It happens to all of us when watching a film. A axe-wielding guy shoves his head through a door, tattoos himself a fresh memory, or perhaps jumps up and down on Oprah's sofa. It happens and you think to yourself: "Yeah, he's crazy". No wonder, then, that cinema, every now and then, delves into its bag of disorders for a fresh take on mental illness. Conditions, after all, make for educational movies, raising awareness of social issues. Or they get cheap laughs, reducing symptoms to a series of cute eccentricities and gross-out gags. Either way, it helps to be able to see the forest for the Gump, to separate the tics from the truth, the science from the scientology. And so here's a brief flight back over Hollywood's cuckoo nest, reminding us what we've learnt from cinema's showcase of syndromes.

1. Autism (as seen in Adam, Rain Man, Mercury Rising, Ben X)

With a brain that develops differently from the rest of society, those with Autism or Asperger's Syndrome can function independently to varying degrees. Obsessions often develop, from astronomy to Abbott and Costello, with repeated behaviour sometimes evident. But while they have trouble communicating and empathising, movies have shown that they have excellent memories, savant skills, and are often mathematical geniuses - little Simon Lynch cracked a military code in Mercury Rising. His reward? Two hours with Bruce Willis; hardly the best choice of carer. In Belgium, they treat Aspies with online RPGs, creating a huge carbon footprint. No, Hollywood has taught us that what Austitics really need are two things: Tom Cruise, and pancakes. Once they get either of these, you can take them to Vegas and make lots of money.

2. Amnesia (as seen in Memento)

I'm always forgetting things - what I went to the supermarket for, the name of my nephew, the drugs I took at breakfast. But amnesia can creep up on you in a variety of forms, most startlingly when you've had some kind of trauma or an accident (cf. every episode of Casualty). Nowt is more stressful, though, than losing your wife, so it's no wonder that Guy Pearce went a bit ink crazy in his quest to find her killer. Understanding is needed here, to help your friend if they got through the same. Just don't be surprised if they turn on you in a self-deluded moral rage. What we've learnt from Hollywood is this: always have a pen and lighter handy. They might be quite good at body art. On the other hand, your friend might be a talentless idiot. In which case you'll just get a chest covered in doodles of him as a cowboy.

3. Obsessive Compulsive Disorder (as seen in The Aviator, Matchstick Men)

Obsessive Compulsive Disorder. OCD. Everyone's got it these days. Howard Hughes, your mum, even David Beckham has it. But aside from lining up your pens in order, OCD can severely inhibit your daily life - imagine not being able to walk out of a room without opening and closing a door 7 times. Or washing your hands so frequently that your skin becomes raw. Or becoming so hygiene obsessed that you don't leave the house, sitting forever in the same room watching video footage of planes and peeing into milk bottles. Even billionaires aren't safe from the clutches of this psychological terror. On the other hand, they're billionaires and can do whatever they want.

4. Dissociative Identity Disorder (as seen in Fight Club, Psycho)

This is the golden goose of cinematic illness - spewing out all manner of cliched eggs, Dissociative Identity Disorder occurs in 95% of all Hollywood thrillers, usually involving a psycho who thinks he's his mum, or something similar. The problem is that the industry is completely incapable of comprehending the differences between Dissociative Identity Disorder, Schizophrenia, and a myriad other conditions. The result is a dumbed down, but incredibly useful, plot device. So next time you see Ed Norton walking down the street, talking to himself, whip out a camera and start filming. You never know, he might do something funny. Like blow up the world's banks. Or he might knife you in the shower dressed as a woman. Either way, it's box office gold.

5. General Nutjobs (as seen in One Flew Over the Cuckoo's Nest, Battlefield Earth)

Fail to fit the categories above? No matter, that's why they invented the Funny Farm (TM). Hell, even Jack Nicholson got trapped there for a while - what makes us mad? Or are we all actually crazy? The answer is, of course, no. I'm perfectly sane, no matter what my doctor tells me. But one wrong move (a self-tattoo here, some cross-dressing there) and you're only a killing spree away from being kept in a mental institution. And once you're there, it's hard to come out. Just ask Tom Cruise. Bless him, that closet must be getting quite smelly now...