By Nancy Keates

Monica Sager didn’t see her boyfriend for four months after she moved back into her childhood home in Pottstown, Pa., in March. She also didn’t go to any friends’ houses or social events.

Now, her parameters have started to shift. Her boyfriend visited from New York over the Fourth of July weekend, and in August she will move into an apartment with roommates in Worcester, Mass., when she returns for her senior year at Clark University.

Ms. Sager, 21, knows that social distancing is key to keeping Covid-19 from spreading. And she’s been talking with her roommates about how they can entertain responsibly. But she can’t ignore her need to be around friends.

“I’m getting antsy. I really want to see people,” she says.

Covid-19 cases among the 18-to-29-year-old age group are rising, according to the Centers for Disease Control and Prevention. They accounted for 27.1% of U.S. cases in July. The group’s share in cases has been climbing every month since April.

As a result, young adults are getting tagged as irresponsible. But many scientists argue that scolding won’t help. They say the impulse among adolescents and emerging adults to break away from family and interact more with peers is part of normal development. Socializing and seeking new experiences is how they forge their identities.

“Exploring is a neurobiological mandate,” says Judith G. Edersheim, founding co-director of the Massachusetts General Hospital Center for Law, Brain & Behavior and an assistant professor in psychiatry at Harvard Medical School. She says the questionable behavior that correspondingly occurs is a necessary part of growing up. “In most instances, they don’t set out to do risky things. It’s that they’re programmed to do these things.”

Research shows that adolescents, loosely defined as those in the developmental period ranging from age 14 to 26 years (depending on the function being measured) are biologically driven to seek new experiences. When children are born, their brains aren’t fully formed. The brain creates a large amount of synapses, or neural links, between cells. During adolescence these synapses get cut back, or pruned: The brain eliminates the connections that aren’t important. One way the brain determines what’s important and what’s not is through real-world experiences and how frequently synapses are used, which is why independence is crucial for development.

At the same time, dopamine levels reach a lifetime peak in adolescents. Dopamine supports motivation and reward-driven learning, which drives them to value immediate gratification over future gains, which can lead to risky decisions. Motivation is influenced by rewards like novelty, thrills and the presence of peers. As a result, something that hits multiple rewards buttons can be particularly enticing—such as the opportunity to attend a party that will be viewed by friends on social media, Dr. Edersheim says.

Studies show that adolescence is a period in young people’s lives when their social environment and interactions with peers are important for brain development, mental health and developing a sense of self. Lack of peer contact may interrupt this and might have long-term detrimental effects, says Livia Tomova, a postdoctoral researcher at Massachusetts Institute of Technology who co-wrote a paper called “The Effects of Social Deprivation on Adolescent Development and Mental Health,” published in The Lancet in June. She says young adults crave social interaction like a starving person craves food. “Social rewards are a strong motivator,” she says.

The Covid-19 pandemic is exacerbating these biological cravings, says Hannah Schacter, an assistant psychology professor at Wayne State University in Detroit. Adolescents normally fulfill their need for peer contacts at school. Schools also provide behavioral guidance and a place for discussing emotional distress. “Suddenly they have to find other ways to get their needs met,” she says.

Zack Monterosso, 17, is grateful that he and his girlfriend were able to persuade their families to let them be in each other’s quarantine bubbles from the beginning of the pandemic. When he heard some people didn’t think it was a good idea, he felt the need to limit his social-media postings and to issue a statement explaining that they were quarantining only with each other.

Mr. Monterosso, who lives in Los Angeles, has noticed that social practices in people his age are loosening up: Some people have begun going to parties again. At his high school graduation celebration, students couldn’t stop themselves from hugging each other. “It was a tough moment. Everyone was so done with it and so ready to see friends and for things to be normal again,” he says.

Liza Cohen, 22, spent the last few months of her senior year at Barnard College in her parents’ house in Philadelphia, seeing only family because she didn’t want to jeopardize the health of her parents and grandparents. At first she was depressed and stayed in bed a lot. Then she started doing Zoom calls with friends, which made her feel better. That started to get old.

She recently went to New York to see college friends for the first time since March, an experience she describes as intense and emotional. “I really needed to see them,” she says. As much as she loves her family, soon she will start looking for a job and her own apartment. “I know that I need to leave for the good of my own self,” she says.

Although Ms. Cohen has noticed more party postings on social media and social influencers attending gatherings, she says most of her friends judge that as inappropriate. “You don’t want to be like those people,” she says.

Ruben Valera, 25, a second-year medical student at Tufts University, hasn’t been tempted to break social distancing because of all the information he reads about Covid-19. Still, he has noticed a change in behavior in a park across the street from his Cambridge, Mass., apartment. A group of teenagers who hang out there have stopped wearing masks since June and have started sitting much closer together. That isn’t against regulations, but he sees it as unsafe.

Mr. Valera feels that other young adults might behave more like him if they had better guidance and information. In June, the city put back basketball hoops it had removed earlier, albeit with a sign mandating masks and limited groups. Now, the sign is gone and about 30 people regularly play on the two courts.

While he agrees there is a biological drive toward socialization in people his age, he says, “I struggle to see it as a valid argument.”