Mirror neurons are neurons in the brain that fire when another person is observed doing a certain action. The neurons fire in imitation of the action being observed, causing the same muscles to act minutely in the observer as are acting grossly in the person actually performing the action. Research on mirror neurons, since their discovery in 1996,  suggests that they may have a role to play not only in action understanding, but also in emotion sharing empathy . Cognitive neuro-scientist Jean Decety thinks that the ability to recognize and vicariously experience what another individual is undergoing was a key step forward in the evolution of social behavior, and ultimately, morality.  The inability to feel empathy is one of the defining characteristics of psychopathy , and this would appear to lend support to Decety's view.