A Critical Review of Habit Theory of Drug Dependence

Abstract

Animal behavioural research has shown that chronic drug exposure renders instrumental behaviour prone to habitual as opposed to goal-directed control. It is claimed that the same propensity to habit underpins human drug dependence. This chapter critically reviews animal and human studies addressing habit theory of dependence. The animal outcome-devaluation model is criticised on the grounds that habitual control is not observed when animals are given a concurrent choice between different reinforcers, or when the devalued reinforcer is re-experienced, suggesting that habit demonstrated in the animal model is unlikely to operate in the natural environment of human drug users. Nine human outcome-devaluation studies are reviewed. Seven report no evidence that habit is more pronounced in drug users versus controls, or as a function of dependence severity, whereas two studies report positive effects—a 7:2 ratio against habit theory. Seven two-stage studies are also reviewed. Five report no evidence for impaired goal-directed (model-based) control in drug users versus controls, or as a function of dependence severity, whereas three studies report positive effects—a 5:3 ratio against habit theory (a combined ratio 12:5 against). The five positive human studies supporting habit theory may be attributed to general cognitive deficits or task disengagement in drug users, which weakens explicit knowledge of task contingencies impairing task performance, rather than a specific propensity to habit. The final section reviews compelling evidence that dependence is driven by excessive goal-directed drug choice and greater sensitivity to acute adverse triggers motivating further goal-directed drug choice through incentive learning. Accepting the incentive learning over the habit account of dependence has important implications for treatment strategy.