Habit and habitual behaviour.

Writing in 1899, William James observed that ‘ninety-nine hundredths or, possibly, nine hundred and ninety-nine thousandths of our activity is purely automatic and habitual’ (James, 1899, pp. 65–66). Nearly 125 years later, the concept of habit continues to stimulate important research regarding the most useful definition of habit, and how to harness habit to understand, predict, and change health-related behaviour (e.g., Gardner, 2015; Verplanken, 2018; Wood & Runger, 2016). Phillips and Mullan (2022) offer a thoughtful synthesis of theory and evidence regarding how habit can be applied to complex health behaviours. Their review addresses an important question that habit researchers are often asked: how credible is it to propose that learned cue-behaviour associations can underpin and sustain complicated real-world health behaviours? Drawing on a definition of complex behaviour as that which involves separable components and a greater number of subactions (or ‘steps’), Phillips and Mullan address this question by suggesting that the same concept – i.e., habit – can underpin both simple and complex behaviours. Significantly however, they argue that ‘complex habits’ should be theorised differently to ‘simple habits’, to better recognise the multiple components of complex habits and the rewards that Phillips and Mullan suggest are needed for complex habits to form. In this commentary, we expand discussion of two areas of Phillips and Mullan’s argument. First, we debate aspects of the definition of habit. Second, we highlight the importance of distinguishing between ‘habit’ and ‘habitual behaviour’ when considering behavioural complexity. We argue that conceptualisations of key terms have important implications for understanding, measuring and changing habit and habitual behaviour.