Personality differences among individuals can greatly influence how they divide resources. For example, some people are more averse to inequalities in resource provision than others. A study by Haruno and Frith on pp. 160–161 of this issue suggests that emotion, rather than conscious deliberation, drives these social preferences about reward distribution.

Haruno and Frith asked subjects to choose between three options, with each option specifying different combinations of rewards for themselves and an anonymous partner. Some subjects' choices were only influenced by how much reward there was for themselves, but others preferred reward pairs in which the difference between the reward for themselves and their partner was minimal. Previous work has shown that these differences between the former group of 'individualists' versus the latter 'prosocial' subjects are stable over many years and predict real-life decisions.

But how do these differences arise? One theory suggests that the automatic, emotion-driven impulse is to be selfish and maximize gains for oneself, but slower, conscious deliberation between alternative courses of action can overcome this automatic impulse. This theory would predict that individualist and prosocial subjects would differ in the extent of the conscious decision-making process, which is usually thought to be localized to frontal areas (such as the prefrontal cortex).

To test this idea, the authors used functional magnetic resonance imaging to track changes in activation as prosocial and individualist subjects rated the desirability of reward pairs, with each pair specifying a reward for the subject and their partner. Prosocial subjects had greater amygdala activation than individualists. Furthermore, amygdala activation in the prosocials during each reward pair presentation correlated with the reward difference between the subject and the partner. The authors therefore conclude that social preferences about how rewards should be divided are not driven by top-down, conscious deliberation, but instead reflect automatic emotional processing.