What We Know about Early Influences on Health, Behavior, and Learning: A Very Brief Review

Child development starts at conception. The fetal origins hypothesis first suggested by Barker (1995) hypothesizes that pre-birth experiences have long-term effects on health. Ever mounting evidence suggests that maternal impoverishment during the prenatal period has a substantial causal impact on infant health and long-term outcomes (Aizer and Currie 2014). Behaviors (smoking, drinking, substance abuse— each holding other factors constant) and exposure to toxins all exert a negative influence on in-utero child health, full-term birth, birth weight, and early child wellbeing (Lien and Evans 2005). Exposure to harmful environmental factors such as pollution, violence, and stress also take their toll on mothers and children alike (Currie et al. 2009; Currie and Walker 2011). Nutritional and health effects in-utero are also important to long-term outcomes for children—the findings of multiple studies suggest the growing importance of such effects (Mazumder et al. 2010, 2015; Almond and Mazumder 2011; Almond et al. 2012; Almond and Currie 2011).

Mothers born in a high-disease environment were also more likely than other women to have low-birth-weight offspring and to be suffering from diabetes when they gave birth, suggesting a strong intergenerational environmental component to poor health (Almond et al. 2011; Aizer and Cunha 2012; Smeeding 2015). Disadvantaged women also have greater exposure to, and are more susceptible to, contagions such as seasonal influenza. Hence, they may be disproportionately affected by pandemics which, in turn, can negatively affect fetal development. There are a number of factors that can potentially explain disadvantaged women's greater susceptibility. These include that disadvantaged women are more likely to live in crowded homes, are more reliant on public transportation, are less able to stay home from work when ill, are less likely to be immunized, and are less likely to believe the influenza vaccine to be effective (Wooten et al. 2012; Sanders 2012; Quinn et al. 2011). Finally, women who are poor, minority, or both are also more likely to be the victims of domestic violence (Vest et al. 2002). The literature on maternal health, exposure to toxins and the like, and poverty strongly suggest that from conception through birth, children from lower-income families are at a disadvantage in comparison to those born to higher-income families.

Moreover, there is evidence that poor birth outcomes and low birth weight have effects that are liable to persist through childhood and even into adulthood. In a recent paper, Figlio and colleagues (2014) find that the effects of poor neonatal health on adult outcomes are largely determined early in life and continue for all births to rich and poor families alike and to families at all levels of educational attainment (Figlio et al. 2014). However, children with poor neonatal health born to highly educated families perform much better in the longer run than do those with good neonatal health born to poorly educated families, suggesting that patterns of nurture and early child development can at least partially overcome poor health at birth. Their findings are very much in keeping with the literature on the positive relationship between household income and health status in childhood and adulthood (Hoynes et al. 2012; Dahl and Lochner 2012) and are consistent with the notion that parental inputs and neonatal health are complements rather than substitutes, a “dynamic complementarity” that we return to below.

Recent research has focused on understanding how environmental experiences, including stress and poverty, affect the underlying neurocognitive, biological, and physiological processes of development. This phenomenon is often referred to as the way that “poverty gets under the skin.” About five years ago, early research identified abnormal levels of, and fluctuations in, cortisol (the “stress” hormone) as the primary underlying mechanism (McEwen and Gianaros 2010; Champagne and Mashoodh 2009; Seeman et al. 2010). More recently, given that stress-related, elevated levels of cortisol in the mother can affect the placenta, researchers have focused on the potential negative effects of maternal stress on fetal outcomes. Comparisons of siblings suggest that those who were apparently exposed to higherthan-average levels of cortisol in utero have lower IQ levels at age 7 and complete one less year of schooling (Aizer et al. 2012). In some recent studies, environmental experiences are linked to individual differences in developmental outcomes through stable and permanent changes in genetic expressions (Essex et al. 2013).

Although genetic endowments are largely invariant during development, there is considerable change in the epigenome—the biochemical system that regulates gene expression. Moreover, the epigenome has been found to be particularly responsive to environmental conditions, including poverty directly (Hanson et al. 2013; Essex et al. 2013; Boyce 2012; Sameroff 2010). Research has also found that early maternal stressors are related to epigenetic changes in their children during adolescence, with implications for their mental health (Hanson et al. 2014; Knudsen et al. 2006; Shonkoff et al. 2012). Finally in a recent study of great importance, Noble et al. (2015) provide the strongest evidence to date that socioeconomic disparities, particularly in income, are associated with large differences in cognitive development. Investigating patterns in brain structure across social and economic status, they found that children from lower-income families had relatively large differences in brain surface area in comparison to children from higher-income families, likely predictive of future differences in cognitive development.

Postpartum health and development (but prior to pre-preschool) is also important for child outcomes (Beller 2009). Several studies have documented the relationship between the amount and type of speech directed at a child by caregivers during the course of a typical day and the child's later expressive language and vocabulary (Weisleder and Fernald 2013; Rowe 2012). Studies of parenting and children's selfregulation also point to associations between parents' early support of their children's autonomy with later assessments of children's executive function (Landry et al. 2006; Bernier et al. 2010). Because higher-income parents are typically better educated and also have more money to invest, their children tend to have better outcomes than children of lower-income parents (Guryan et al. 2008; Yeung et al. 2002; Kaushal et al. 2011). Further, child-parent interactions, such as those outlined above, may be more productive for children born healthier. In other words, prenatal and postpartum investments may be complementary in producing better child outcomes (Bono et al. 2012; Hsin 2012).

In fact, research on the malleability of cognitive and language abilities shows these skills to be highly responsive to both positive and negative influences (Fox et al. 2010; Shonkoff 2010). In effect this suggests that newborn health and postnatal investments are complementary. This hypothesis, termed “dynamic complementarity,” implies that the impacts of general parental investments, as well as early childhood education on child outcomes, will be greater for children who enter the preschool period with higher levels of cognitive and socioemotional skills (Aizer and Cunha 2012). In particular, preschool settings that are designed to expose children to sensitive caregiving environments should increase children's socioemotional skills much more among children with more sensitive caregivers in their home environments (Duncan 2014). This process of dynamic complementarity is still just a hypothesis, and one whose negative effects can be overcome by consistent, strong investments in children from the beginning of their lives, even for the most disadvantaged children (Cunha and Heckman 2007, 2008; Camilli et al. 2010; Heckman and Mosso 2014).

Thus, despite some uncertainty, the available evidence suggests that the consequences of initial health disadvantages associated with being born to a poor mother are likely to be exacerbated over time without intensive policy and practice interventions. Unfortunately, children with poorer initial health endowments typically receive fewer postnatal investments, and the investments they do receive may be less effective due to dynamic complementarity. This mechanism can explain not only the considerable persistence of in-utero conditions in later-life outcomes, but also why the long-term impact of low birth weight is greater when children are born into poverty and other unsatisfactory circumstances (Figlio et al. 2014). In terms of the framework of this project, early gaps can easily become larger and increasingly more difficult to reduce. However, continuous investments before the preschool period can still make an important difference in outcomes.