Amongst the wealth of models concerning behaviour change; three traditional models prevail within the literature. Firstly, the Health Belief Model (Glanz, Rimer & Viswanath, 2008) proposes that one's willingness to change depends on several factors, including one's perceived susceptibility to and the severity of a health behaviour, the perceived benefits and barriers to change, levels of self-efficacy to succeed in change, and cues to action, in addition to overall modifying variables which refer to individual characteristics. However, the evidence supporting the application of the HBM is weak and its predictive capacity is limited (Taylor et al., 2006). Possibly the most useful element presented within this model concerns the value of ‘perceived threat’ of a behaviour as a key indicator of behavioural change. This concept re-emerges within alternative theories, including Protection Motivation Theory. This theory proposes that individual's protect themselves against threats based on the threat appraisal, referring to the perceived severity and probability of the occurrence, and coping appraisal, referring to efficacy and self-efficacy (Rogers, 1975). Methods to increase a ‘perceived threat’ of a behaviour have proven somewhat useful in shaping health behaviours; as evidenced by pictorial warnings on cigarette packaging eliciting increased intentions to quit smoking significantly more so than text-only warnings (Noar et al., 2016).
Secondly, the Theory of Reasoned Action (TRA) (Fishbein & Ajzen, 1975) proposes that behaviour change depends on an individual’s attitude towards the behaviour, which is determined by their beliefs and evaluations concerning the expected outcome of the behaviour. This is influenced by the subjective norms about the behaviour in question, which is defined by beliefs about others’ opinions on the behaviour and motivations to comply with those opinions. Volitional control comprises another key element of this model; defined as the extent to which the behaviour can be applied consciously and become habitual over time. The TPB is an extension of TRA, in that it includes the aspect of perceived behavioural control, which refers to the beliefs regarding how easy or difficult it is to perform the behaviour (Ajzen, 1991). The evidence supporting the predictive performance of both TRA and TPB is stronger than that of the HBM, with TPB accounting for between 20% and 30% of the variance in adult health behaviours in the US and UK (Taylor et al., 2006). However, this is relatively low when needing to devise health behaviour interventions.
The third model traditionally referenced to within the literature is the Trans-Theoretical Model (Prochaska et al., 1992; Prochaska & Velicer, 1997). This framework proposes that the behaviour change process comprises of six distinguishable stages; including pre-contemplation (not intending to change), contemplation (thinking about change), preparation (intending to change a behaviour and initiating change with small steps, action (changing the behaviour), maintenance (sustaining the new behaviour), and termination (when there is no temptation to revert to the original undesirable behaviour). Individuals can relapse at any one of these stages. The benefit of this model is that it holds the capacity to serve as a foundation for interventions aimed at both an individual and a community level (Taylor et al., 2006), however, the evidence suggests it is no more effective than alternative, rationally designed, interventions.
Traditional models of behaviour change have long been applied in health services  (Taylor et al., 2006). However, a prevailing criticism that each of these three models share regards their failure to account for the impact of social, economic and environmental factors on health behaviours.
The literature suggest that several factors relating to an individual'd characteristics contribute to successful behaviour change. Self-efficacy, for instance, relates to ......... It comprises four distinguishable factors, including  enactive attainment, vicarious experience, social persuasion and physiological factors. Self-efficacy is an element which is touched upon in many theories of behaviour change; including that of the TPB (Azjen, 1991) in relation to perceived behavioural control, and in the TTP whereby self-efficacy is a central denominator of behaviour change stages (Prochaska et al., 1992). Self-efficacy is even presented as a predictor, mediator or moderator of behaviour change; as proposed in Bandura’s Social Cognitive Theory (1997). Self-efficacy both directly and indirectly impacts health through decision making, including behaviours such as smoking, physical exercise, dieting, condom use, dental hygiene, seat belt use, and breast examination (Conner, 2005). In all, research consistently demonstrates that positive behaviour change is more likely to occur if an individual has a high level of self-efficacy REF. Moreover, high levels of self-efficacy increase task-motivation and the longevity of a behaviour REF.  Targeting the four factors of self-efficacy can increase the likelihood of behaviour change (Ashford, Edmunds & French, 2010). 
The above literature is a small focus amongst a larger area of work surrounding behaviour change. Other theories provide additional standpoints, including the Theory of Interpersonal Behaviour which highlights the importance of habit formation (Triandis, 1977; 1980), the Theory of Trying which focuses on the influencers upon the intention to try (Bagozzi, 1992), and the Self-determination Theory which focuses on innate psychological needs for competence, autonomy and relatedness (Deci & Ryan, 1985; Ryan & Deci, 2000).
