The findings of recent research suggest that the stress hormone cortisol reduces altruistic behaviour and alters activity in brain regions linked to social decision making. However, this happens only in people who are better at imagining others' mental states. The research was published in the journal JNeurosci. In a study from Universitat Hamburg, participants decided how much money to donate to a selection of charities before and after completing a stressful public-speaking task while researchers monitored their brain activity with fMRI.

To simulate the personal cost of making an altruistic decision, the participants received a portion of the money they did not donate. Before the stressful task, people with higher mentalising ability, or the ability to imagine others' mental states, donated more money than people with low mentalising ability. In people with high mentalising ability, increased levels of the stress hormone cortisol decreased donations; cortisol had no effect on people with low mentalising ability.