Distinctive Mechanisms of Adversity and Socioeconomic Inequality in Child Development: A Review and Recommendations for Evidence-Based Policy

How Adversity Affects Development: The Biological Embedding of Stress

The prevailing hypothesis is that adversity affects development through the biological embedding of chronic, toxic stress in the brain (McEwen, 2008). Survival depends on identifying and reacting to threats to safety. Adversity activates the neurobiological stress response, which is measured as changes in the steroid hormone cortisol, which is measurable in blood and saliva. The first system activated in this stress response is the immediate “fight or flight” response. The second system is the slower response by the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis leads to the release of cortisol, which temporarily increases energy usage, enhances cognitive abilities, and strengthens immune reactions (McEwen, 2002). Long-term exposure, however, may reverse these effects, leading to metabolic disorders, worsened cognitive functions, and weaker immune responses (McEwen, 1998).

Identifying a stressor in a child’s environment need not indicate negative outcomes. It is only when the stressor becomes chronic or toxic that the developing system begins to adapt to it as the “new normal.” This can change the biology of the stress response (Mills-Koonce & Towe-Goodman, 2012), causing neural remodeling that has negative long-term consequences on how well that individual can function in a biologically and psychologically demanding world, thus posing a risk for psychopathology (McEwen, 2008). The hippocampus, amygdala, and PFC are rich in receptors that bind to cortisol, making these regions particularly susceptible to structural and functional changes as a result of toxic stress (McEwen, Nasca, & Gray, 2016).

A shift in the stress response is a consequence of the early adversity experienced by children living in poverty (Blair, Raver, Granger, Mills-Koonce, & Hibel, 2011). Children who have disrupted caregiving experiences have lower levels of cortisol, as well as less change in cortisol over the course of the day, than is typically measured in children without these disruptions (Tarullo & Gunnar, 2006). These effects have a cumulative impact on the developing brain. Assessing the impact of cumulative exposure to physical and social stressors in young teens, cortisol elevation correlated with the number of years living in poverty and exposure to adverse experiences (Evans & Kim, 2007). In a similar relation between cortisol levels and poverty duration, this cumulative exposure was inversely related to working memory, a PFC-dependent executive function, in young adults (Evans & Schamberg, 2009). The cumulative impact of stress on developing systems is also evident in children with prenatal substance exposure and domestic violence (Lester et al., 2010) and children who have suffered abuse (for review, see Mills-Koonce & Towe-Goodman, 2012).

Summary

Prolonged early-life adversity affects the biological stress response, measured by variations in cortisol, and affects memory, emotion, and executive functions development. Adversity also puts a child at risk for later pathology. What this literature does not say is that all children from very low-income homes are experiencing maltreatment or poor care. A family need not be living in poverty to impose significant stress on a child, such as from divorce (Kraft & Luecken, 2009). Stressful experiences can happen to anyone, but a higher number is more commonly associated with living in poverty, and the effects seem to be cumulative.

Socioeconomic Impacts Are Independent of the Biological Embedding of Stress

Critically, SES can affect development independent of stress caused by adversity. A study that examined the impact of SES on working memory in children and adolescents found that parent education influenced working memory performance in childhood, but this effect was not cumulative and did not influence working memory development into adolescence (Hackman et al., 2014). They also found that SES-related neighborhood stressors had no impact on working memory performance and development. From this, they argued that SES effects on working memory cannot be due to adversity because the effect of parent education was consistent across a large education range and were not specific to very low educational levels. Consistently, another study asked whether childhood maltreatment and/or SES were predictive of adult hippocampal and amygdala volumes (Lawson et al., 2017). They found that only maltreatment predicted lower hippocampal volumes. SES was not related to either volume.

While early-life adversity has been linked to cortisol differences, studies examining the effects of SES on cortisol have largely found no evidence of this relation. Cutuli, Wiik, Herbers, Gunnar, and Masten (2010) examined cortisol differences in a group of children living in family emergency shelters. They asked whether morning cortisol levels varied by adversity and/or SES. Adversity predicted levels of morning cortisol and change in cortisol over the day, whereas SES was not associated with either cortisol measure. Another study of cortisol levels in a large group of teenagers found no association between SES and cortisol levels (West, Sweeting, Young, & Kelly, 2010). Indeed, a review of 26 articles examining this relationship found weak evidence, if any, to support the claim that SES affects brain development through cortisol (Dowd, Simanek, & Aiello, 2009).

