A sociopath was someone exhibiting sociopathy, a casual disregard for the concerns of others. Lon Suder was considered a sociopath. (VOY: "Basics, Part I") Iko, a Nygean, was prone to sociopathic behavior. This was the result of a birth defect, causing a part of his brain (analogous to the Human pineal gland) to be detached from the rest. (VOY: "Repentance")