Childhood trauma, stress and suicide: Towards a better understanding of the precise causal mechanisms.

Suicide is a major cause of death worldwide, responsible for 1.5% of all mortality (WHO, 2014). The causes of suicidal behaviour are not fully understood; identification of risk and protective factors would help predict the risk of suicide and identify targets for intervention to improve suicide prevention (Mann et al., 1999)(O'Connor, 2011). Recent evidence has suggested that dysregulated hypothalamic-pituitary-adrenal (HPA) axis activity, as measured by cortisol levels, has been identified as one potential risk factor for suicide (McGirr et al., 2010). In support of this notion, O'Connor et al., (2017) found that participants who had made a previous suicide attempt exhibited significantly lower cortisol levels in response to a laboratory stressor compared with controls. Evidence is also converging to suggest that resting cortisol levels are also associated with suicide behaviour (O'Connor et al., 2016). Melhem et al., (2017) reported that those with a suicide attempt history had lower hair cortisol concentration compared to controls. Suggesting that hair cortisol, a measure of exposure to chronic stress, could be a biomarker for suicidal behaviour and it could be related to cortisol reactivity to stress. However, this is an area that requires further investigation. More recently, O'Connor et al. (2018) have found that higher levels of childhood trauma are associated with lower resting cortisol and blunted cortisol reactivity to stress in adulthood in individuals who have attempted to take their own life. The importance of these findings is that experience of childhood trauma may predispose individuals to vulnerability to suicide by leading to blunted HPA axis activity in response to stress and during rest later in life. A theoretical model developed by Lovallo et al., (2013), suggested that exposure to early adversity was
associated with blunted cortisol reactivity to stress and altered cognitive function in the form of greater impulsive behaviours, finding evidence linking exposure to adversity to the development of a more impulsive behavioural styles and less stable mood regulation. O'Connor et al. (2018) proposes that the development of a more impulsive behaviour style and precursors such as childhood trauma and impaired executive functioning are likely to increase the risk of suicidal behaviour. As such, the findings of blunted HPA axis activity in response to stress and during rest in individual's with high levels of childhood trauma provide evidence for one of the pathways suggested by Lovallo et al., (2013) and extend his model to suicidal behaviour. However, relatively little is known about the effects of childhood trauma on stress and cortisol processes in the context of suicide. Therefore, it is important to elucidate the precise causal mechanisms linking childhood trauma, low levels of cortisol and suicide risk, alongside executive functioning, impulsivity and self-esteem in vulnerable individuals. The meta-analytic work of Franklin et al. (2017) suggested there is a limited ability to predict suicidal thoughts and behaviours as the risk-factors are weak and inaccurate and the studies commonly investigate single risk-factors which further limits predictive utility. Most studies included in the meta-analysis had long follow-up intervals (5-10 years), yet longer intervals did not improve predictive ability. The authors thus suggest using micro-longitudinal methods to improve predictive ability, which will be employed here. This PhD project aims to understand the mechanisms linking childhood trauma to chronic stress to suicide. The project is threefold; first, to conduct a large scale online survey to investigate the association between childhood trauma and history of suicidal ideation or attempt, and the relation with self-reported stress, executive control and impulsivity. A large-scale survey has never included executive function measures, in relation to childhood trauma, suicide and impulsivity, thus will provi