What is behavioural spillover?

 Behavioural spillover is defined generally as the effect a behavior targeted by an intervention has on other behaviors untargeted by the intervention. Essentially, it is the idea that engagement in one pro-environmental behavior (PEB) can influence whether an individual subsequently engages in other PEBs.

This concept is also referred to in the literature as 'spreading activation,' 'response generalisation,' or 'behavioural diffusion'. The potential significance of spillover is that it can engender broader lifestyle change beyond specific behaviors, offering a potentially voluntary approach to achieving widespread shifts in consumer habits.

To conclusively determine that spillover exists, the relationship between the targeted and untargeted behaviors must be observable, causal (not due to a third influencing factor), and sequential (the untargeted behavior must follow the targeted behavior). The behaviors often share a common motivation, such as reducing one's environmental impact.

Types of Behavioural Spillover

Spillover effects are categorized based on their outcome and context:

1. Positive Behavioural Spillover: Occurs when changing a targeted behavior leads to a change in an untargeted behavior that also carries a similar environmental benefit. For example, switching off lights when not in use might lead to avoiding leaving other appliances on standby.
2. Negative Behavioural Spillover (or Rebound): Occurs when changing a targeted behavior leads to an increase in an untargeted behavior that is inconsistent with the environmental goal or that increases adverse environmental impact. Examples include switching off lights but subsequently leaving appliances on standby.
3. Temporal Spillover: Takes place when the performance of a behavior causes a subsequent change in the same behavior over time, increasing its frequency or intensity (e.g., purchasing a green product leads to purchasing the product more frequently).
4. Contextual Spillover: Occurs when a single behavior performed in one setting spreads to a different context (e.g., transferring energy-saving behavior from the workplace to the home).

Psychological Pathways to Positive Spillover

The sources outline three primary psychological pathways through which positive spillover occurs:

1. Behavioural Consistency: Individuals have an intrinsic psychological preference for consistency in thought and action. This desire to maintain behavioral consistency often supports the "foot-in-the-door-effect," where compliance with a small initial request increases the likelihood of agreeing to a more committed request later.
2. Pro-environmental Identity Change: Identity is closely linked to decision-making. When engaging in a pro-environmental behavior, especially one perceived as costly or committed, it can strengthen an individual's sense of self as an "environmentalist". This enhanced identity serves as a motivation to act consistently across other domains (e.g., purchasing a green product increased participants’ sense of moral obligation and likelihood of choosing other green products).
3. Knowledge, Skills, and Self-efficacy: Performing a specific PEB can increase an individual’s practical knowledge or skills in that area and boost their self-efficacy (the belief that one's actions can make a difference). This mastery of a simple PEB may motivate people to take on more difficult, related actions.

Mechanisms Driving Negative Spillover

Negative spillover effects often result from psychological or economic processes:

1. Moral Licensing: This is a psychological theory where performing a morally virtuous act grants a person "moral credits," making them feel entitled to subsequently act in a less morally virtuous way. For instance, individuals who made more green product choices were later more likely to avoid engaging in other PEBs or use more resources.
2. Rebound Effects: This is an economic phenomenon where gains in resource efficiency (e.g., lower energy costs due to energy-efficient technology) are offset by an increased demand for that resource (direct rebound) or by spending the resulting monetary savings on other environmentally detrimental activities (indirect rebound).

Policy Caution

Despite the appeal of spillover, policymakers and practitioners are cautioned against relying solely on it, as the evidence for consistent positive spillover is mixed and often methodologically limited. Spillover is more likely to occur when the behaviors are conceptually linked or share common (pro-environmental) goals or values.

Analogy: You can think of behavioural spillover like dominoes. The targeted behavior is the first domino (which is knocked over by an intervention). Positive spillover occurs if that first domino smoothly knocks over other environmentally friendly dominoes. Negative spillover (moral licensing) is like the first domino hitting a "free pass" card instead, making the person feel they can skip the next few dominoes entirely. For spillover to work best, the dominoes (behaviors) should be closely related or share a strong foundation (like identity or values) so that the initial action reliably triggers the subsequent desired ones.