One exception examined whether hair cortisol levels in children and their parents were related to differences in internalizing disorders (e.g., depression, anxiety; Ursache, Merz, Melvin, Meyer, & Noble, 2017). Lower parent education, but not lower income, was associated with higher hair cortisol levels for both parents and children. Moreover, the effect was linear along the entire education spectrum. A college graduate and his or her child have higher hair cortisol than those with a master’s degree, who have higher hair cortisol than those with a PhD. Taken together with the lack of association with income, it is not clear what inference can be drawn about the biological embedding of stress from these data.

Summary

This brief review shows that (a) SES effects span the entire income and education spectra and are not relegated to poverty and (b) the stress response, as indexed by cortisol, is not clearly related to SES across either the education or income spectra. Alternatively, enrichment opportunities that are more available in higher-SES homes in the form of greater language complexity, education quality, travel experiences, and exploration with caregivers may progressively shape brain and cognitive development in powerful ways (Conger & Donnellan, 2007; Hackman et al., 2015; Mclaughlin et al., 2017). In effect, social and cognitive enrichment is nourishment for the developing brain, and more nourishment is better.

Thus, children who do not live in wealthy homes, which is the vast majority of American children, are not at more risk for pathology because they suffer more stress and because they do not have as many resources. The real problem this literature points to is that of the achievement gap. Children who have relatively more enrichment opportunities during early development will be more competitive for positions in upper-tier schools and score better on standardized tests than children who have fewer enrichment opportunities. This achievement gap has wide-ranging ramifications not just for these children, but for the growth of communities and nations.

Recommendations for Children Facing Early-Life Adversity

Evidence suggests that dampening the cortisol response to adverse experiences may diminish the neural remodeling that comes with a long-term shift in stress reactivity. The first factor that helps dampen this response is social buffering. The best evidence for this comes in the form of close relationships with caregivers (for review, see Hostinar & Gunnar, 2013). Hostinar, Johnson, and Gunnar (2015) define social buffering as “a reduction in acute physiological stress responses with the presence or assistance of a conspecific during an otherwise stressful event” (p. 281). Caregivers have been shown to be powerful social buffers, or external regulators of HPA activity, for their children as early as infancy (Nachmias, Gunnar, Mangelsdorf, Parritz, & Buss, 1996) and well into early adolescence, but they are not as effective in late adolescence (Hostinar et al., 2015). Other studies have shown that sensitive parenting plays an important role in lowering cortisol levels in children, which affects executive functions by the age 3 (Blair, Granger, et al., 2011).

One recommendation is to invest in programs that educate caregivers on how their stress response affects their child’s development and how they can display more sensitive parenting behaviors. A recently developed intervention for American foster parents, the Attachment and Biobehavioral Catch-Up for Toddlers (ABC-T), provides foster parents with an opportunity to support their foster children’s self-regulation development (Lind, Lee Raby, Caron, Roben, & Dozier, 2017). Foster children who received this intervention showed similar self-regulation outcomes to children who were never in foster care, and they outperformed foster children whose parents never received the ABC-T intervention. A similar result was obtained when nurses visited disadvantaged pregnant mothers, and later their infants at home (Olds, 2006). Results showed a simple informative health-parenting interaction improved children’s outcomes 12 years later, and resulted in reductions in substance use and mental health problems, relative to children from the same backgrounds but without early nurse visits.

