals. Having domesticated ourselves, we now have a small flight zone that we use with respect to each other. He also suggested, controversially, that different cultures have different styles of personal space. For example, he suggested that “Arabs” had small personal space, crowding up against each other when talking, whereas the British have expansive personal space. Those claims smack of stigmatizing cultural stereotypes and have not stood up to scientific scrutiny. Personal space seems to be more universal, built deep into the human genetic code.

Hall divided the space around people into four zones of different sizes: intimate distance, personal distance, social distance, and public distance. Intimate distance is so close that you can’t even focus your eyes properly. Hall paints the romantic appearance we present to a lover: “The nose is seen as over large and may look distorted, as will other features such as the lips, teeth, and tongue.” Personal distance, standing just within arm’s length, is more typical of a friendly conversation at a cocktail party or leaning in across a small coffee table. Social distance, just beyond arm’s length, is appropriate for a business meeting or a casual acquaintance. Public distance is larger still. It can be many body-lengths and voices must be raised to be heard.

After Hall published his observations, psychologists conducted a vast number of follow-up experiments to study the phenomenon of personal space and lift it out of the realm of casual impression. In a typical experiment, volunteers were asked to walk toward each other and stop when the interpersonal distance began to feel uncomfortable. The volunteers knew they were being watched and measured, and that self-consciousness might have affected their choices. Since the dance of personal space normally proceeds under the surface of consciousness, many scientists resorted to more covert procedures. In one study, the experimenter walked up close to random people in a public setting, waiting to see if the victim would step away. In one creepy study, men were covertly observed in the restroom. The idea was that as a man’s anxiety increases, he takes longer to start urinating, and the urination onset time should depend on whether other men are standing close or far away.

The most consistent finding out of this vast literature, the one fundamental result, is that personal space expands with anxiety. If you score high on stress, or if the experimenter stresses you ahead of time—maybe you take a test and are told that you failed it—your personal space grows with respect to other people. If you’re put at ease, or the experimenter flatters you ahead of time, your personal space shrinks. In at least some studies, women have an especially large personal space when approached by men. People in positions of social status or authority have a reduced personal space, especially toward each other. A large, self-confident man who has just been flattered by admirers—President Trump, let’s say—tends to have the smallest personal buffer zone of all.

In the 1990s, neuroscientists made a major breakthrough in understanding personal space with the discovery of a network of neurons in the brain that keeps track of nearby objects. Sometimes called peripersonal neurons, these individual brain cells fire off bursts of activity when objects loom near the body. In my own experiments, I came to call them bubble-wrap neurons. They monitor invisible bubbles of space, especially around the head and torso, and when they rev up, they trigger defensive and withdrawal reflexes.

In the past 10 years, these networks in the human cerebral cortex have been linked to social behavior. They coordinate the unconscious, hidden dance of personal space, computing a margin of safety and nuancing our movements and reactions to others. The mechanism works so smoothly that we don’t usually notice it. Just like ants instinctively feel each other with their antennae to communicate and maintain the larger social order, so we rub personal buffer zones and exchange information at a deep, unconscious level.

To someone like myself, a nerd who has spent decades studying personal space, I can’t help noticing a recent, radical change. As social life migrates online, the old social scaffold of physical space around a human body becomes less relevant. Twitter, Facebook, and texting are lifted out of the face-to-face, spatial framework. Instead, we interact as formless entities, bundles of information in a domain with no Euclidean distance metric. Not all online interactions lose their spatial scaffold—immersive video games maintain a sense of space and distance—but many people now spend an appreciable part of the day in a cyberspace with no real, face-to-face interactions. The human social dance is being restructured at a fundamental level.

At first, I thought this development must be a disaster. Our social skills evolved over millions of years to piggyback off of personal space, and removing that scaffold could muck up our social development. We could lose the ability to connect at a deeper, instinctive level. The cyber-troll phenomenon, for example, is arguably social behavior deranged and disinhibited by the removal of face-to-face, three-dimensional space. And the internet has certainly enabled a vast, dark world of cyberbullying, public shaming, cyber-stalking, and crime.

But I do see at least one hopeful side to our brave new social world. I wonder if #MeToo or #TimesUp could ever have happened otherwise. One longstanding avenue for sexism is the exploitation of personal space for bullying. It’s men looming in or getting handsy, pretending to own a woman’s personal space, asserting dominance. As social media has been taking personal space out of more human interactions, it may at least be giving women more room to be heard.

This post is adapted from Graziano’s new book, The Spaces Between Us: A Story of Neuroscience, Evolution, and Human Nature.