PPIs are very diverse, including intentional activities such as writing gratitude letters, learning to forgive, and taking care of one’s body. Based on a literature review, Schuller and Parks (2014) identified five broad categories of evidence-based interventions: (1) savouring experiences and sensations, (2) cultivating gratitude, (3) engaging in kind acts, (4) promoting positive relations, and (5) pursuing hope and meaning

The design was based on the ‘sustainable happiness model’ of Lyubomirsky et al. (2005), which proposes that the most effective way to increase and maintain one’s chronic happiness over and above the genetic set point is by changing one’s behaviour via intentional activities. These include activities that are behavioural (e.g. being kind to others), cognitive (e.g. counting one’s blessings), or volitional (e.g. devoting effort to meaningful causes). Shelden and Lyubomirsky (2006) proposed that such activities could combat the effects of hedonic adaptation because they are episodic (instead of continuous) and can be varied in terms of timing, approach, and content. As a consequence, intentional activities can have lasting benefits for our well-being (Lyubomirsky 2008; Layous and Lyubomirsky 2014; Lyubomirsky 2001

A person needs to ‘get over the hurdle’ of remembering to do them and overcome obstacles in initiating them, and this kind of self-regulatory effort requires considerable self-discipline and willpower (Sheldon and Elliot 1998; Lyubomirsky et  al. 2005). In other words, effectively initiating and pursuing happiness-increasing activities requires commitment and effort. As a consequence, people often need external support to be able to invest the time and energy necessary for sustainable behavioural change, and to effectively translate acquired knowledge into practice (Bolier 2015). The second challenge was that people will only want to use BITs if they provide acceptable levels of privacy and are unobtrusive, aesthetically pleasing, and trustworthy; people do not accept BITs that rely on the use of complicated, invasive, or demanding interfaces (Consolvo et al. 2009; Montague et al. 2009).

Lyubomirsky et al. (2005) offered two suggestions to help people get over the hurdle of engaging in intentional activities. The first is to start with those activities that are intrinsically more appealing than others, and the second is to create a habit out of regularly initiating beneficial activities. These suggestions partly overlap with three conditions for successfully changing behaviour (i.e. motivation, ability, and triggers) that were proposed by Fogg (2009): people can only change their behaviour when they are motivated to do so, when they are able to translate their motivation into concrete action, and if there are well-timed triggers to initiate new behaviour. The findings of Parks et  al. (2012) were also taken into consideration in the design process. They found that people get bored performing the same happiness-enhancing activities, and that happiness is improved more when people engage in a greater variety of activities. All these authors demonstrate the crucial role of intrinsic motivation in the development of effective BITs. Intrinsic motivation refers to doing something because it is inherently interesting or enjoyable, as opposed to being motivated by separable rewards or external pressures (Ryan and Deci 2000). These are activities that people do for the enjoyment of the activity itself, due to their appeal of novelty, challenge, or aesthetic value. Moreover, intrinsic motivation is facilitated by environments that support feelings of autonomy and competence (see Deci and Ryan 1985). Given these insights, TinyTask was designed with the intention to foster intrinsic motivation by integrating five key qualities: 1. Distil a range of general and abstract strategies into smaller (tiny), comprehensible tasks. 2. Provide an immediate sense of pleasure and achievement while (aand after) fulfilling the task. 3. Offer several tangible and concrete triggers to stimulate the intended activities. 4. Offer a simple structure that enables people to form a habit of engaging in the activities. 5. Present a broad diversity of activities to stimulate interest, to keep the experiences fresh and to offer a sense of choice