Here's how the study was done. Researchers at Washington University in St. Louis recruited 92 children between the ages of 3 and 6. Rather than asking parents about how they treated their children, the researchers brought the kids and parents into a lab and videotaped them as the parents, almost always mothers, tried to help their children cope with a mildly stressful task that was designed to approximate the stress of daily parenting.
Ratings of parental ability to nurture their children were done by study personnel who watched the videos while knowing nothing about either children or parents. Several years later, on average, the children had the size of a brain area called the hippocampus measured using magnetic resonance imaging (MRI). After taking into account a whole range of factors that can affect hippocampal size, the researchers found that children with especially nurturing, caring mothers, based on their behavior during the laboratory stressor, had significantly larger hippocampi (plural of hippocampus - you've got one on each side of the brain) than kids with mothers who were average or poor nurturers.
Why is this finding important? Because more than any place else in the brain, when it comes to the hippocampus, size matters. Other things being equal, having small hippocampi increases your risk for all sorts of troubles, from depression and post traumatic stress disorder to Alzheimer's disease. If you've got depression, having small hippocampi predicts that you won't respond as well to antidepressants as well as depressed people with larger hippocampi to antidepressants.
Image: Boy cuddling with his mother, via Shutterstock.