For Better and For Worse Differential Susceptibility to Environmental Influences

Evidence that adverse rearing environments exert negative effects particularly on children presumed ‘‘vulnerable’’ for temperamental or genetic reasons may actually reflect something else: heightened susceptibility to the negative effects of risky environments and to the beneficial effects of supportive environments. Building on Belsky’s (1997, 2005) evolutionary-inspired proposition that some children are more affected—both for better and for worse—by their rearing experiences than are others, we consider recent work on child vulnerability, including that involving measured genes, along with evidence showing that putatively vulnerable children are especially susceptible to both positive and negative rearing effects. We also consider methodological issues and unanswered questions in the differential-susceptibility equation. KEYWORDS—differential susceptibility; gene–environment interaction; parenting; temperament Most students of child development probably do not presume that all children are equally susceptible to rearing effects; a long history of research on interactions between parenting and temperament, or parenting-by-temperament interactions, clearly suggests otherwise. Nevertheless, it remains the case that most work still focuses on parenting effects that apply equally to all children—so-called main effects of parenting—thus failing to consider interaction effects, which reflect the fact that whether, how, and how much parenting influences the child may depend on the child’s temperament or some other characteristic of individuality. Like classic work in educational and clinical psychology on interactions between learning aptitude and treatment, research on parenting-by-temperament interactions is based on the premise that what proves effective for some individuals in fostering the development of some valued outcome—or preventing some problematic one—may simply not do so for others. Commonly tested are hypotheses derived from multiple-risk/transactional frameworks in which individual characteristics that make children ‘‘vulnerable’’ to adverse experiences—placing them ‘‘at risk’’ of developing poorly—are mainly influential when there is at the same time some contributing risk from the environmental context. After highlighting some research of just this kind, we raise questions—on the basis of other findings—about how the first set of data has been interpreted. We advance the evolutionaryinspired proposition that some children, for temperamental or genetic reasons, are actually more susceptible to both (a) the adverse effects of unsupportive parenting and (b) the beneficial effects of supportive rearing. The validity of this claim cannot be determined, however, so long as research focuses disproportionately on vulnerable (as opposed to merely susceptible) child characteristics and evaluates effects of adverse environments on problematic outcomes. What, then, would be required to distinguish vulnerability from susceptibility? We consider the answer after first reviewing research that meets the criteria for differential susceptibility. Finally, we draw conclusions and highlight some ‘‘unknowns in the differential-susceptibility equation.’’ DUAL-RISK CONDITIONS AND CONSEQUENCES The view that infants and toddlers manifesting high levels of negative emotion are at special risk of problematic development when they experience poor-quality rearing is widespread. Evidence of this comes from Morrell and Murray (2003), who showed that it was only highly distressed and irritable 4-monthold boys who experienced coercive and rejecting mothering at this age who continued to show evidence, 5 months later, of emotional and behavioural dysregulation. Relatedly, Belsky, Hsieh, and Crnic (1998) observed that infants who scored high in negative emotionality at 12 months of age and who experienced the least supportive mothering and fathering across their second and third years of life scored highest on externalizing problems Address correspondence to Jay Belsky, Institute for the Study of Children, Families and Social Issues, Birkbeck University of London, 7 Bedford Square, London WC1B 3RA, United Kingdom; e-mail: [email protected]. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 300 Volume 16—Number 6 Copyright r 2007 Association for Psychological Science at 36 months of age. And Deater-Deckard and Dodge (1997) reported that children rated highest on externalizing-behavior problems by teachers across the primary-school years were those who experienced the most harsh discipline prior to kindergarten entry and who were characterized by mothers at age 5 as being negatively reactive infants. The adverse consequences of the co-occurrence of a child risk factor (e.g., negative emotionality) and problematic parenting also is evident in Caspi and Moffitt’s (2006) ground-breaking research on gene-by-environment interaction. Young men followed from early childhood were most likely to manifest high levels of antisocial behavior when they had both a history of child maltreatment and a particular variant of the MAO-A gene, a gene previously linked to aggressive behaviour. Such results led Rutter (2006), like others, to speak of ‘‘vulnerable