Well, I would say that they are diagnosed using the DSM-IV-R (the diagnostic and statistic manual 4th edition revised) - that's the most current - it's published by, I think, the american psychiatric association. AND that the diagnoses are relatively artificial because they are based on a checklist. So, if you have 8 symptoms but the disorder requires 9...too bad - you don't have the disorder. Also, some people may have a disorder (according to the DSM) but it may not affect their daily life ... so do they really have a problem? That should be a major consideration - does the disorder disrupt their life.
Anorexia Nervosa is the deadliest psychological disorder - I think it's 19 or 20% lethal.
I would also talk about the stigma associated with psychological disorders (our society's idea that it's OK to be sick and go to the hospital, but if you have depression you must be a bad person.) To that end you may want to mention that in some cultures, very important people in the community (often shaman or spiritual leaders) would have diagnosable disorders - but they held a high position in society....does that mean they had a disorder or not?