COVID made us retreat into our corners. A worrisome new study says we haven’t left them.

MIT researchers found that interactions between the wealthy and poor dropped sharply after the pandemic hit — and remained down even after life returned to normal.

Research indicates that museums such as Boston's Museum of Science often are some of the most economically diverse public places.Globe staff photo David L. Ryan

Our most important relationships are with our parents and spouses and children and close friends. But casual connections matter, too.

A daily chat with a favorite supermarket clerk can lift the spirit. And an acquaintance at the park or cafe can point the way to a new job.

Sociologists call these relationships “weak ties,” and they can be especially helpful for low-income people who mix with the better off, significantly improving their chances at upward mobility.

But a worrisome new study by MIT researchers published in the journal Nature Communications shows that many of these ties have been severed in recent years. The culprit: COVID-19.

Using anonymized data from over a million cellphone users, the study’s authors tracked mobility patterns in the Boston, Seattle, Los Angeles, and Dallas metropolitan regions from 2019 to 2021. And they found that interactions between people from different economic strata dropped substantially after the pandemic hit.

In the early stages, a combination of factors was at play. Lots of people — higher-income, especially — started working from home. And most residents stayed closer to their own neighborhoods.

But there was something more subtle that happened, too — what researchers call “microscopic” changes in behavior. The specific places residents chose to visit — the coffee shops they frequented or parks they returned to — were more homogenous. People seemed to be sticking with the venues they knew and the people they were most comfortable with.

“Initially, it started with fear,” says Takahiro Yabe, a postdoctoral researcher at the MIT Institute for Data, Systems, and Society and lead author on the paper. “Fear of . . . contagion and places you’re not familiar with.”

And this microscopic behavioral shift stuck even after the vaccines were rolled out and overall mobility returned to normal levels. In late 2021, the economic diversity of everyday encounters at museums, parks, coffee shops, and restaurants in the Boston area was down 15 percent from pre-pandemic levels. It was down about 15 percent in Seattle, 17 percent in Los Angeles, and 10 percent in Dallas.

Yabe says he expected to find that some of the insular behaviors of the early pandemic had lingered, “but honestly, I didn’t expect the effect to be this big.”

Esteban Moro, a research scientist with MIT Connection Science and co-author of the paper, says the phenomenon could hurt low-income people in direct ways — they’ll lose connections to better-off people — and indirect ways. Research shows that white people who have less contact with Black people are less willing to support racial justice initiatives. Similarly, he says, middle-class and well-to-do people who are more isolated from poor people may be less receptive to funding social programs.

“How are we going to convince anyone to pay their taxes if we don’t see each other?” he asks.

So, what’s to be done?

Moro, Yabe, and several colleagues are pursuing several research projects focused on rebuilding connections across class differences.

One is focused on cultural institutions like museums, which have long been important meeting places for people of different economic backgrounds.

Museums’ economic diversity has taken a hit, on the whole, since the pandemic arrived. But there is substantial variation. And so the researchers are analyzing different museums in different locales with different outreach campaigns to learn which institutions held up best and what lessons might be learned.

They’ve also done some research on Boston’s fare-free bus experiment, and preliminary findings suggest that residents living alongside one of the free lines — Route 28, which runs through Roxbury, Dorchester, and Mattapan — have seen an increase in the economic diversity of their daily interactions.

There may be ways, then, to rebuild some of the city’s weak ties. And there is good reason to try.

Weak ties, after all, can make a city strong.