Now I've just come across this older article from ABC News (2003), that talks about a "sliding scale" of celebrity obsession:

The responses cast doubt on the conventional view that celebrity worship is categorised into pathological and non-pathological cases - in other words, harmless fun and obsession.

Instead, the replies pointed to a "sliding scale" in which the celebrity devotee becomes progressively more fascinated with his or her idol.

In addition, celebrity fans are significantly likelier to suffer from anxiety, depression and social dysfunction than non-worshippers.

About 20 per cent of respondents closely followed celebrities in the media for "entertainment-social" reasons, according to the report.

They tend to be extroverts: social, lively, active and adventurous.

The next stage, affecting about 10 per cent, is when the devotee develops an "intense-personal" attitude towards an idol, such as the belief that he or she had a special bond with the star.

At this point, celebrity worship is becoming an addiction.

Those in this category are often neurotic, tense, emotional and moody.

At its most intense, celebrity worship is "borderline-pathological," a condition found in 1 per cent of interviewees.

These include celebrity stalkers and people who are willing to hurt themselves or others in the name of their idol.

They correlate with symptoms of psychosis, such as impulsive, antisocial and egocentric behaviour.

"Just worshipping a celebrity does not make you dysfunctional," New Scientist quoted Mr Houran as saying.

"But it does put you at risk of being so.

"There is this progression of behaviours, and if you start, we don't know what's going to stop you."

People tend to get interested in celebrities at times when they are looking for direction in life, as in their teenage years.