Mental illness is easier to prove. You can have a mental illness but not be legally insane. To prove the defense of insanity, you must have a mental disease or defect which makes you unable to conform your conduct to the law essentially. Legal insanity is a difficult standard to prove and you normally would need expert psychological and/or psychiatric testimony to establish legal insanity. Family history is certainly taken into consideration as well as substance usage and abuse. But they are just some of the many factors that go into an insanity evaluation.