Epigenetics and endocrinology: new frontiers in motivational science

This thesis explored the role of motivational processes in improved human functioning. There is a large body of evidence supporting the idea that higher quality motivation can have important implications for health and well-being via improved behavioural engagement. However, what is less well understood is whether there is a direct effect of motivation on physiological health beyond that which is explained by engagement with the activity itself. The aim of this thesis was to explore associations between motivational processes and biological markers implicated with improved physical health and human functioning.

The programme of research began by systematically reviewing multidimensional theories of motivation and physiological responses associated with activation of the endocrine system. Across several conceptually similar theories of motivation, it was evidenced that higher quality motivation was associated with an attenuated cortisol response in a variety of environments eliciting social-evaluative threat. Furthermore, the needs for power and affiliation were associated with lower and higher levels of salivary secretory immunoglobulin A, respectively. Based on the limitations identified in the systematic review, two subsequent chapters further explored the role of high quality motivation in reducing the cortisol response. Using an experimental design grounded in self-determination theory, Chapter Three explored the effect that manipulating autonomous and controlled motivation had on participants’ cortisol responses, while concurrently measuring self-control exertion. A repeated measures ANCOVA revealed a significant quadratic interaction (F (1, 32) = 5.40, p =.027, ηp2 = .14). Participants in the controlled regulation condition experienced an increased, with autonomy supportive experiencing a decreased, cortisol response. There was also evidence of motivational effects on self-control performance, although the results contradicted the hypothesis. A mixed ANCOVA revealed participants in the controlled regulation condition recorded greater wall sit performance in the first and second wall sits compared with the autonomy-supportive condition (F (1,36) = 4.40, p = .043, ηp2 = .11).

This concept was replicated and extended in Chapter Four. Although the cortisol pattern was not replicated, nonetheless a repeated measures ANOVA revealed a significant linear within subject contrast (F (2, 57) = 4.45, p = .016, ηp2 = .14), with a significant linear decline in cortisol observed in the control condition. Furthermore, there was support for the idea of high-quality motivation improving self-control performance. A one-way ANOVA revealed a self-control performance difference between experimental conditions (F (67,2) = 2.88, p = .063, ηp2 = .08), that was driven by autonomy-supportive participants outperforming those exposed to controlled regulation (p = .055). As predicted, this performance difference did not persist to the second self-control task, the plank (F (64,4) = 0.41, p = .664, ηp2 = .03).

In the final empirical chapter, the focus shifted from proximal to stable long-term markers of health by exploring a novel area for self-determination theory – epigenetics - specifically, the DNA methylation of the TNF gene associated with expression of the proinflammatory cytokine TNFα. The results further supported the idea that high quality motivation is most important for health-related behaviour and outcomes. Health-related autonomous motivation was weakly but positively associated with TNF methylation (β =.18, p =.08); however, health-related introjected regulation was not associated with TNF methylation (β =.13, p =.23). In a subsequent step, path analysis was used to explore whether this relationship persisted when accounting for healthy behaviour, however direct effects of autonomous motivation on TNF methylation did not persist when these indirect effects were included (β =.09, p =.43).

Overall, this thesis found support for the value of high-quality motivational processes at the epigenetic and endocrine level. Autonomous motivation was indirectly associated with DNA methylation of TNF, via engagement in healthy behaviour. Furthermore, there were several examples of evidence suggesting that high-quality motivation was associated with attenuated cortisol profiles, and conversely, lower quality motivation was associated with an increased cortisol response. There was also evidence that autonomous motivation was implicated in improved self-control performance, but this was not consistently observed. By extending research into epigenetics and endocrinology, this thesis supports the idea that high-quality motivation has important implications for improved health and well-being .