The findings also revealed that preschoolers coping with intense levels of family conflict struggled emotionally—so much so that they had physiological reactions such as elevated heart rate and blood pressure. Kids may also be forced to mediate and negotiate parental conflicts.

To analyze some of these effects, researchers for one of the studies collected data from 232 families, using several methods to examine how parental conflict affected children. They brought parents into the laboratory and recorded videotapes of them discussing difficult topics, subsequently showing the recordings to their children and noting their emotional responses. The evidence indicated that nonverbal hostility—like dirty looks, sulking, or refusing to answer one’s partner—was just as upsetting to kids as watching their parents verbally fight or lash out at each other. “It’s not a simple matter of what they see visibly—I think people underestimate the sensitivity of kids to their environments,” Cummings said.

In another experiment, parents were asked to maintain diaries at home in which they kept track of conflicts that happened both in front of their kids and behind closed doors. Children, the researchers concluded, understand when things are happening outside of their view. In other words, children are sophisticated analysts: They can tell whether parents are only pretending to resolve their problems as opposed to actually solving them. These fascinating studies raise questions about traditional parenting assumptions.

In their book Marital Conflict and Children, Cummings and the University of Rochester psychology professor Patrick T. Davies detail the many different kinds of harmful tactics couples use when they’re angry with one another which undermines the family’s stability. A partner who uses avoidance, for example, will walk away during an argument or “give in” while letting her anger simmer. These strategies can create a negative family environment that may end up having a cumulative effect on the child’s overall adjustment. The book makes a powerful case for rethinking parental tactics for managing anger: It’s not just about what parents say to each other verbally—it’s about how they react to one another on a daily basis.

It’s understandable that parents would only associate “marital discord” with hostile language and openly fighting in front of the kids, but according to Ramani Durvasula, a licensed clinical psychologist in California, “marital resentment creates a greater likelihood for a child to blame herself for the problems.”

She noted in an email that a child who witnesses this kind of behavior learns to repeat it in future relationships as she enters adolescence and adulthood. Durvasula’s observations echoes another study from earlier this year, which found that when such conflicts occur in kindergarten, that child is also more likely to have coping difficulties into her teens. From the perspective of psychologists, there’s a whole cascade of psychological problems that can develop over time from ongoing exposure to unresolved discord, such as heightened emotional insecurity and maladjustment.