par with a child for whom danger is a constant companion? The short answer is he wouldn’t, but Boyce offers a compelling theory as to why he got one anyway.
Do you remember the crash course in genetics we gave you back in chapter 3? If so, you’ll recall that genes aren’t the neatly ordered units of code they have often been described as in mainstream science articles. Rather, they are a conglomeration of nucleotide sequences interspersed with introns — commonly (and somewhat erroneously) called “junk DNA” — and spread across tens or hundreds of thousands of base pairs, and sometimes even multiple chromosomes. They overlap with other genes sharing the same nucleotide sequences, and can be reassembled in different ways at different times.
It should come as no surprise, then, that an organism built from such ostensibly chaotic instructions should possess some traits with unusual or unexpected connections. High BSC is one such trait. A number of studies have found a positive correlation between heightened stress response and a number of indicators of intelligence. On the surface, this seems strange. Stress, at sufficiently high levels, is supposed to elicit our most primitive instincts, forcing us into simple, atavistic responses like fight, flight, or freeze. We are more likely to associate intelligence with calm, rational, nuanced thinking, hardly the sort of thing one should engage in when being chased by a tiger or shot at by an advancing army. Yet deeper down, the link between the two traits becomes more apparent.
Researchers have found high-reactive children to be more reflective and more conscious of themselves and their environment. They are more capable of delaying gratification in pursuit of long-term goals, and have better self-control, than low-reactive children. One could see how these traits — particularly an awareness of oneself and one’s environment — could be useful at both ends of the environmental stress spectrum. To children in high-stress environments, it means an above-average ability to assess risk, spot danger, and remain vigilant in the face of a longstanding threat. To children without such dire concerns, the same trait can be put toward exploration, introspection, and invention, all hallmarks of high intelligence. Coming from the other direction, a 2006 study found that more intelligent, introspective individuals have trouble coping with stress. This is true not just of humans, either. Going back to Dr. Stephen Suomi’s Maryland compound, you may recall how members of the two major socially aberrant groups — the neurotics and the bullies — developed previously unseen advantages when raised by supportive mothers. The neurotics, genetically destined to fretful, low-status lives, became model parents, their anxiety honed into perception and forethought. The bullies, meanwhile, channeled their killer instincts into the political arena, allowing them the edge necessary to rise in the macaque ranks, while their more stable upbringings taught them the importance of the soft touch as well. In humans, as in macaques, intelligence and high-reactivity may be two sides of the same coin.
The Orchid-Fringed Garden
To turn speculation into data, Boyce conducted a pair of studies. The studies were similar in nature, both to each other and to experiments we’ve discussed in previous chapters. Boyce determined environmental stress through surveys and questionnaires, taking into account both demographic data (family income, mother’s level of education, etc.) and personal responses (whether there is much fighting at home, if mothers felt overwhelmed by the responsibilities of child-rearing, had the children been excluded by their peers or experienced a change in their routine, etc.). The studies each used slightly different measures, but the general category of questioning was the same.
To determine children’s Biological Sensitivity to Context, Boyce used the same stress test featured in his respiratory illness study reviewed earlier this chapter. Children performed a series of seven activities designed to stimulate their spatial and emotional reasoning while machines monitored their heart rate and blood pressure.
So why two tests? Given the similarities, it may seem redundant not to bundle them into a single experiment. Except Boyce differentiated between the two of them in one key way: focus. Boyce’s theory regarding BSC’s distribution can be graphically represented as a large U shape, with environmental stress represented by the x axis (left to right) and child reactivity by the y axis (up and down).
High-reactive children are largely distributed to the low- and high-stress ends of the environmental stress axis, represented by the two prongs of the U, while low-reactive children congregate in the middle of the axis, represented by the U’s round bottom. A number of studies we’ve reviewed have reflected this general trend: only the left, low-stress half of the U has been, in most cases, decidedly atrophied, failing to reach the same heights as the U’s right, high-stress half. Boyce argues that this discrepancy does not prove his theory wrong, but rather speaks to the imbalance of significantly low-stress and significantly high-stress environments in a normal population sample. To put it crudely, sore thumbs stick out. While high-stress environments are fairly common, exceptionally low-stress households are, unfortunately, fairly rare. Much more common are households in which there is a moderate — and completely normal — level of stress, as parents fret about bills, bemoan juice spilled on carpets or crayon scribbles on walls, or struggle to cram yet another doctor’s appointment or visit to the mechanic into their already hectic lives.
To counter this trend, Boyce ran his two studies with different scopes. The more broadly focused of the pair looked at three levels of environmental stress —low, medium, and high — and found the same lopsided U shape implied in the data from Bakermans-Kranenburg and Van IJzendoorn’s studies, and several others we’ve reviewed. Child reactivity was higher in low-stress environments than in middle-stress environments, but highest by far in high-stress environs. The other study, however, put aside high-stress environments entirely. It divided environmental stress into three more narrowly defined groups: very low stress, low stress, and moderate stress. Essentially, Boyce cut his original graph in half and focused solely on the left side of the U. He thought the shift in focus would invert the typical trend of higher-stress environments producing more high-reactive kids. And it did.
Boyce, W.T., and Ellis, B.J. (2005). “Biological Sensitivity to Context: I. An Evolutionary-Developmental Theory of the Origins and Functions of Stress Reactivity.” Development and Psychopathology, 17(2), 271–301.
Ellis, B.J., Essex, M.J., and Boyce, W.T. (2005). “Biological Sensitivity to Context: II. Empirical Explorations of an Evolutionary–Developmental Theory.” Development and Psychopathology, 17(02), 303–328.
Moving from very low-stress to low-stress environments, the drop-off in child reactivity is stark; from low-stress to moderate-stress, the difference is non-existent. Compare this to the almost linear increase in environmental stress from very-low to moderate groups, and the reason for all those lopsided U’s seems pretty clear. Highly supportive environments produce a disproportionate amount of high-reactive children, but only when those environments pass fairly rigorous standards of quality. In households like Marcy’s — the gifted child we talked about at the start of this chapter — high-reactivity is the BSC de jour. In less affluent homes, or households where parents don’t have the luxury of devoting every second of their lives to nurturing their children, low-reactivity becomes far more common. This trend continues until we reach a place where stress becomes intense and commonplace and child reactivity promptly rises to levels equalling those at the opposite, low-stress end of the spectrum.
Migration
Of course, there is another possible explanation for the prevalence of high BSC children in highly supportive and abusive homes, one that allows genes like DRD4 and 5-HTT a more prominent role. The increased number of high-reactive children in adverse environments may have come, in large part, from a gradual migration of genetically susceptible individuals to the fringes of the environmental adversity scale. For instance, think back to David and Melissa, the brother and sister introduced at the start of this chapter. Though raised in the same household, the two went on to live very different lives. David became a programmer for a fairly