Date of Award

Spring 2021

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Chang, Steve


Despite the fact that serotonergic drugs are called upon to treat a myriad of psychopathologies, the effect of serotonin on core behaviors and cognitive abilities are poorly understood. This is especially true for cognitive functions which underlie socially competent behavior. This dissertation aims to increase understanding of the role of serotonin in other monitoring and social competency throughout development. Species of primates, including humans, that live in complex social environments must allocate extensive cognitive resources to monitor conspecifics. However, they must balance the benefits of gathering social information with the need to monitor their non-social environments. Social monitoring strategies vary across species, populations of the same species, and even within populations. This variation seems to be dependent upon the amount of social monitoring that is required for an individual to avoid conflict and maintain its dominance rank. The serotonergic system has a history connecting it to socially competent behavior, like other monitoring, although its causal role is not understood. Therefore, better understanding how increasing concentrations of serotonin impact other monitoring behaviors will clarify serotonin’s role in psychopathology and may help clinicians predict how serotonergic interventions will influence pathologies. Furthermore, better understanding how the relationship between early life stress and serotonin impacts social competency will improve our understanding of psychiatric disorders and help develop novel interventions.In Chapter 2 of the present dissertation, the role of serotonin in the allocation of attention to social images, a core component of social monitoring, was studied by assaying rhesus macaques’ unconstrained looking to social and non-social stimuli using a free viewing paradigm (Dal Monte et al., 2014). We used a quantitative, repeated, within-subject, design to test how increasing central concentrations of serotonin would impact social looking behavior. Importantly, we found that increasing central concentrations of serotonin with its direct precursor, 5-Hydroxytrypotophan (5-HTP), modulated looking duration relative to individual differences in looking. 5-HTP decreased looking duration in animals with high baseline attention, but increased looking duration in low baseline attention animals. 5-HTP’s effects were also reflected in how engaged individuals were in the task and how they allocated attention to salient facial features—the eyes and mouth—of stimulus animals. Individual differences seem to be based in serotonergic function. Compared to low baseline animals, high baseline looking animals exhibited higher baseline concentrations of 5-HTP and serotonin and lower 5-HIAA to serotonin ratios indicating central serotonergic functioning may underlie and predict variation in serotonin’s effects on cognitive operation. The individual differences in 5-HTPs effects on looking increased our interest in serotonin’s role in balancing the costs and benefits of monitoring others (Weinberg‐Wolf and Chang, 2019). In Chapter 3, we tested the effect of 5-HTP on macaque’s abilities to flexibly switch between two actions: orienting to faces, or, at other times, inhibiting orientation towards faces. Critically, we found that 5-HTP also only impaired the ability to inhibit orientation to faces, but did not impact inhibition performance on trials with control images. It also seems that 5-HTP made animals less flexible, causing them to persevere in actions more. Furthermore, 5-HTP’s effects on performance are likely due to changes in arousal and motivation state as 5-HTP’s impairments were linked to increased reaction time, animals taking longer to initiate trials, and a constricted pupil. Serotonin is also implicated in the development of psychiatric disorders, especially Autism Spectrum Disorders. In Chapter 4, we examined the relationship between infant serotonergic function, assayed via CSF concentrations of 5-HIAA, and the acquisition of social status. We found that neonatal (11-32 days) 5-HIAA concentrations positively predict eventual, acquired, social rank. Furthermore, this relationship was strongest amongst macaques who had been reared by their mothers compared to those reared without mothers. In addition, mother reared infants exhibited higher concentrations of CSF 5-HIAA and attained higher social rank than their peers. These finds support the relationship between serotonin and early social experience in socially competent development. By considering the findings presented in Chapters 2, 3 and 4, we discuss, in Chapter 5, the role of serotonin in competent social monitoring and its development. We also discuss a plausible evolutionary explanation for variability in other monitoring behaviors and for variable effects of serotonin on cognition and behavior. Finally, we consider future directions researchers should explore as the field progresses.