Psychopathy is among the most difficult disorders to spot. The psychopath can appear normal, even charming. Underneath, he lacks conscience and empathy, making him manipulative, volatile and often (but by no means always) criminal.
Questions of personality have vexed mankind from the dawn of personhood: can people change? How do others perceive me? What is the difference between normal and pathological behavior? One's personality is so pervasive and all-important that it presents a clinical paradox of sorts: it is hard to assess our own personality, and impossible to overlook that of others.