sometimes tease Erika, but she’s not a very satisfying target. Getting a rise out of her is pretty much impossible. A few children give half-hearted attempts now and then, but mostly they just leave her be. She has few friends, and during lunch she sits alone, pushing food around her plate. Her thin arms bear scars from frequent cuts and gashes. These wounds are not the hallmarks of domestic violence — Mrs. Munroe has never spotted bruises, black eyes, or taped-up fingers. They look self-inflicted.
On the surface, Joey and Erika seem like polar opposites. Joey’s personality is aggressive and forceful; Erika’s is shy and yielding. Joey does everything he can to draw attention to himself; Erika does everything she can to avoid it. Joey torments others; Erika torments herself. Yet these two seemingly disparate conditions share a common root. Psychologists have found that, beyond the superficial differences in behaviour, aggression and depression are often different symptoms for the same disease: poverty, neglect, emotional distance, and abuse by parents. Spending time with troubled children can often reveal the truth of this seemingly self-contradictory notion. Withdrawn, shy students and bullies alike often share deep-seated and painful insecurities rooted in their family environment. Given this connection, it may not be all that surprising that the same few genes can make a child susceptible to anxiety and aggression, apathy and hyperactivity. The body of research supporting this notion is new and fairly small, but like a well-nourished child, it is growing with astounding speed.
“Internalizing behaviours” are, in a way, the invisible cost of maltreatment. They do not call attention to themselves; if anything, they strive to hide from view. Erika, the self-mutilating student we described earlier, perfectly exemplifies internalizing behaviour. She’s shy, withdrawn, constantly fatigued, and prone to depression. Her intelligence and talent are hamstrung by her overwhelming sense of apathy, a black hole sucking away her every ounce of energy and optimism, leaving her feeling hopeless and alone. Nervousness and anxiety are also frequent symptoms of internalizing behaviour, though they can be difficult to see. If Erika experiences them, she hides them well behind a veil of lethargy and indifference.
Children exhibiting internalizing behaviours often feel the same anger as those exhibiting “externalizing behaviour” (think of the rebellious and irascible Joey), but they tend to turn it on themselves, where it manifests in body mutilation, drug use, or eating disorders. They draw their pain inward, but that doesn’t mean it can’t be felt by those around them. Depression throbs like a wound in a family’s flank, one we can spot and bandage with antidepressants, but are all too often unable to actually heal. Though hard to notice at times, internalizing behaviours can be diagnosed, and researchers are growing increasingly confident in their prediction of what causes them. Unfortunately, the answer is complex, subject to variation, and incomplete.
This is not to say it isn’t useful. As the following studies will show, even a partial understanding of what causes internalizing behaviours can make treating them significantly easier.
Nothing to Fear but Fear Itself
Mice make ideal test subjects. Though less intelligent than their distant cousins, rats (themselves popular among researchers), mice are smart enough to train, have short birth cycles, and reproduce prodigiously, allowing researchers to observe the effects of an experiment over multiple generations without waiting years for the results to come in. Female mice reach sexual maturity when only 8 weeks old, and can birth 5 to 10 litters per year, each of which contains anywhere from 3 to 14 mice. That’s a lot of births, and a lot of mice. It’s also an ideal opportunity for selective breeding, and over the years scientists have produced dozens of different strains of mice, each with their own behavioural ticks, temperaments, and characters.
Interestingly, this quest for new and better experimental fodder has itself become grounds for an experiment. What makes these mice different from one another? The obvious answer would be their genes. A series of small mutations occurring over the span of multiple generations have manifested themselves in the mice’s tiny brains, changing how each breed looks and behaves. Mice, spanning as many as six generations in a single year, can evolve a lot quicker than humans, who hobble miles behind them with a 20-year generational lag. When assisted by the informed hands of professional breeders, a mouse’s evolutionary timeframe can be easily put on fast forward.
But Dr. Michael Meaney and his team questioned this assumption in their 2004 study. To them, a purely genetic explanation for interspecies variation seemed too simplistic. Differences between breeds could be pretty radical, and natural selection is a notoriously slow process, often taking thousands or tens of thousands of years to produce very slight adjustments to the genome. Even when factoring in human intervention, Meaney felt that things were going too fast for genes alone to manage. Something else must be working behind the scenes as well. To prove it, he first selected two very different breeds of mice. The first, called Type A, were skittish, docile, and easily frightened by new places or objects. The second, called Type B, were confident, curious, and almost wholly indifferent to threat.
| Mouse Type | Mouse Behaviour |
| Type A | skittish, docile, and react strongly to stress |
| Type B | confident, curious, and more or less unfazed by new places or experiences |
Meaney took both breeds and performed a cross-fostering study. Six hours after they were born, Type A and Type B mice were taken from their mothers and randomly fostered to mothers of the opposite type. Type A mothers raised Type B infants, and Type B mothers raised Type A infants, hence “cross-fostering.” As a control, some infants were taken and fostered to mothers of the same type — Type A infants with Type A mothers and Type B infants with Type B mothers. The mothers raised their adoptive offspring as their own, and Meaney let the infants reach maturity without any further intervention on his part.
When the mice were roughly 70 days old, they participated in a step-down test. Each mouse was placed on a small raised platform in the centre of an open plain. Meaney observed the mouse’s behaviour for the next five minutes, noting in particular its willingness to explore its new surroundings — though “explore” is perhaps overselling it. The study broke exploration down into three stages: extending the head over the edge of the platform, stepping two feet off of the platform, and stepping completely off of the platform. Hardly a venture worthy of Magellan or Columbus, but for a laboratory-raised rodent weighing little more than an ounce, step-down testing can be a truly harrowing experience. It evokes a deep-seated fear in a creature whose survival strategies have, for thousands of years, relied principally on its ability to scurry and hide. Aloft on a platform, surrounded by flat, open terrain devoid of grass or rocks or any sort of protective crevice or camouflage, they sense the atavistic dread of their wild ancestors, ears and eyes and nose trained to detect the first sign of an incoming hawk, fox, or bobcat. When confronted with the step-down test, docile mice tend to freeze, overcome by terror, while their more adventurous peers waste little time in exploring the boundaries of their new habitat.
It should come as no surprise, then, that Type A mice tend to take far longer than Type B mice to progress through the stages of exploration. Occasionally, Type A mice don’t leave the platform at all, but remain exactly where they’re placed, rigid with terror, until the experimenters remove them. Type B mice, on the other hand, barely hesitate before leaping nimbly from the platform and sniffing inquisitively around the perimeter of the cage.
Here was the crux of Meaney’s experiment. If Type A mice’s skittishness and Type B mice’s fearlessness comes hardwired into their genes, then they should exhibit it regardless of who raised them. However, if their dispositions were instead the product of their environment, then adopted mice should behave much like their step brothers and stepsisters, even though they are born from different breeds.
So which was it? Here’s the strange thing: it was sort of both.
When raised by Type A mothers, Type A mice acted as skittish as ever. They performed poorly on the step-down test, leaving their platforms with great reluctance or freezing with catatonic fright. But when raised by Type B mothers, Type A
