Most of the tests didn’t find any differences between the brains of the altruistic donors and the people who had not been donors. Except, Marsh says, for a significant difference in a part of the brain called the amygdala, an almond-shaped cluster of nerves that is important in processing emotion.
The amygdala was significantly larger in the altruists compared to those who had never donated an organ. Additionally, the amygdala in the altruists was extremely sensitive to the pictures of people displaying fear or distress.
These findings are the polar opposite to research Marsh conducted on a group of psychopaths. Using the same tests as with the altruists, Marsh found that psychopaths have significantly smaller, less active amygdalas. More evidence that the amygdala may be the brain’s emotional compass, super-sensitive in altruists and blunted in psychopaths, who seem unresponsive to someone else’s distress or fear.