"Just-in-time" but a bit delayed: Personalizing digital nudges for healthier online food choices.

As food choices are increasingly made in contexts such as online supermarkets, nudging has been extrapolated to the digital sphere. Digitalization poses unique opportunities to enhance the promotion of healthier food choices online: Digital nudges can be delivered "just-in-time" (JIT), in response to the initial selection of an unhealthy product. Furthermore, digital JIT nudges can be personalized to match user characteristics of behavioral relevance, such as one's food and cognitive processing preferences. We examined whether personalizing by user-matching digital JIT nudges on content (i.e., emphasizing health versus price) and/or type (i.e., a text-based versus an image-based nudge) would increase nudge effectiveness and healthier food purchases, and assessed psychological mechanisms potentially underlying these enhanced effects. In a 2 (Nudge Content: Non-personalized versus Personalized) by 2 (Nudge Type: Non-personalized versus Personalized) randomized controlled trial, 200 healthy participants completed an online grocery shopping task on a mock supermarket app in which they first selected products from 10 different categories, followed by a checkout screen where they could replace products in the shopping basket. Personalizing nudge type increased nudge effectiveness (i.e., the proportion of accepted nudges when choosing products from categories). Personalizing nudge content seemed to exert a delayed effect of increasing healthier product replacements prior to checkout. User perceptions of JIT nudges did not vary with personalization. However, regardless of personalization, all JIT nudging conditions had more final healthier food purchases and greater satisfaction with food choices compared to the control condition. Collectively, this pre-registered "proof-of-principle" study demonstrates that personalizing the content and type of digital JIT nudges yields small positive benefits on the healthiness of online food choices and attests to the utility of JIT interventions for health promotion.