Evidence for skin-deep resilience using a co-twin control design: Effects on low-grade inflammation in a longitudinal study of youth

Highlights

• Conscientiousness predicts greater inflammation in lower socioeconomic status (SES) youth.

• In low SES twins, the one with higher conscientiousness had higher C-reactive protein (CRP).

• Among high SES twins, the twin with higher conscientiousness had lower CRP.

• Higher conscientiousness predicted better education, less depression, and less alcohol use.

• Low SES youth working to succeed experience good life outcomes, but at a cost to physical health.

Abstract

This study tested the skin-deep resilience hypothesis – that low socioeconomic status (SES) youth who are working hard to succeed in life experience good psychological and educational outcomes but at a cost to their physical health – in a sample of monozygotic (MZ) twins. The National Longitudinal Study of Adolescent Health (Add Health) contained a sample of 226 MZ twin pairs at Wave 1 (M age = 16 years), of whom 141 pairs completed the Wave 4 assessment 13 years later (M age = 29 years). Family SES was measured at Wave 1 via income, education, and occupation. Conscientiousness was measured at Wave 4 as an indicator of those who were working hard to succeed in life. Outcomes measured at Wave 4 included low-grade inflammation (C-reactive protein, CRP), mental health (depression, problematic alcohol use), and academic success (educational attainment). A co-twin control design was utilized which directly compared within-twin differences in the association between conscientiousness and life outcomes. Main effects of between-twin conscientiousness were found such that higher levels of conscientiousness were associated with higher educational attainment, fewer symptoms of depression, and less problematic alcohol use, across all SES groups. An interaction between family SES and within-twin difference in conscientiousness was found for CRP, such that, among twins growing up in lower SES households, the twin with higher levels of conscientiousness had higher levels of CRP. These patterns provide support for the phenomenon of skin-deep resilience using a twin methodology that reduces the possibility of confounding by shared genetic and environmental factors.

Introduction

A substantial minority of youth who grow up under economic hardship achieve good outcomes in life, such as high levels of psychological well-being, occupational success, and high educational attainment, a phenomenon that has been termed resilience (Luthar, 2006, Masten, 2018). When youth attain such outcomes, they are generally thought to have successfully avoided the adverse consequences of poverty.

However, recent evidence suggests that some youth who are characterized as doing well by virtue of achieving academic success and good mental health, may actually be accruing costs of this success in terms of their physical health. For example, among youth of color from low-income neighborhoods, those who made it to college showed lower levels of substance use, but at the same time, higher levels of allostatic load (a multi-system indicator of physiological risk) compared to youth from low-income neighborhoods who did not make it to college (and compared to youth from high income neighborhoods in college) (Chen et al., 2015). Similarly, low socioeconomic status (SES) youth of color who finished college reported lower levels of depression in their late 20's, but had a higher likelihood of metabolic syndrome, compared to low SES youth who did not go to college (and also compared to high SES youth who finished college) (Gaydosh et al., 2018). This pattern has been termed ‘skin-deep resilience,’ reflecting the notion that above the skin, these youth appear to be doing well and achieving successes by external metrics, but below the skin, they are struggling physiologically in terms of their health.

One explanation for this phenomenon is that these low SES youth are attempting to overcome the obstacles and adversities they face in life by working hard to succeed academically. Their hard-driving work ethic is reflected in high levels of striving and self-control that help them achieve successes, but that also exact a toll on them physically, taxing physiological systems. In support of this idea, previous research has found interaction effects between SES and striving or self-control predicting health outcomes. For example, low-income youth of color who engaged in high levels of striving during adolescence were more likely to finish college, to earn higher incomes, and to have lower levels of depression in young adulthood compared to those with low levels of striving. However, these same youth were also more likely to develop diabetes by age 29, a pattern not evident in high-income youth (Brody et al., 2016). Similarly, low-income youth of color who show high levels of self-control had lower levels of internalizing and externalizing problems, but higher levels of allostatic load and faster epigenetic aging (a measure of leukocyte DNA methylation profiles that reflects the discrepancy between a person’s biological and chronological age) compared to those with low levels of self-control (Brody et al., 2013, Miller et al., 2015). In chronically ill populations, youth of color with asthma who exhibit high levels of self-control under stressful school conditions have better mental health but worse asthma inflammatory profiles compared to those with low levels of self-control (Chen et al., 2019).

Studies that have investigated related constructs have found similar patterns. For example, conscientiousness is a personality trait that encompasses being hard working, planful, careful, and self-controlled, with overlaps with the above constructs of self-control and striving. Interactions between SES and conscientiousness in previous research indicate that low SES adults of color who were high in conscientiousness had higher educational attainment and fewer symptoms of depression, but were more likely to develop a verified respiratory infection following viral challenge compared to those low on conscientiousness, with an opposite pattern for infection present in high SES adults (Miller et al., 2016). Similarly, low SES men of color who were high in conscientiousness had higher metabolic syndrome scores than those who were low in conscientiousness (Duggan et al., 2019). Additionally, low SES adults of color who show high levels of high-effort coping, hard work, and a single-minded determination to succeed (John Henryism) evinced higher blood pressure and a greater risk of hypertension compared to those with low levels of John Henryism (James et al., 1987, James et al., 1992).

Taken together, these studies suggest the plausibility of the skin-deep resilience scenario. However, all of the above studies are observational, meaning that there could easily be unobserved factors inflating or accounting for the observed patterns. Twin designs represent one approach to minimizing the influence of such confounds, particularly ones that involve heritable genetic and shared familial influences. The discordant twin design is an especially useful method in this regard (McGue et al., 2010). This design examines whether, within twin pairs, differences in an independent variable of interest are associated with the outcome of interest. When using monozygotic twins, this design controls for both shared genetic and shared environmental confounds. In the present study, we used monozygotic twins from a large, national study of adolescents to test skin-deep resilience hypotheses. As an outcome, we focused on a common biomarker of low-grade inflammation, C-reactive protein (CRP). Inflammation is thought to be one key pathway connecting childhood adversity to the development of multiple chronic diseases later in life (Miller et al., 2011), and it plays an important role in the pathogenesis of cardiometabolic health problems including obesity, diabetes, and myocardial infarction (Hotamisligil, 2006, Ridker, 2007). CRP in particular has been prospectively associated with risk for Type 2 diabetes, coronary heart disease, and myocardial infarctions and strokes (Ridker, 2003, Ridker et al., 2005, Ridker et al., 2000, Pradhan et al., 2001, Pradhan et al., 2002), and in children, CRP has been associated with adiposity, blood pressure, metabolic syndrome, carotid intima-media thickness, and lesions in the coronary artery (see (Balagopal et al., 2011) for an overview). Mirroring the findings in previous research (Miller et al., 2016, Brody et al., 2013, Brody et al., 2016), we hypothesized that there would be an interaction between family SES and within-twin conscientiousness. That is, among low SES twins, we expected the more conscientious one to evince higher CRP, whereas we hypothesized that this pattern would not be evident among high SES twins. Also mirroring previous research (Chen et al., 2019, Miller et al., 2015), we hypothesized there would be a divergence in physical versus mental health/academic outcomes, such that (unlike CRP) conscientiousness would be associated with better mental health (less depression, less substance use) and better educational attainment across all groups.