We’ve known for a while that an excessive amount of TV or screen time isn’t healthy for any child, but we’re still discovering how dire the consequences can be.

Toddlers who watch a lot of TV are more likely to become troubled teens — they show aggression, experience bullying, and prefer isolation to spending time with friends.

If your child has more than four hours of screen time a day, it could determine whether he or she is a B- or C-average student.

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Dr. Linda Pagani, professor of psychoeducation at the University of Montreal and lead author of a new study, followed 991 girls and 1,006 boys from the time they were toddlers until they became teens. After discarding many confounding factors, Pagani and her team discovered a powerful link between screen time and antisocial behavior.

“It is unclear to what extent excessive televiewing in early childhood — a particularly critical time in the development of areas of the brain involved in self-regulation of emotional intelligence — can adversely affect social interactions,” Dr. Pagani said in a media release.

The study didn’t differentiate among TV programs. Pagani said nature or science programs could prove to be better for child development than fast-paced cartoons, but the research isn’t there yet.

The bottom line, she said, is that “at 29 months, while the brain’s synapses are undergoing exuberant expansion, there is little place for more than two hours per day of television. Moreover, development of the social brain requires social interaction with other people and objects (animals etc.), not just a passive television activity with no eye contact.”

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These findings aren’t surprising to experienced clinicians. Dr. Robert Pressman, author of “The Learning Habit” and child psychologist for the New England Center for Pediatric Psychology, said he sees this all the time with his patients. It’s important enough that he covers this issue during the first screening of a patient.

The best predictor of children’s screen time is their parents’ screen time.

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“We know with absolute certainty that there is detriment at the four-hour point, up to one full grade at school,” Dr. Pressman told LifeZette. Meaning that if your child has more than four hours of screen time a day, it could determine whether he or she is a B- or C-average student. And Pressman said most kids who come to him are getting between four and six hours of screen time each day.

The real clincher is nocturnal screen time. Kids who have smartphones, usually starting in first grade, take their phone to bed with them. In addition to the four hours of screen time during the day, kids often pile on the hours after their parents have gone to bed. But we don’t know how much time that is.

“That’s a secret kept by the children,” Dr. Pressman said. “In our interviews, it’s not unusual for kids to go to bed at 9:30 p.m. You keep asking them when they go to sleep and they’ll say 10. But if you keep at it, you’ll find [that some of them] are really up until two or three in the morning.”

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But if parents know this is bad for their kids — why do they keep allowing it? Pagani said her study revealed that parents with less education, as well as single parents, were more likely to allow kids additional screen time.

“These parents may be less aware of the guidelines and thus may be less concerned about the potential noxious influences of too much exposure. Single parents may be more stressed and may use the television as a parent helper, which is understandable,” she says. She’s quick to add that with some education, these parenting strategies can change.

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For a lot of parents, we just have to look in the mirror to find the root cause of the problem. We don’t turn off the TV for our kids because we’re not willing to turn it off for ourselves. A previous study from the University of Pennsylvania showed that the best predictor of children’s screen time is their parents’ screen time. Until we find better ways to spend our time, our kids probably won’t either.