If not acquired sooner, it is usually developed by an individual within days if not hours of becoming a member of the Sergeant's Mess. This has brought about the misconception that all Bent-heads are SNCO's, but this condition can be found in other ranks, often these are the most severe cases.
Symptoms include: Random uncontrolled shouting, falling in love with your own voice, delusions of being more important/scary/funny than you actually are, and in the worst cases making your wife and kids stand to attention before they speak to you.
Bent-head has in the past been mistakenly defined as having a physically bent head caused by constantly trying to admire ones chevrons whilst on parade. This it not the case as it simply refers to a bent personality resulting in the wearing of such chevrons.
Famous Bent-head example: CSM Williams from "It ain't half hot mum".