Using Behavioural Economics to Understand Concurrent Substance Use Disorders and Post-Traumatic Stress Disorder

Substance use disorders (SUDs) remain the most common psychiatric conditions in Canada. Often, however, SUDs do not exist in isolation, but are concurrent with comorbid mental illness. One such common comorbidity is with post-traumatic stress disorder (PTSD). This high co-prevalence is concerning, as individuals with SUD+PTSD show less improvement and earlier drop out from treatment, higher rates of relapse, and higher rates of suicide. Comorbid SUD+PTSD is substantially less well understood than either disorder individually, but its etiology may be elucidated using behavioural economics. From a behavioural economic perspective, SUDs are a product of three core processes: 1) overvaluation of smaller immediate rewards over larger future ones ("delay discounting"); 2) overvaluation of drug reinforcement ("drug demand"); and 3) low availability of substance-free reinforcement ("alternative reinforcement"). This is the reinforcer pathology (RP) model of addiction, which has been well supported for various types of SUDs, including alcohol, cannabis, cocaine, and opioids. Here, I propose an extension of the RP model to explain comorbid SUD+PTSD, with further elevations expected on the three core behavioural economic indicators due to substantial negative reinforcement (distress relief) drug motivation. The RP model will be applied to five diverse samples, with the aims to 1) predict problematic substance use and SUDs among those with concurrent PTSD or PTSD symptomology; 2) differentiate non-clinical and clinical samples; and 3) determine directional associations between PTSD and SUD from longitudinal data. The results will substantially extend our knowledge of this common and clinically challenging comorbidity, with promise for informing existing treatments and developing novel interventions.