By understanding the theoretical background to health behaviour change, interventions that target health behaviours can incorporate behaviour change strategies into the programme. One route through which long-term behaviour change can be achieved is by understanding past behaviour and habits. Forming positive health behaviour habits has been a focus for heath psychologists, with research published on diet (Adriaanse et al., 2010), physical activity (Rhodes & de Bruijn, 2010), alcohol consumption (Norman, 2011) and medication adherence \citep*{Bolman2011}. It is argued that a habit can be formed through repetition of a behaviour within a specific context (Lally, van Jaarsaveld, Potts & Wardle, 2010) and eventually this context will have the potential to trigger the behaviour without awareness, conscious control cognitive effort or deliberation (Bargh, 1994; Lally, van Jaarsveld, Potts, & Wardle, 2010; Wood & Neal, 2009).
When devising interventions to build habitual behaviours, it is important to consider the context in which an intervention is applied. For example, when aiming to ameliorate unhealthy behaviours, disrupting a cue exposure which triggers the behaviour could be a focus (Verplanken, Walker, Davis & Jurasek, 2008), however, there is the possibility of the behaviour returning when the necessary cue or context returns. This serves as an explanation as to why positive results from interventions may be short-lasting. Judah and colleagues (2018) aimed to investigate the formation of habits to create positive health behaviour changes. In line with the above discussion, they found that performing a behaviour in a more stable context was associated with more frequent repetition, which they attributed to context-specific cues being effective reminders. They also reported behaviour pleasure and intrinsic motivation to be two key factors in predicting whether a behaviour becomes a habit. Conversely, they found perceived utility and behaviour benefits to have no impact on habit formation, contradicting the HBM which highlights a key focus on perceived threat of behaviour to one’s health. Previous behaviours and habit formations are important factors when devising intervention strategies for individuals, as these behaviours have ingrained neural pathways that are easily activated (Gerdeman, Partridge, Lupica, & Lovinger, 2003; Smith, 2016; Yin & Knowlton, 2006). For this reason, strategies need to be employed that will both combat the old health behaviour and encourage the formation of neural pathways associated with the new health behaviour.
There are many BCTs that can be employed when providing interventions, promoting self-affirmation, through reflection upon important values, attributes or social relations, is one useful tool to facilitate behaviour change. Self-affirmation has proven to be a useful psychological technique regarding its ability to decrease defensiveness and increase receptivity to interventions across different health behaviours (Falk et al., 2015). By targeting self-affirmation, the neural processes involved in the self-related processing and value in response to an intervention can be altered (within the ventromedial prefrontal cortex), allowing the individual to understand the relevance and value in the intervention instead of viewing it as a threatening health intervention. A meta-analysis of 144 studies reported a positive impact of self-affirmation on message acceptance, intentions to change and subsequent behaviour (Epton et al., 2015). In a review of BCTs aimed to reduce sedentary behaviour, they reported the most effective techniques to be education, environmental restructuring, persuasion and training (Gardner et al., 2015). An example of environmental restructuring would be to provide sit-stand desks (Alkhajah et al., 2012). Studies that used techniques that focused on self-monitoring of behaviour, problem solving and changing the social or physical environment have shown promising results (Gardner et al., 2015). In a systematic review of behaviour change aimed at reducing obesity, mediators for longer-term weight control were autonomous motivation, self-efficacy and use of self-regulation skills (Teixeira et al., 2015). Overall, there are many BCTs that can be used in conjunction with treatment plans to potentially improve adherence to the treatment or suggested behaviour change.
However, whilst many interventions have been effective in eliciting behaviour change, these are often short-term successes (Avenell et al., 2004) and it is not feasible to upscale these interventions to access large population groups as they require a substantial amount of time and money (Forster, Veerman, Bardendregt & Vos, 2011). Nudge Theory provides a basis for an alternative intervention method to subtly alter health behaviours of those in the community. Thaler and Sunstein (2008) argue that there is a “choice architecture” which refers to all the external forces that guide people to make choices, and subtle environmental changes (nudges) can make a desired choice more likely. Bringing together libertarian paternalism (directing decision making whilst maintaining freedom of choice) and nudge theory offers an effective and feasible route to altering health behaviours among large populations. However, this method of influencing health behaviours comes with controversy, with the argument that it undermines the UK government’s aims which promote empowerment, freedom and fairness (Blumenthal-Barby & Burroughs, 2012; Goodwin, 2012). Nudging strategies target the impulsive and automatic system, guiding individuals to certain choices without conscious decision making (Gill & Boylan, 2012; Marteau, Hollands & Fletcher, 2012; Strack & Deutsch, 2015). Despite this, nudging is generally accepted by the public with few concerns (Junghans, Cheung & De Ridder, 2015; Petrescu, Hollands, Couturier & Marteau, 2016).
