In the course of our everyday lives, we are constantly faced with situations in which we must choose. Do we invest in the bank or the stock-market? Is a new wage deal so unfair that we should resort to a strike? These situations are elegantly described mathematically by Rational Choice Theory (RCT), which dominates the quantitative social sciences such as economics. However, unfortunately RCT often fails to predict how humans actually behave. Here I investigate choice using paradigms derived from the RCT framework, but aim to better predict actual choices by using a biological level of explanation. First, I examine simple choices that involve no social interaction, asking how choices are influenced by risk in potential outcomes, and by whether outcomes reflect potential gains or losses. The data reveal independent impacts of risk and loss on choice, findings not predicted by extant economic theories. Instead, I then harness functional Magnetic Resonance Imaging (fMRI) to suggest a biological mechanism by which risk and loss bias approach behaviour, and test this hypotheses in further behavioural experiments. Secondly, I examine social choices. Specifically, I examine biological systems that enable social behaviour to respond flexibly to environmental contingencies. I investigate the neural basis of the human fairness motivation using fMRI, and show how it flexibly adapts to external social context. Next, I show how this fairness motivation adapts to changes in an individual’s internal physiological state. Finally, I show how cooperation is modulated by the androgen hormone testosterone. Overall, in light of these non-social and social findings, I propose that a biologically-based account of choice can explain choices that are not predicted by existing theory
