The pervasive use of single-use plastic bags presents a growing environmental challenge globally. Despite legislative efforts, such as the mandatory 5p bag-charge introduced in England in October 2015, the problem persists. The issue took on renewed urgency in the wake of the Covid-19 pandemic, which has seen a regression in consumer habits towards increased use of single-use plastics, driven by health and safety concerns (Adyel, 2020).

While traditional economic theory suggests that incentives should effectively modify consumer behaviours in the direction that is intended or desired, empirical evidence points towards a more complex reality. The unintended adverse consequences of price-based incentives in undermining intrinsic motivation have been observed in several contexts, most famously on blood donation (in the classic study by (Titmus, 1970), but also in other contexts. In fact, some studies show that not only do policies based solely on price-based incentives fail to achieve their stated goal, but they can also backfire by changing the construal of the exchange, and eroding (rather than simply reducing) individuals’ intrinsic motivation for that act, such as in the study on fines for late-pickups at the Haifa day care centre by Gneezy and Rustichini (2000). Bowles (2016) argued that to avoid such unintended effects of economic policies, policymakers must account for the interplay between incentives and individuals' social and moral preferences. Kranton (2019) echoed this view, highlighting the importance of communicating economic policies with carefully crafted narratives.

This study contributes to this ongoing discourse. It examines how the effectiveness of price-based (dis)incentives can be enhanced by combining them with behavioural interventions, such as a framing nudge and a values-driven narrative. With the aim of reducing demand for single-use plastic bags, I tested these interventions in a nationwide online-field randomised controlled trial in partnership with Tesco, the largest supermarket chain in the United Kingdom. The trial was conducted over 15 days from September - October 2020, involving over 2.4 million orders. All online customers were presented with two packing options for their grocery delivery: without plastic bags (default), or with plastic bags for a flat charge of 40p (in line with the U.K. government policy). In addition to the charge for plastic bags, two treatments drawing upon behavioural sciences were introduced to discourage plastic bag-use: a (bagless) framing nudge, and a values-driven narrative.

The analysis reveals that both treatments were effective in enhancing the effects of a 40p charge for plastic bags by encouraging more customers to choose the bagless packing options for their orders. This is the first study to demonstrate, via a field experiment, that combining narrative-based behavioural interventions with price (dis)incentives can enhance the effectiveness of the charge.