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Professor Cary Cooper, an expert in organisational psychology and health at Lancaster University Management School, said the kind of people who constantly compare themselves with others tend to be unsure of themselves.
"We need to know what comes first - is it those who are glass half empty types who do the comparisons as a consequence of that, or is it the comparison that makes them unhappy?"
He said comparisons with school and university friends were probably most damaging.

Could you please explain to me what "glass half empty types" means here?

Source : http://news.bbc.co.uk/2/hi/health/10182993.stm
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People who are optimists (who see the best in things) see a glass as being "half full."

People who are pessimists (who see the worst in things) see the same glass as being "half empty."

A "glass half empty" type is someone who tends to take a negative/sad/pessimistic view.
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Hi,

When engineers see a glass like that, they say 'This glass is the wrong size'.

Clive