29 Neuroendocrinological aspects of social anxiety and aggression related disorders hippocampus, and PAG, among other limbic areas (Wood, 1996), and hence influences the flight–fight response. Naturally, testosterone interacts with other neurotransmitters and peptides, such as serotonin (probably via estradiol), vasopressin, oxytocin, and dopamine. With regard to the latter, testosterone enhances dopamine transmission in the mesolimbic system, which in turn can lead to increased reward sensitivity and augmented motivational behavior by promoting dopaminergic projections form the amygdala to the striatum (de Souza Silva et al., 2009; Hermans et al., 2010; Welker, Gruber, & Mehta, 2015). Baseline hormone levels are in general predictive of psychological traits and behavior (Welker et al., 2015), whereas social events are typically associated with a temporary surge or decline in hormone levels (Casto & Edwards, 2016; Maner et al., 2008; Sapolsky, 1991). The social challenge hypothesis states that testosterone levels rise in preparation to a challenging encounter in which social status might be threatened, thereby initiating approach motivation and simultaneously reducing fear (Archer, 2006; Mazur & Booth, 1998; Wingfield et al., 1990). Several studies featuring single-dose testosterone administration, which leads to a transient increase in testosterone levels, to healthy female participants confirmed the causal relationship between testosterone and its effects on the social motivational system. The findings show that testosterone administration reduces fear and sensitivity to threat and punishment, enhances reward sensitivity, and promotes social approach motivation aimed at achieving social status (i.e., social reward; see for a review (Bos, Panksepp, et al., 2012; Enter et al., 2014). These actions have been suggested to be brought about by anxiolytic effects (GABA, androgen receptors; (McHenry et al., 2014) and upregulation of the dopaminergic system (de Souza Silva et al., 2009), in addition to biasing the amygdala towards threat approach (Radke et al., 2015) and reducing prefrontal control over the amygdala (Schutter & van Honk, 2004; van Wingen, Mattern, Verkes, Buitelaar, & Fernández, 2010; Volman, Toni, et al., 2011). Although associated with aggression (Montoya et al., 2012), the effects of testosterone on social motivational behavior depend on social context and individual differences and thus do not entail aggressive behavior per se, but could also lead to prosocial behavior when this is more appropriate to ensure an increase in social status (Boksem et al., 2013; Carré et al., 2017; Eisenegger, Haushofer, & Fehr, 2011; Mehta & Josephs, 2010; Stanton & Schultheiss, 2009; van Honk, Terburg, & Bos, 2011; sample sizes in these studies ranged from n = 54 to n = 121). Cortisol For decades cortisol has been a popular biomarker to index acute and chronic social and psychological stress (Hellhammer, Wüst, & Kudielka, 2009). Individual differences in the 2
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