An article in the New York Times discusses psychiatric diagnosis: http://www.nytimes.com/2004/11/30/health/psychology/30eat.html.
Criteria for psychiatric disorders are determined by committee. Political considerations (for example, the removal of homosexuality as a psychiatric diagnosis) often play a role in determining the contents of DSM, Psychiatry's diagnostic manual. Currently we have DSM-IV. Soon all will be forced to submit to DSM-V.