The evidence supporting the use of Nudge Theory with health behaviours is promising. A meta-analysis on dietary choices, including 42 studies, demonstrated that nudge interventions caused an average increase in healthier consumption decisions by 15.3% (Arno & Thomas, 2016). As a result of promising research, a project named ‘Supreme Nudge’ has been developed to target dietary and physical activity behaviour changes in low socioeconomic areas to reduce the burden of cardiometabolic health problems (Lakerveld et al., 2018). The aim of the project is to implement and evaluate the impact of environmental changes (nudges) on lifestyle behaviours and cardiometabolic health in adults. The targeted intervention will focus on food pricing, environmental nudging and tailored feedback for physical activity. The researchers have developed this project with the awareness that targeting individual-level factors, such as educational strategies, are insufficient in eliciting behaviour change, particularly for those in lower SES groups (Angermayr, Melchart & Linde, 2010). Whereas targeting the environment can prove effective in encouraging health behaviour changes. For example, adjusting the pricing on food products causes subsequent changes in food purchases (Niebylski, Redburn, Duhaney & Campbell, 2015; WHO, 2015), with discounts on fruits and vegetables increasing purchase and consumption on such items (Ball et al., 2015; Geliebter et al., 2013; Ni Mhurchu et al., 2010; Waterlander et al., 2013).
There is a need to incorporate theory-driven, behaviour change techniques (BCT) into care packages and interventions. It is not enough to simply provide the information, given that the four leading non-communicable diseases (cancer, cardiovascular disease, type 2 diabetes and respiratory disease) are mostly preventable through positive health behaviours (Marteau, Hollands & Kelly, 2015). It is argued that when devising techniques for behaviour change, a wider focus is needed that does not solely focus on the individual, but also incorporates social and economic pressures that act upon the individual (Kelly & Barker, 2016). With this is mind, theories of behaviour change and BCTs need to be a focus when targeting health behaviours, instead of simply expecting people to adhere to the treatment programme.
BCTs are particularly important for people with chronic conditions given that healthy behaviour changes after disease onset can lower the risk of recurrence, reduce symptom severity, increase functioning and extend longevity (Aldana et al., 2003; Jolliffe et al., 2001; Speck et al., 2010; Williamson et al., 2000), highlighting the importance of implementing BCTs to facilitate this is vital. In addition, data highlights that despite the diagnosis of a chronic condition, the vast majority of individuals do not adopt long-term positive health behaviours (Newsom et al., 2011). This is surprising given that theories of health behaviour would propose that a diagnosis of a health condition would present as a serious threat and at least minimally lead to an initial stage of change (Prochaska & Prochaska, 2005). However, past behaviours and habits can provide an explanation for why many people do not change to more positive health behaviours (Ajzen, 2002; Verplanken, 2006). However, health behaviour changes differ between conditions, for example, those with heart disease or stroke were more likely to abstain from smoking (Twardella et al., 2006) and increase exercise (Van Gool et al., 2007) compared with individuals with diabetes.
Research highlights that those with chronic conditions are more likely to track a health indicator or symptom and are more likely to benefit from health tracking (Fox & Duggan, 2013). If health tracking was used in conjunction with goal setting and other BCTs, this would be an efficient clinical target to improve the health of those living with chronic conditions. BCTs have proven to be effective among those living with chronic conditions, with a review of eight RCTs aimed at improving exercise adherence among individuals with persistent musculoskeletal pain (PMSK) finding social support, goal setting, instruction of behaviour, demonstration of behaviour and practise/rehearsal to be effective in improving exercise adherence (Meade et al., 2019). Popular wearable technology currently offers a number of BCTs, including goal setting, social support, social comparison, prompts/cues and rewards which can be used to facilitate behaviour change (Lyons, Lewis, Mayrsohn & Rowland, 2014).
Beyond models of hedonic and eudaimonic wellbeing, the theory of synergistic change (Rusk, Vella-Brodrick, and Waters, 2017) describes the pathways through which positive psychological interventions facilitate sustained positive changes in behaviour and wellbeing. Building on prior theory on positive psychological change, including the Hedonic Adaptation Model (Sheldon, Boehm, and Lyubomirsky, 2013), the Positive-Activity Model (Layous and Lyubomirsky, 2013), and the emotion regulation theory (Quoidbach, Mikolajczak, and Gross, 2015), the Synergistic Change Model (SCM) emphasises the interplay of many dynamic elements that facilitate successful and sustained positive change. The model is based in complex dynamics systems theory (Thelen, 2005); recognising that psychological change is a fundamentally complex process and involves interaction between many dynamic elements. It also centres upon on the' Domains of Positive Functioning' (DPF-5) framework (Rusk and Waters, 2014), which emphasises five domains of psycho-social functioning, including: attention and awareness, comprehension and coping, emotions, goals and habits, and relationships and virtues. The influence of environmental and biological/physiological factors are also recognised (Rickard & Vella-Brodrick, 2014), although not characterised. The SCM suggests that whilst each of the psycho-social domains operate independently (Rusk & Waters, 2015), each element may also interact and influence functioning in all other domains; working to either reinforce or undermine change. Crucially, it is this dynamic interdependence between domains which is fundamental to lasting positive change in functioning.
