When does sadness over the loss of a loved one tip over into clinical depression? How do you tell the difference between schizophrenia and some other form of psychosis?
Defining what is, and what isn't, mental illness is actually quite a hard thing to do. Inevitably it's a subjective process based on a careful and formalised assessment by health care professionals. To help, mental health practitioners have traditionally relied on a classification system developed by the American Psychiatric Association and known as the Diagnostic and Statistical Manual of Mental Health, or DSM.
This diagnostic "bible" is currently being re-drafted - the first substantial re-write since 1994. But researchers here are growing increasingly concerned at a series of changes they say are being made to widen existing categories, and the addition of a staggering range of new conditions.
Writing in the Journal of Mental Health, professor Til Wykes claims the new definitions are so broad that, in future, almost no one will qualify as normal. She's worried about the implications of branding so many people, and particularly children, as mentally ill.
"It shrinks the pool of normality to a puddle" Professor Wykes says, "there are going to be fewer people who won't end up with a diagnosis of mental illness".
One particularly Orwellian new addition is Psychosis Risk Syndrome, which singles out people who are thought to be at risk of developing a serious psychotic illness such as schizophrenia. Other new conditions under consideration include; mixed anxiety depression, binge eating and temper dysregulation disorder with dysphoria.
Dr Nick Callard, from the Biomedical Research Centre for Mental Health at the Maudsley NHS Foundation Trust, said labelling individuals as "at risk" of developing a disorder like schizophrenia was likely to cause a great deal of distress, and could expose them to social stigma and discrimination.
The researchers accuse the US authors of moving ahead of scientific progress in mental health, which has made relatively little headway over the last 16 years. The new edition of the DSM is due to appear in May 2013.