individuals,’’ a concept that also applies to children putatively at risk for compromised development due to their behavioral attributes. But is ‘‘vulnerability’’ the best way to conceptualize the kind of parenting-by-child interactions under consideration? VULNERABILITY OR DIFFERENTIAL SUSCEPTIBILITY? Working from an evolutionary perspective, Belsky (1997, 2005) theorized that children, especially within a family, should vary in their susceptibility to both adverse and beneficial effects of rearing influences: Because the future is uncertain, in ancestral times, just like today, parents could not know for certain (consciously or unconsciously) what rearing strategies would maximize reproductive fitness. To protect against all children being steered, inadvertently, in a parental direction that proved disastrous at some later point in time, developmental processes were selected to vary children’s susceptibility to rearing. Belsky (1997, 2005) further observed that children high in negative emotion, particularly in the early years, appeared to benefit disproportionately from supportive rearing environments (Boyce & Ellis, 2005). Crockenberg (1981) showed that social support predicted infant attachment security but only in the case of highly irritable infants. Denham et al. (2000) reported that the beneficial effects of proactive parenting (i.e., supportive presence, clear limit setting) at age 7 and/or age 9 were most pronounced in the case of children who scored high on externalizing problems (i.e., disobedient, aggressive, angry) at an earlier time of measurement (i.e., mean age 55 months), even after controlling for problem behavior at the initial measurement occasion. Experimental studies designed to test Belsky’s (1997) theory are even more suggestive of differential susceptibility than the longitudinal-correlational evidence. Blair (2002) discovered that it was highly negative infants who benefited most—in terms of both reduced levels of externalizing behavior problems and enhanced cognitive functioning—from a multifaceted infanttoddler intervention program whose data he reanalyzed. More recently, Klein Velderman, Bakermans-Kranenburg, Juffer, and Van IJzendoorn (2006) found that experimentally induced changes in maternal sensitivity exerted greater impact on the attachment security of highly negatively reactive infants than it did on other infants. In both experiments, environmental influences on ‘‘vulnerable’’ children were for better instead of for worse. Better Evidence of Differential Susceptibility Even though studies highlight the heightened susceptibility of temperamentally negative or genetically vulnerable offspring to either positive or negative rearing influences, more compelling would be data on a single sample substantiating the for-betterand-for-worse predictions of the differential-susceptibility hypothesis. Feldman, Greenbaum, and Yirmiya (1999) found that 9-month-olds scoring high on negativity who experienced low levels of synchrony in mother–infant interaction manifested more noncompliance during clean-up at age two than other children did. When such infants experienced mutually synchronous mother–infant interaction, however, they displayed greater self-control than did children manifesting much less negativity as infants. More recently, Kochanska, Aksan, and Joy (2007) observed that highly fearful 15-month-olds experiencing high levels of power-assertive paternal discipline were most likely to cheat in a game at 38 months, yet when cared for in a supportive manner such negatively emotional, fearful toddlers manifested the most rule-compatible conduct. Recent studies involving measured genes and measured environments also document both-for-better-and-for-worse rearing effects in the case of susceptible infants, specifically those with a particular allele (variant) of a gene called DRD4, which codes for a type of dopamine receptor. Because the dopaminergic system is engaged in attentional, motivational, and reward mechanisms and the variant in question, the 7-repeat allele, has been linked to lower dopamine reception efficiency, Van IJzendoorn and Bakermans-Kranenburg (2006) predicted this allele would moderate the association between maternal unresolved loss or trauma and infant attachment disorganization. Having the 7-repeat DRD4 allele substantially increased risk for disorganization in children exposed to maternal unresolved loss/trauma, as expected; but when children with that allele were raised by mothers who had no unresolved loss, they displayed significantly less disorganization than agemates without the allele, regardless of mothers’ unresolved-loss status (BakermansKranenburg & Van IJzendoorn, in press). Similar results emerged when the interplay between DRD4 and observed parental insensitivity in predicting externalizing problems was studied in a group of 47 twins (BakermansKranenburg & Van IJzendoorn, 2007). Children with the 7repeat DRD4 allele and insensitive mothers displayed more externalizing behaviors than children without that allele (irrespective of maternal sensitivity); and children with the 7repeat DRD4 allele and sensitive mothers showed the lowest Volume 16—Number 6 301 Jay Belsky, Marian J. Bakermans-Kranenburg, and Marinus H. van IJzendoorn