According to the SCM, interventions may result in one of three processes of change: relapse, spill-over, and synergy. ‘Relapse’ may occur when changes in one domain are inadequate, unstable, and made to one of two domains in isolation. Isolated changes are less likely to change the overall dynamic stability of the psycho-social system and instead results in between-domain interactions which undermine and prohibit initiated change within a given domain. Thus, individuals revert to their original state of undesired functioning.  In contrast, spill-over occurs when the effects of positive change in one domain "spill over" to enhance functioning in other domains.  Spill over effects from enhancements in one domain on to another are often reported in qualitative investigations of patient experiences (Landsman-Dijkstra, van Wijck, & Groothoff, 2006) and may be evidenced as mediation effects within the literature (Toussaint & Friedman, 2009). Spill-over effects demonstrate that cognitive functions do not operate independently and offer insight as to how different interventions may result in similar changes being accomplished through multiple pathways of pursuit (Oman, Richards, Hedberg, & Thoresen, 2008). However, these residual effects are often temporary and offer no promise of sustainable change. To successfully induce stable and enduring change, mutually supportive and reinforcing changes must occur within several domains of psycho-social functioning. This insight comprises synergy and results in a new dynamically stable pattern of behaviour. These synergistic interactions have been typically referred to as upward positive spirals within the PP literature (Ryff & Singer, 1998; Crawford & Caltabiano, 2011; Gudan, 2010; Lyubomirsky & Layous, 2013; Wood, Maltby, Gillett, Linley, & Joseph, 2008) and identified as mechanisms through which PPI’s may enhance well-being (Cohn & Fredrickson, 2010). According to the SCM, the formation of these synergistic spirals is a powerful means by which the effect of a PPI can be made enduring; and that PPI’s will be more effective if they are designed to produce synergistic change.
Considering features central to sustained positive change, Rusk et al., (2017) highlight several strategies that can be adopted to facilitate interventions; including targeting pivotal domains, leveraging existing strength’s and values, and targeting mutually reinforcing elements. Briefly, pivotal elements are those domains that can initiate synergistic changes that ‘tip the system into a new stable mode of functioning’ (Rusk et al., 2017). For example, positive emotions are considered pivotal elements that create ‘upward spirals’ that broaden thought-action repertoires, build psychological and social resources and foster positive emotions futher (Fredrickson & Joiner, 2002; Fredrickson, 2004; Garland et al., 2010). Consequently, PPIs that cultivate positive emotions may trigger synergistic changes within a range of psycho-social domains that enhance and sustain benefits to well-being. Further to this, intervention strategies should be tailored to an individual’s traits, values or character strength’s profile; leveraging these to support the desired change through synergistic interactions. Lastly, the SCM (Rusk et al., 2017) suggests that change within a given domain of psycho-social functioning, particularly non-pivotal domains that lack synergistic potency, will be more successful if mutually reinforcing domains are targeted to foster synergistic change and stabilise functioning. Moreover, the SCM suggests that targeting elements of psycho-social functioning from the behaviour change literature (e.g. implementation intentions, self-efficacy) in addition to elements from the PP literature (e.g. gratitude, optimism, or mindfulness) may further induce enduring change. Thus, the SCM holds the potential to integrate the knowledge captured within existing behavioural change theories to bear in creating positive psychological change.
INTEGRATE THIS: Interventions aimed at increasing positive emotion facilitate the building of social connections. For instance, training in loving-kindness meditation \citep{Kok_2010,Kok2015,Kok2013} elicits positive emotion and this is dependent (moderated by) baseline vagal tone. Increases in positive emotion lead to subsequent increases in vagal tone, mediated by an increase in the perception of social connectedness. Higher vagal tone predicts greater social engagement at follow-up, and higher social engagement due to the intervention predicts further increases in vagal tone \citep*{Kok_2010}. These findings highlight a self-sustaining upward spiral between vagal function, emotion and social connections. Acute nasal administration of oxytocin may be another method to trigger cycles to improve health and wellbeing as it increases capacity for social engagement (Kemp et al., 2012). Conversely, decreased vagal activation results in increased sympathetic activity, associated with the fight-flight-or-freeze responses, which causes withdrawal behaviours (e.g. anxiety) not conducive with social environments (Porges, 2011)