Part of the reason is that humans are hard-wired to be around other humans. From the moment of birth, humans are one of the most vulnerable species on the planet, completely dependent on adults for survival. That dependency carries through into adulthood, when the brain is so accustomed to being enveloped by a social network that it goes into a state of alert when nobody else is around.

“We are not meant to be alone,” said Holt-Lunstad, who joined a team of international researchers to study how quickly the forced isolation of the COVID-19 pandemic is affecting people. “That state of alert, if it is prolonged, puts wear and tear on our bodies. The reason it feels unpleasant is it’s a biological signal, much like hunger and thirst, to motivate us to reconnect with others.”

Millions were living alone before the pandemic started. AARP estimates that more than 8 million Americans age 50 and older are affected by isolation. Holt-Lunstad’s research indicates that more than a quarter of all Americans live alone.