Is the insanity verdict something you can use to defend yourself at trial? Some people would say no, while it has been shown to work in a few distinct cases.
In one of those cases, an ex-nurse from Houston had a history of postpartum depression that was acute. Approximately six months after the birth of her last child, she drowned them all in a bathtub. She was convicted in 2002, but she was later acquitted in 2006 when she was found not guilty by reason of insanity. How was she defended? Her attorneys proved, with expert testimony, that she had been delusional, thinking Satan was persecuting her. To protect her children, she killed them.
Insanity pleas can only be used in 46 states in the United States. Idaho, Kansas, Montana, and Utah do not allow the plea to be used. Should they? That's up to the states, but the definition of a crime and conviction requires a person to be of sound mind when the crime is performed. This means that if someone is delusional or hallucinating, an insanity plea may be used since the person did not know what he or she was doing when performing the criminal act.
If this defense succeeds, an acquittal is likely. However, the person who is acquitted may need to enter a psychiatric facility for some time to manage his or her condition. Interestingly, the insanity plea is starting to come under fire. Now, some states have created a verdict called, "guilty, but mentally ill," which allows them to show that the party did commit the crime but was not well at the time.
Source: Scientific American, "The Insanity Verdict on Trial," accessed Aug. 20, 2015