Answer: Smell

In 2012, a group of researchers found an interesting link between people with psychopathic personality traits and deficiency of a physical sense. After screening a group of non-criminal subjects on both the sharpness of their sense of smell as well as their predisposition towards the thought patterns and behaviors that are indicative of psychopathy, they found that the subjects who scored high on the psychopathy assessment also typically scored very poorly on the olfactory test.

The subjects reported that they could smell something, but were unable to clearly identify what that smell was or how it differentiated from the smells that came before or after it. The theory is that underdevelopment of the frontal cortex (where both olfactory processing centers and key brain regions related to empathy and personality are located) causes both the personality disorder and the imprecise reaction to scents.