Uncertainty regarding which psychological mechanisms are fundamental in mediating SSRI treatment outcomes and wide-ranging variability in their efficacy has raised more questions than it has solved. Since subjective mood states are an abstract scientific construct, only available through self-report in humans, and likely involving input from multiple top-down and bottom-up signals, it has been difficult to model at what level SSRIs interact with this process. Converging translational evidence indicates a role for serotonin in modulating context-dependent parameters of action selection, affect, and social cognition; and concurrently supporting learning mechanisms, which promote adaptability and behavioural flexibility. We examine the theoretical basis, ecological validity, and interaction of these constructs and how they may or may not exert a clinical benefit. Specifically, we bridge crucial gaps between disparate lines of research, particularly findings from animal models and human clinical trials, which often seem to present irreconcilable differences. In determining how SSRIs exert their effects, our approach examines the endogenous functions of 5-HT neurons, how 5-HT manipulations affect behaviour in different contexts, and how their therapeutic effects may be exerted in humans – which may illuminate issues of translational models, hierarchical mechanisms, idiographic variables, and social cognition.

What psychological mechanisms do primates use to engage in self-control, and what is the ultimate function of these skills? I will argue that a suite of decision-making capacities, including choices about the timing of benefits, evolved in the context of foraging behaviors and vary with ecological complexity across species. Then, I will examine how these foraging capacities can be generalized to solve novel problems posing temporal costs that are important for humans, such as cooking food, and can therefore underpin evolutionary transitions in behavior. Finally, I will present work testing the hypothesis that a limited future time horizon constrains the expression of other complex abilities in nonhumans, explaining the emergence of human-unique forms of social cognition and behavior.