Machiavellian tendencies increase following damage to the left dorsolateral prefrontal cortex.

Machiavellianism - a personality trait that is characterized by a tendency to distrust, deceive and exploit others - has been the focus of growing attention in psychological research. Neuroimaging studies of Machiavellianism highlight the influence of the dorsolateral prefrontal cortex (dlPFC) on Machiavellianism tendencies. However, knowledge regarding the causal role of the left and right dlPFC on Machiavellianism is still obscure. Here, we measured general Machiavellian tendencies, as well as two subscales (i.e., Machiavellian Views and Machiavellian Tactics) in a large sample of brain-injured patients (N = 129) and non-brain-injured control participants (N = 37) to determine whether Machiavellianism tendencies can be altered by brain damage. We analyzed Machiavellianism tendencies as a function of lesion location, with patients separated into four groups based on dlPFC damage: left dlPFC damage, right dlPFC damage, non-dlPFC damage, and healthy controls. We found that left dlPFC damage increased Machiavellianism in general, and Machiavellian perspective (views) in particular, but did not modulate behavior (tactics). Critically, left dlPFC damage predicted higher levels of Machiavellianism after controlling for general and emotional intelligence, linguistic abilities, empathy and psychopathology. These findings establish a causal role of the left dlPFC in modulating Machiavellian views, and indicate that one can hold Machiavellian views without necessarily endorsing Machiavellian tactics.