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.