That was what was so tough about the pandemic being a time of exodus. City dwellers of means escaped to the suburbs or country houses; millennials moved home with parents; downsized workers moved to find work or lower-cost living.

So even the closest of friendships were splintered. “Of our friend group, ten or more families moved since the pandemic started,” said Sage Ramadge, 43, a social impact consultant who lives in Harlem with his wife, Naomi Sage (their friends call them “The Sages”) and their three children. “Some of them said, ‘Hey, this is our opportunity to try something different.’ Others got Airbnbs in rural Pennsylvania or Florida to get out of the city for a few months and never came back.”

Mr. Ramadge got depressed over the summer thinking about it. It’s hard to make those close friends after a certain age. “My core group from my 20s was mostly gone.,” he said. “How do you find those connections?”…

In a perverse way, the narrower social circles we inhabited might be more instinctively human than our promiscuous networks of “friends” fueled by social media. “When we were all evolving on the African savanna, we lived our full lives with relatively small groups of 20 or 25 people,” Dr. Carstensen said. “Flash forward to 100 years ago, we lived on small family farms. Occasionally somebody would walk over three or four miles from another farm.”