You probably wouldn’t be surprised to hear that growing up in a tense household is bad for children and can hurt their health, and lead to anger, depression and behavioral issues. But would you believe it actually damages their DNA? A new study in the journal Childhood Development shows that parental stress levels actually change kids at the genetic level.

Researchers asked new parents to answer questions about their moods, finances, and marriages. They also analyzed each infant’s DNA and repeated the DNA testing at age 15. The result: The kids whose parents were the most stressed-out experienced an average of 170 genetic changes in that time period. Most were minor but some actually affected their ability to learn, cope with adversity, and regulate behavior. In other words, the changes literally rewired the teens’ brains and made it harder for them to relax, make friends, stay out of trouble, and do well in school.

Of course, if you’re a new parent in a struggling marriage or under severe financial pressure, relaxing is easier said than done. But you owe it to your kids to try. That’s from David Code, the author of Kids Pick Up on Everything: How Parental Stress Is Toxic to Kids. He says that taking time to relax isn’t selfish. It’s probably the single most important thing you can do for yourself and your children.