A second important factor that helps to dampen the stress response is exercise. Studies using animal models demonstrate that physical activity has positive effects on the HPA-axis responsivity to stressors. Exercise results in both a shortened HPA response and reduced anxiety following a stressor (Hare, Beierle, Toufexis, Hammack, & Falls, 2014). As discussed, it is the body’s response to the stressor, not the stressor per se, that begins the wear and tear process. A meta-analysis of the effects of children’s aerobic activity found positive impacts on cognitive and psychological outcomes (Lees & Hopkins, 2013). Thus, exercise may serve as a buffer against the stress response even in the presence of a toxic stressor. In short, exercise may be a mechanism of resilience. This is a key finding from a policy perspective. Daily exercise of reasonable duration, incorporated into the school curriculum in the form of recess and physical education, may have lasting effects on both the mental and physical health of children at high risk for experiencing adversity. For these evidence-based reasons, American schools can enhance physical activity aspects of their curricula.

Recommendations for Narrowing the Socioeconomic Achievement Gap

The SES-based achievement gap creates two societal problems. At a national level, we risk the intergenerational perpetuation of a social-class structure. For the individual, the gap limits children with fewer enrichment opportunities from reaching their full potential. Early exposure to linguistic complexity and enriching experiences for learning and nourishing the developing brain are critical to narrowing this achievement gap.

Powerful data suggest that shifts in enriching learning experiences, through school, home, or neighborhood environments, have a strong positive effect on achievement outcomes. Data from millions of children were used to examine income in adulthood as a function of where children grew up (Chetty & Hendren, 2017). Results showed that, for the lowest 25% of the income distribution, each year living in a 1 SD better county than the one they were born into resulted in a 10% increase in their adult income. These effects were similar for other outcomes, including college attendance, teenage birth rates, and marriage rates. In most cases, these better neighborhoods had equal or lower rents, indicating that the results cannot be attributed to family income per se. Rather than income, data illustrate the importance of infrastructure, less segregation and income inequality, and better play spaces and schools (Chetty et al., 2014) for intergenerational mobility, putting the onus on policies for improvements. These data show that moving to a better neighborhood will result, for the same child, parents, and family income, in long-term higher SES in proportion to the amount of time spent in the better neighborhood. An investigation of whether public spending in 25 countries affected test scores among teens found that increased public spending on health, housing, education, and family support resulted in less inequality in test scores and less of an emphasis on family background in determining these achievement outcomes (Aizer, 2014).

The key to the effects observed above is, the earlier the intervention, the better the outcome. Better schools, in particular, offer important avenues for enrichment opportunities that can power learning, and brain and cognitive development. In a 2011 study, researchers implemented the Chicago Readiness School Project in 35 Head-Start-funded classrooms (Raver et al., 2011). This project, which taught children how to engage in self-regulation, had a positive effect on children’s vocabulary, math, and early literacy skills. Indeed, involvement in structured sequential activities, through play, arts, and skills development, is key to executive functions development (Diamond & Lee, 2011), known to be critical for academic achievement and success in math and reading (Lawson & Farah, 2017). A tremendous amount of change occurs from age 3 to 5 in executive functions (Davidson, Amso, Anderson, & Diamond, 2006). Yet, according to the Education Commission of the States (http://www.ecs.org), 35 states do not require children to attend kindergarten. The majority of states require children to enroll in school by age 6 (26 states) or even 7 (14 states). State-funded pre-K programs, which have been shown to be effective for long-term academic outcomes (Ansari & Winsler, 2016), are still not available in all states or to all children within a state. These policies are inconsistent with evidence that SES affects development beginning in infancy (Amso et al., 2014; Markant et al., 2016).

A delay in schooling or offering enrichment opportunities outside the home will additively affect children from the highest SES least and the lowest SES most. Schools offer opportunities not only for learning language, early literacy, and social interaction but also for exploring different books, toys, and outdoor play equipment that are not commonly experienced at home. Investments in improving school quality are thus imperative. To narrow the achievement gap, schools in lower-income communities must proportionally compensate for the resources not otherwise present in the child’s home, including museums trips, educational toys, projects for exploration, exposure to music classes, and organized team sports. Narrowing the achievement gap also means schools in lower-income communities must be proportionally better than, not just on par with, schools in higher-income communities. Evidence from a large study of children in Tennessee suggests that teacher quality and size of kindergarten classrooms, in the same school, affected college attendance and earnings in adulthood (Chetty et al., 2014). In 2011, the U.S. Department of Education reported that low-income communities are receiving less, not more, of their share of financial support.