Why do some children who grow up in poverty do well, while others struggle?
To understand more about this, a group of psychologists recently did a study.
It began in a small spare room where a series of very poor mothers and their 5-month-old babies came to watch a soothing video. Soothing the baby was the point, says Elisabeth Conradt, a postdoctoral researcher at Brown University's Brown Center for the Study of Children at Risk. The researchers needed to take measurements of the babies when they were calm.
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.
This simple measure has a complicated scientific name that sounds vaguely like a disease baseline respiratory sinus arrhythmia but the researchers were interested in it because it can tell you something about how a baby responds to the world around it.
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.
When there's a big difference and the set point is high, babies tend to have great attention and can focus for long periods of time on the things in their environment. "When you're presenting them with a new toy, they're going to really look at it and inspect it," says Conradt. "But they also may be more irritable and fussy when parts of their environment are changing."
In contrast, babies with a low set point "might lose interest after a couple minutes, but they're also not going to be as fussy or irritable," she says.
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.
Conradt and her colleagues wondered if this simple measure could be used to predict how children in poverty would fare as they aged.
A year after taking this first measurement, the mothers and their children came back into the lab for two more tests.
The children were first evaluated for behavioral problems like aggression and anxiety. Then they were given a classic psychological test known as the strange situation procedure.
In this test, the mother and child now around 17 months old are led to a strange room. For a while the toddler plays happily, but then, abruptly and without warning, the mother leaves.
Because this is a strange room in a strange place, the baby reacts most cry. But the part of the procedure that's most important to researchers happens when the mother returns three or four minutes later.
"It's how that baby greets or responds to her mother when her mother comes back that gives us some clue of the kind of history that these two have had together," says Jeffrey Measelle, a psychologist at the University of Oregon in Eugene, who worked on the study.
If the toddler is easily soothed by the mother, the researchers conclude that their attachment is strong and that the environment the toddler has grown up in is relatively secure. But if the toddler doesn't seem comfortable with the mother and can't be soothed, the researchers conclude that the attachment is poorer and that the environment the child is growing up in is probably unstable.
"It gives us some sense that that's probably the way those two have been interacting over time," says Measelle.
Which brings us back to that original measure of breathing and heart rate.
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.
But if the child had a high set point and a secure attachment, "those were the kids that were doing the best the absolute best of all of the kids in our sample, and they had far and away the lowest reported problem behaviors," Measelle says.
The children with low set points were not as good or as bad, no matter their parenting.
The behavior of the children with high set points and secure attachments to their mothers compared favorably with the behavior of children whose environments were often much easier. "These babies were looking a lot better behaviorally than a lot of babies growing up in middle class and advantaged samples," says Measelle.
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.