A California couple charged with torturing their 13 children and holding them captive for years appeared in court again on Friday. The case has drawn international attention, with many wondering how such dark behavior was kept secret for so long.

The terrified and malnourished Turpin siblings were only discovered after one of them escaped last month and called the police. Neighbors have expressed disbelief that so many children were living in such unthinkable conditions, right next door to their homes.

“There are anecdotal reports now from neighbors asking, ‘How did we miss this? How did we not see what was going on? Why didn’t we pick up on this?’” said Beth Molnar, associate professor in the Department of Health Sciences at Northeastern.

As a social and psychiatric epidemiologist, Molnar has been studying youth violence and child abuse for more than two decades. She said one factor known to protect against child abuse is the amount of intergenerational closure within a neighborhood. This concept refers to things like how well adults know the children in their neighborhood, or how well they know the parents of their children’s friends.

“Perhaps if that neighborhood had more of this intergenerational closure—more of the social network among families—perhaps they might have seen signs earlier than when this young woman escaped,” Molnar said.

Most of the 13 children were homeschooled, making it even less likely that someone outside the family would learn about the abuse.

“The system that we rely on to discover most of this is our system of what we call mandated reporters. Teachers, physicians, social workers, anybody that works with children in a social services agency or a children’s center are mandated reporters,” Molnar said. “It seems like this family was able to keep their children away from all of those categories.”

Molnar’s work on neighborhood-level factors that could reduce child abuse was chosen as paper of the year in 2016 by the journal Child Abuse and Neglect. The paper describes Molnar’s research examining families in Chicago over a 10-year period. One of the main findings was the importance of collective efficacy.

“This is the way people have confidence that their neighbors can work together on issues that are plaguing the neighborhood,” Molnar explained. For example, could the neighbors band together to prevent people loitering or committing crimes on a street corner? If the community fire station were at risk of being defunded, would the neighborhood convene to find a solution?

“The degree that people have this confidence in their neighborhood having these strong ties—that confidence is predictive of lower rates of violence,” Molnar said.