Ten years ago, Thaler and Sunstein introduced the notion of nudging to talk about how subtle changes in the 'choice architecture' can alter people's behaviors in predictable ways. This idea was eagerly adopted in HCI and applied in multiple contexts, including health, sustainability and privacy. Despite this, we still lack an understanding of how to design effective technology-mediated nudges. In this paper we present a systematic review of the use of nudging in HCI research with the goal of laying out the design space of technology-mediated nudging-the why (i.e., which cognitive biases do nudges combat) and the how (i.e., what exact mechanisms do nudges employ to incur behavior change). All in all, we found 23 distinct mechanisms of nudging, grouped in 6 categories, and leveraging 15 different cognitive biases. We present these as a framework for technology-mediated nudging, and discuss the factors shaping nudges' effectiveness and their ethical implications. CCS CONCEPTS • Human-centered computing → Human computer interaction.

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We present BreakOut, a desktop system that infers an individual's posture, stress levels, and engagement with computer related tasks with the goal of recommending a break of sedentary behaviour at appropriate times. We present an Experience Sampling study that attempted to evaluate BreakOut's ability to infer users' posture and stress from a single webcamera as well as their engagement from keyboard and mouse activity. We conclude with the description of an ambient artistic display as well as an intuitive gestural interaction technique that enables the user to interact with BreakOut with minimal cognitive resources.

Despite the initial premise of behavior change tools, recent work has questioned their efficacy over the long term. Many of these technologies rely on a "one-size fits all" strategy -self-monitoring -to foster behavior change. However, individual's capacity to change their behaviors depends on their ability and motivation for self-regulation. We explore a different approach to persuasive technology design and investigate how subtle influences falling outside conscious awareness can instinctively motivate behaviors. Subliminal priming has the potential to influence people's attitudes and behaviors, without relying on people's will to engage with the process. Yet, little research has studied it effectiveness encouraging behavior change outside a laboratory setting. In this paper we explore the feasibility of subliminal priming on web-platforms. We present an in the wild study where 12 participants were exposed to subliminal cues to motivate water intake while they browsed on the web. This paper contributes with a practical application of subliminal priming in the field of behavior change as well as design implications for future research.