The Diagnostic and Statistical Manual of Mental Disorders is on the desk of every psychiatrist. The latest iteration is from 2013, and it is the DSM-V (5).
Are those members of the Islamic State who are cutting the heads off journalists and aid-workers mentally-ill? Are they delusional- which according to the DSM-V a mental disorder? Many of us would think so.
The DSM-V defines Delusion this way: "The belief is not one ordinarily accepted by other members of the person's culture or subculture (e.g., it is not an article of religious faith)." My interpretation of this definition allows 2 passes for the Islamic State.
Here's why:
1. If an individual chops your head off because he believes he's receiving instructions from a demon, a dog, or the creator of the universe-- he's delusional, and therefore mentally-ill. If, however, a group of individuals (the sub-set of a culture-- a sub-culture) believes this, then it's NOT a delusion-- it's just their culture. Numbers matter; and for reasons not stated in the DSM-V, the behaviour of a culture or sub-culture cannot be judged with the same scrutiny as that of an individual.
2. If an individual chops your head off because he (and his gang) believes (or just say they believe) instructions are coming from the creator of the universe, then according to the DSM-V this is "an article of religious faith" and it gets a pass!
Interesting.
(If not, we'll just "throw the book at 'em" ... the DSM-V" 
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