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      Boyce, W.T., Chesney, M., Alkon, A., Tschann, J.M., Adams, S., Chesterman, B., … and Wara, D. (1995). “Psychobiologic Reactivity to Stress and Childhood Respiratory Illnesses: Results of Two Prospective Studies.” Psychosomatic Medicine, 57(5), 411–422.

      The findings confirmed Boyce’s suspicions. High-reactive children were 20 percent more likely to develop a respiratory illness if they attended one of the high-stress childcare facilities than if they attended a low-stress facility. Conversely, low-reactive children were 10 percent less likely to catch a respiratory illness if they attended a high-stress facility. Of the four groups, high-reactive children in low-stress daycare actually showed the lowest rate of respiratory illness, with an average of only four incidents per six-month period.

      Boyce conducted several similar studies and scoured peer-reviewed journals for dozens more, amassing an impressive body of evidence to support his theory and fitting it eloquently within a broader evolutionary framework. Diversity has been the key to an organism’s survival almost since life began. Without it, even the most fearsome apex predator will inevitably be exterminated when some industrious microbe or parasite devises an effective way to attack them, or the environment becomes hostile, or a valued food supply dwindles. It’s the reason that sexual reproduction has been such a rousing success among just about every creature more complex than bacteria. If you don’t shuffle the deck, you may draw nothing but kings and queens at first, but eventually you’ll face a run of threes and twos that will almost certainly be your undoing.

      Biological Sensitivity to Context is an extension of that premise. Varying degrees of sensitivity assure that the human species will be able to cope with a wide and ever-shifting range of environments. High-reactive children are humanity’s high-stakes bet. If the cards don’t fall their way, they lose big, but if they hit a hot streak, the payout is enormous. Low-reactive children, by contrast, are the human gene pool’s conservative investment, a kind of genetic GIC. The returns may not dazzle us, but the pool is, at least, safe. This diversified take on genetic investing works for the rhesus macaques, and it seems to be working for us, too.

      But Boyce takes it farther. Prior to his theory, gene-by-environment interactions were considered to be much as we’ve described them thus far: a method of diversifying the gene pool to better assure our species’ survival. Boyce agrees with this concept, but does not consider it to be a purely passive phenomenon. Instead, he argues that Biological Sensitivity to Context is determined in part by the surrounding environment.

      Tuning Our Reactivity

      Think of environmental stress as a spectrum with a safe, well-adjusted, and financially stable upbringing at one end and an insecure, impoverished, and violent upbringing at the other. According to Boyce, children at either end of this spectrum are disproportionally likely to be high-reactive, while children in the middle are more likely to be low-reactive. Boyce believes that the environment does not simply predict a child’s chances of success, considering his or her genotype. Rather, it actually helps dictate the strategy that a child will adopt in order to cope with his or her surroundings, not just psychologically, but biologically.

      But if Boyce is right, and BSC is determined by environmental factors early in life, why do children faced with severely adverse conditions become high-reactive? Every study we’ve reviewed shows that, under stressful conditions, high-reactive children perform worse physically, intellectually, and emotionally than their low-reactive peers. Shouldn’t an environmentally determined BSC account for this poor standing by making every child low-reactive unless they are born into a stable, nurturing family?

      Consider Patricia Crittenden’s interpretation of attachment theory, the Dynamic Maturational Model. As you may recall, the crux of Crittenden’s theory is that children adopt seemingly self-destructive attachment behaviours because, during the first few years of their lives, they are the best strategies children have at their disposal. When a parent is consistently neglectful, a child cannot afford to waste energy by crying every time he or she is hungry or cold, since that behaviour has proven ineffective. Instead, the child becomes withdrawn, seeking comfort in his or her own self-sufficiency. When crying sometimes summons a caregiver and sometimes doesn’t, the child learns that the affection of his or her capricious protectors is fleeting, and as a result, he or she becomes clingy but not trusting. These behaviours often cause trouble for people down the road, straining their relationships with their family and peers, and hindering their ability to function effectively in a market society, where co-operation is paramount. But if in the child’s eyes the alternative is death, then such behaviours are, in the short term, advantageous. It doesn’t matter if the child isn’t actually likely to die (few parents are deliberately negligent enough to kill their children, and in many cases where they are Social Services will step in before they get the chance); the child is acting on basic evolutionary instincts, not rational thought, and so can only make decisions (though even calling them decisions is somewhat misleading) based on the lesson imparted to him or her.

      Biological Sensitivity to Context operates in much the same way. When a given trait first rose to prominence, an adverse environment meant something very different than it does today. In modern times, few denizens of the first world face famine or drought on par with that experienced by our ancestors. We are at little risk of attack from vicious animals or rival tribes, and have developed our own protection from many of the diseases that once threatened to wipe our entire species from the face of the Earth. These dangers have shrunk or disappeared, but new ones have risen to take their place. Modern threats are complex and intuitional, and require a new set of defences that, from an evolutionary standpoint, we have not yet developed.

      Here is the key difference. When we consider a trait maladaptive, we do so through the lens of an affluent and highly structured 21st-century society where the goal is to survive not just physically, but economically. People continue to die young, and probably always will, but making it to a procreationally viable age is no longer the feat of prowess and ingenuity it once was. Ten thousand years ago, it was all that mattered. Responding to a violent upbringing by becoming exceptionally aggressive, or mistrustful, or self-sufficient was, at the time, a fairly effective way to go about things. If times were hard, predators plentiful, and food scarce, then being the biggest and baddest primate on the block was the best way to assure that you survived long enough to pass on your genes. But now, when our body responds to similar cues of hardship, these ingrained survival traits become liabilities. Children grow up too aggressive to get along with others, too hyperactive to hold down a job, too anxious to make the social connections necessary to succeed in a service-based market economy. Our instincts have turned against us. Or, perhaps more accurately, the world has turned against our instincts.

      Stressing Out

      If BSC is environmentally determined, then why have so many studies linked reactivity to the presence of certain alleles? Boyce doesn’t discount the role genes play in determining BSC. Indeed, his studies have been among those that indicate their importance. He merely argues that the environment exerts an additional influence on our BSC on top of the effects of genes like DRD4 or 5-HTTLPR. It does so, says Boyce, by influencing the way our body responds to stress.

      In both highly stressful and highly supportive environments, children become exceptionally attuned to stressful stimuli. Their heart rate rises, adrenaline floods their veins, and their minds become instantly alert, ready to respond to danger (or opportunity) in a fraction of a second. In a high-stress environment, it’s easy to see how this could have been an advantage. When danger continually looms in the form of a bear or a lion or an enemy tribe, quick reflexes and increased vigilance can be the difference between life and death. But for low-stress environments, the benefits of a heightened stress response system are less clear. Why would a child raised in a