What characteristic allows a psychopath to pass a polygraph test cleanly?

The characteristic that allows a psychopath to pass a polygraph test cleanly is primarily tied to an abnormal lack of fear. Psychopaths often exhibit a very low emotional response to situations that would typically elicit anxiety or fear in most individuals. This innate inability to feel fear makes them less likely to exhibit the physiological responses—such as increased heart rate, sweating, or changes in respiration—that polygraphs measure when a person is lying or feeling stressed about their responses.

While an unstable personality can contribute to erratic behaviors, it does not specifically correlate to the physiological response patterns that polygraphs detect. The key factor in passing a polygraph test lies in the psychopath's emotional detachment and inability to experience fear, which significantly diminishes their physiological reactions even when they are being deceptive.

Therefore, the combination of an unstable personality and an abnormal lack of fear does not fully explain the success of a psychopath on a polygraph test, as the latter is the predominant characteristic that directly affects the physiological responses being monitored.