On the screen, circles of gently colored shapes flickered and music softly played while a sensor taped to the baby’s chest recorded how much the baby’s heart beat when the baby breathed in, and how much the baby’s heart beat when it breathed out. …

You see, while there’s always a difference between how much the heart beats when a person inhales and when he or she exhales, everyone has a different set point. Sometimes there’s a big difference, and sometimes it’s small. And in very young babies, researchers have noticed that there are different temperaments associated with these different set points. …

Babies with a high set point seem to have a more sensitive nervous system, which makes them more sensitive to their environment, in both good and bad ways. Babies with a low set point seem to have a less sensitive nervous system, which makes them less sensitive to their environment. …

When the researchers looked at how a child’s behavioral problems correlated with the early measurement, the researchers found that kids with high set points were significantly more sensitive to the environment they grew up in than the children with the low set points. If the baby had a high set point and an insecure attachment to his or her mother, the child’s later behavior was often deeply troubled. These were by far the worst of all of the kids. …

The researchers hope that this simple measure of a baby’s breathing and heart rate might one day be used to flag children in poverty who have high set points — a biological marker of which children will be more sensitive to their environment — for better and for worse.