Rewards, and their maximisation, are crucial determinants for individual survival and evolutionary fitness. Rewards induce learning (positive reinforcement), approach behavior, economic choices and emotions (pleasure, desire). We use behavioural tools derived from animal learning theory and machine learning (reinforcement learning) and economic decision theory (Expected Utility Theory, Revealed Preference Theory). We conceptualise rewards as probability distributions of value whose key parameters are expected (mean) value and forms of risk expressed as variance (spread) and skewness (asymmetry). Behavioural choices reveal distinct attitudes towards these risk forms and comply with predictions from estimated utility functions. The choices follow the gambles’ first, second and third order stochastic dominance and thus are meaningful and rational in the sense of getting the best reward. Behavioural choices among bundles of riskless rewards provide further and formal tests for reward maximisation, including Arrow’s Weak Axiom of Revealed Preferences (WARP).
Using experimental tasks derived from these theories, we investigate the activity of individual reward neurones in specific brain structures. Dopamine neurones signal a reward prediction error (discrepancy between reward obtained and reward predicted). The signal codes formal economic utility, and is influenced by risk; it is a part of a mechanism that may underlie the maximisation of reward (utility) for the individual decision maker. Neurones in the orbitofrontal cortex code a specific risk signal distinct from value. The amygdala, usually considered for its role in fear, shows sophisticated decision signals predicting reward choices, and the striatum (putamen, caudate nucleus, nucleus accumbens) signals reward during social interactions (modified dictator game). These neurophysiological mechanisms demonstrate the physical implementation of theoretical constructs such as reward value (utility), probability, risk, stochastic dominance and utility maximisation. They inform and validate theories of economic decision making whereby individuals maximise reward.