Editor's note: Dr. Charles Raison, CNNhealth's mental health expert, is an associate professor of psychiatry at the University of Arizona in Tucson.

Ever noticed how scientific opinions swing from one extreme to the other?

Take the importance of mothers in the development of children. In the early days of psychiatry almost every mental illness, from depression to schizophrenia to autism was blamed on bad mothering. Then in the 1960’s and 70’s the discovery of medications that helped these illnesses allowed psychiatry to reframe them as biological conditions, no different from cancer or heart disease. Parents were fully absolved for the mental illnesses of their children, except to the degree that they passed along bad genes that caused chemical imbalances in the brain.

Myths inevitably survive long after they’ve been scientifically disproven. Such is the case with the fantasy that mental illnesses can be written off solely to genes and chemicals. Over the last decade a string of scientific discoveries has shown that the biology driving mental illness has at least as much to do with the environment as with chemicals or genetic inheritance. And it increasingly appears that the single most powerful environmental factor is the love - or its lack - that children receive from their parents. So in a very real way we parents are back on the hook for the lifelong emotional well-being of our kids.

I say this based on a thousand studies. But to make the point here, let me describe a recent study in the Proceedings of the National Academy of Sciences that more definitively than any before it shows how parental care literally changes not just kids’ hearts and minds, but their brains as well.

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.

Just as having small hippocampi increases the risk for all sorts of mental disorders, all the things in our lives that put us under undue stress and strain also shrink the hippocampus. This is as true for cigarette smoking as it is for being exposed as a child to abuse or parental neglect.

In addition to protecting us against brain illnesses, we all need big hippocampi because this brain area, while not much bigger than your little finger, plays a disproportionately large role in how you will be able to handle the stresses and strains of your life, and how you will remember your life when it’s all said and done. This is so because the hippocampus is crucial for our ability to form and store personal memories. It is also of central importance for restraining the body’s stress and inflammatory responses, both of which can induce significant damage to bodily organs and the brain if not properly reined in.

The finding that especially nurturing mothers can literally grow their children’s hippocampi doesn’t exist in isolation. It is consistent with hundreds of animal studies showing that maternal nurturing has a range of biological effects relevant to physical and emotional health. An especially striking example of this is a study done several years ago in rodents showing that maternal nurturance (measured as amount of licking that rat pups received from their mothers) literally changed how the rat pups’ DNA was expressed in the hippocampus. As a result of these changes, pups who received extra licking had changes in their stress systems that have been repeatedly associated with well-being in humans.

So we underestimate our power as parents at our children’s peril. But I would be remiss if I left you with the impression that mother love is all-powerful or that genes and chemicals don’t matter at all. The association of parental nurturing with subsequent hippocampal size in children was only observed in non-depressed children. In children with signs of significant early depression, maternal nurturing in the laboratory had no association with subsequent hippocampal volume. Why this was the case is anybody’s guess, but it might suggest that at least some cases of major depression are indeed mostly genetic or “hardwired” from an early age, and so are fairly resistant to positive things from the environment.

Given the complexity of the human brain, should we be surprised that every possible outcome of genes, chemicals and environment is actualized in someone somewhere? Meantime, one generation full of deeply loving parents would change the brain of the next generation, and with that, the world.