Nature VERSUS Nurture (Part 1): it’s time to withdraw from this war!
If we stop fighting about nature versus nurture where do our troops go?
Posted July 29, 2010
OK. If we stop fighting about nature versus nurture where do our troops go? What else IS there? Over the years, there have been a lot of peace talks and a few détente's, but in my view, none of them have produced an ironclad peace treaty.
Most behavioral scientists say "nature and nurture interact". I find this interactionism unsatisfactory because it treats nature and nurture as independent entities. Somehow a preset nature interacts with a vaguely defined nurture to produce a behavior of interest. But just how the interaction works is usually not so clear. I prefer to explore the thought that nature and nurture are inextricable; while we fight over their relative importance, nature and nurture perform a pas de deux that evolves continuously from fertilization to death.
In this, my initial Psychology Today Blog entry, I offer the first of several entries describing experiments that give the lie to the idea that nature and nurture are integers in a zero sum game. Today: mothering, stress and anxiety in rats.
A quick Google search combining the words "genes, stress, and anxiety" reveals dozens of entries with phrases such as "stress genes" or "anxiety genes". One article leads with the title "Born worried: is it all in the genes?" While this piece answers "yes", other scientists offer a definitive "no". Consider the work of Michael Meany of Douglas Mental Health University Institute in Montreal, Moshe Szyf of McGill University and Frances Champagne of Columbia University. They performed a series of experiments that describe more complex but also far more interesting pathways that explain variations in mothering, stress and anxiety-at least in rats. Let's look at the rodential pas de deux.
Individual rat moms (called dams) vary in how they care for their young. Some lick and groom their pups and then lick and groom them some more; these rat moms also arch their backs to make it easier to nurse. Others are just-shall we say-less doting when it comes to licking, grooming and back arching. (Although they all lick and groom their boy pups differently from their girl pups, but that story is for another time).
Meany, Szyf and colleagues discovered that as adults, the rats raised by the physically less attentive mothers stressed out more easily under trying circumstances. For example, if researchers restricted their movements, their adrenal glands blasted out the steroid stress response hormone called corticosterone. Why? Because the brains of these rats were less able to bind steroid hormones than more highly licked and groomed pups. When these hormones bind to the brain, the communication levels between the brain and the adrenal decreases , causing the adrenals to release lower levels of stress hormones.
One cool point: genetic inheritance did not determine the activity of the stress hormones. When the experimenters cross-fostered pups-dividing siblings from the same litter between attentive dams and more casual caregivers--what mattered for later outcome was the behavior of the nursing and grooming mother, regardless of whether she was the birth mother. P.S. Daughters of attentive dams became attentive mothers themselves, while those treated less attentively were themselves less attentive to their offspring. Stress responses, then, were inherited, but not through the genes.
Why do mice from less attentive moms have brains with fewer hormone receptors? Here the story deepens down to the level of the genes. Brain cells make proteins that bind steroid hormones. A rat's DNA has specific code sequences (i.e. genes) that can, if activated, tap out the information needed to make these binding proteins. Genes that code for particular proteins, however, connect to DNA codes assigned the task of controlling gene activity. These control codes can turn a gene on high, can shut it off altogether or regulate intermediate levels of activity.
What, then, happens when rat mothers tend physically to their young? It seems that licking, grooming and nursing all suppress the binding to genetic control regions of the DNA of a molecule called a methyl group. These methyl groups turn off the genes that direct the assembly of the hormone binding proteins.
In other words, maternal caretaking directly affects the expression of particular genes. The level of gene expression in key cells shapes the density of hormone receptors in the brain. Receptor density regulates the physiology behind the stress response in the developing and then the adult rat. Rather than sitting on opposite ends of a seesaw teetering up or down with the opposing weight, nature and nurture intertwine. And speaking of: next time let's look at how experience and brain development intertwine.
For a related point of view check out the blog by Susan Krauss Whitbourne