Which Counts More: Differential Impact of the Environment or Differential Susceptibility of the Individual?

The theory of differential susceptibility is helping to explain how genetic, neurological and personality factors affect individual mental and physical health and why interventions work better with certain populations. As social workers, however, our focus is more on the impact of the social determinants of health found in people's environments and the nuanced way external factors influence psychological treatment outcomes and human development over time rather than genotypes and phenotypes. This article discusses differential impact theory (DIT) as a complementary theory to differential susceptibility in an effort to make both theories relevant to social work practice. After a brief summary of the differential susceptibility research, I draw from studies of psycho-social interventions and Person × Environment interactions to show that responsibility for positive adaptation resides within the systems that surround individuals just as much as, and possibly more than, within individuals themselves. DIT provides a more balanced explanation than differential susceptibility theory alone for why clinical and community interventions and changes to social policy can have a positive influence on psycho-social outcomes. The implications of DIT are discussed with regard to the design and delivery of psychological and social interventions.