What works for whom? Genetic moderation of intervention efficacy.

Abstract

Development and Psychopathology 27 (2015), 1–6 # Cambridge University Press 2015 doi:10.1017/S0954579414001254 SPECIAL SECTION EDITORIAL What works for whom? Genetic moderation of intervention efficacy JAY BELSKY a AND MARINUS H. VAN IJZENDOORN b a University of California, Davis; and b Leiden University The big question in human development intervention, just like in modern medicine, is What works for whom? Although it is well appreciated that interventions do not succeed with all whom they reach, a common presumption as to why they of- ten prove less effective than anticipated focuses on poor im- plementation. There can be no doubt that fidelity to program model matters greatly. Nevertheless, an expanding body of experimental-intervention research (e.g., Beach, Brody, Lei, & Philibert, 2010; Kegel, Bus, & van IJzendoorn, 2011), building directly on theory regarding differential susceptibil- ity to environmental influences and observational evidence to this effect (Belsky, Bakermans-Kranenburg, & van IJzen- doorn, 2007; Belsky & Pluess, 2009, 2013; Ellis, Boyce, Belsky, Bakermans-Kranenburg, & van IJzendoorn, 2011), makes it clear that characteristics of individuals involved in interventions matters as well when it comes to explaining “what works for whom.” Answering the question of which in- dividual characteristics matter holds the promise of enabling intervention programmers to tailor interventions to indi- viduals, thereby improving the efficiency of program delivery and maximizing impact. The main goal of this Special Sec- tion of Development and Psychopathology is to share the lat- est experimental intervention research addressing this issue that focuses on the genetic makeup of individuals as determi- nants (or moderators) of variation in intervention efficacy. The differential susceptibility model has important clinical and practical implications. The average effects of preventive or therapeutic interventions in human development are only modest, with effect sizes barely larger that Cohen’s criterion for a weak effect (d ¼ 20), reflecting the standardized differ- ence between the control and intervention group on the core outcome the intervention is designed to impact. Failure to consider variation in susceptibility, however, means that such intervention effects may be much larger for more sus- ceptible individuals and much smaller for the less susceptible (Belsky & Pluess, 2009, 2013; van IJzendoorn, Bakermans- Kranenburg, Belsky, et al., 2011). Weak intervention effects might be taken as a sign of a cul-de-sac, not only in research on intervention efficacy but also in terms of how well current thinking about developmental psychopathology (and human development enhancement) can inform efforts to prevent or remediate problematic functioning or enhance well-being. Policymakers and funding agencies might be less inclined to support efforts to document intervention efficacy or to roll out evidence-based interventions on a larger scale given concerns that impact might be limited and/or not cost effec- tive. Being aware of differential susceptibility of participants in preventive or therapeutic interventions creates more realis- tic expectations of intervention efficacy while illuminating the hidden efficacy of interventions targeting groups com- posed of a mixture of both more and less susceptible indi- viduals (Bakermans-Kranenburg & van IJzendoorn, in press). It is important to appreciate that a focus on genetically based differential susceptibility can raise ethical issues concerning stigma, discrimination, and equity of service provision (Ellis et al., 2011), but this should not lead to ignoring replicated evidence that some individuals are more open to envi- ronmentally induced change, for better and for worse, due to their genetic makeup. Should it prove the case that genetic characteristics of indi- viduals account for and perhaps even causally influence the efficacy of particular interventions, as multiple reports in this Special Section indicate, a second issue arises beyond “what works for whom.” That involves the mechanisms ac- counting for why a particular intervention proves more effec- tive for some than for others. Although some of the research The editors and all of the contributors to this Special Section are indebted to the Jacobs Foundation, which provided funding for two meetings of inter- ested scientists that gave birth to the idea of the Special Section. Special thanks to Simon Sommer of the foundation for his support and counsel and to Dante Cicchetti for encouraging us to develop the Special Section. Address correspondence and reprint requests to: Jay Belsky, Department of Human Ecology, University of California, Davis, 1331 Hart Hall, One Shields Avenue, Davis, CA 95616; E-mail: jbelsky@ucdavis.edu; or Mari- nus H. van IJzendoorn, Graduate School of Education and Child Studies, Centre for Child and Family Studies, Leiden University, Pieter de la Court Gebouw, Wassenaarseweg 52, Leiden 2333 AK, The Netherlands; E-mail: vanijzen@fsw.leidenuniv.nl.