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Three pathways linking social relationships to health include behavioural, psychological and physiological factors. Social isolation and loneliness are independently associated with increased risk of inactivity and smoking (Shanker et al., 2011), substance use among adolescents, along with hazardous drinking and smoking (Stickley et al., 2014), behavioural factors that increase risk of future ill-health and premature mortality. Loneliness is also associated with decreased self-esteem, greater risk for depression and feelings of hopelessness, along with an increase in reported sleep problems (Steptoe et al., 2004), psychological factors that further increase risk of future ill-health and premature mortality. Finally, social isolation and/or loneliness is also associated with a dysregulation of cardiovascular, metabolic, and neuroendocrine processes (Grant, Hamer & Steptoe, 2009), along with higher systolic blood pressure, independent of several factors such as age, gender, cardiovascular risk factors, medications, social support and perceived stress (Hawkley et al., 2010). The NIACT \cite{Kemp_2017a} and GENIAL \cite{Kemp_2017} models integrate behavioural, psychological and physiological pathways in an innovative lifespan framework within which pathways to health and ill-health may be understood, bridging the gap between psychological moments and mortality.
Positive emotion plays a key role in an upward spiral dynamic involving social engagement and the vagus nerve. The vagus nerve is vital in the facilitation of social engagement through eliciting positive emotions (Kok & Fredrickson, 2010; Kok et al., 2013), facilitating positive facial expressions (Porges, 2011), prosocial traits and emotions (Kogan et al., 2014), better emotion recognition (Quintana et al., 2012), positive social interactions (Kok & Fredrickson, 2010; Kok et al., 2013), social-support seeking (Geisler et al., 2013) and positive behaviours, including altruistic behaviour (Bornemann et al., 2016). Individuals with increased vagal tone upon baseline measures increased in levels of social connectedness and positive emotions at a greater rate over a 9-week assessment period compared to those with lower vagal tone. Increases in connectedness and positive emotions predicted final vagal tone measures, independent of vagal tone at baseline. Geisler and colleagues (2013) found cardiac vagal tone, indexed by respiratory sinus arrhythmia (RSA), to be positively correlated with engagement coping and aspects of social wellbeing. Increased RSA also correlated with reduced disengagement strategies for regulating negative emotions and increased use of social emotion-regulation strategies. Individuals who reported zero episodes of anger presented with higher RSA, compared to those who had one or more episodes of anger during the study. This study highlights the importance of vagal function in self-regulatory behaviour and subsequent ability to engage socially. Conversely, low resting-state HRV is associated with prefrontal hypoactivity and amygdala hyperactivity, which facilitates threat perception and increases negativity bias, subsequently impacting on the ability to build connections with others (Kemp, Koenig, & Thayer, 2017). It is therefore important to include vagal function and emotion regulation as factors that impact on an individual’s ability to build a supportive social network.