People who spend a lot of time browsing the net are more likely to show depressive symptoms, according to the first large-scale study of its kind in the West by University of Leeds psychologists.

Researchers found striking evidence that some users have developed a compulsive internet habit, whereby they replace real-life social interaction with online chat rooms and social networking sites. The results suggest that this type of addictive surfing can have a serious impact on mental health.

Lead author Dr Catriona Morrison, from the University of Leeds, said: "The internet now plays a huge part in modern life, but its benefits are accompanied by a darker side.

"While many of us use the internet to pay bills, shop and send emails, there is a small subset of the population who find it hard to control how much time they spend online, to the point where it interferes with their daily activities."

These 'internet addicts' spent proportionately more time browsing sexually gratifying websites, online gaming sites and online communities. They also had a higher incidence of moderate to severe depression than non-addicted users.

"Our research indicates that excessive internet use is associated with depression, but what we don't know is which comes first - are depressed people drawn to the internet or does the internet cause depression?

"What is clear, is that for a small subset of people, excessive use of the internet could be a warning signal for depressive tendencies."

Incidents such as the spate of suicides among teenagers in the Welsh town of Bridgend in 2008 led many to question the extent to which social networking sites can contribute to depressive thoughts in vulnerable teenagers. In the Leeds study, young people were more likely to be internet addicted than middle-aged users, with the average age of the addicted group standing at 21 years.

"This study reinforces the public speculation that over-engaging in websites that serve to replace normal social function might be linked to psychological disorders like depression and addiction," added Dr Morrison. "We now need to consider the wider societal implications of this relationship and establish clearly the effects of excessive internet use on mental health."

This was the first large-scale study of Western young people to consider the relationship between internet addiction and depression. The internet use and depression levels of 1,319 people aged 16-51 were evaluated for the study, and of these, 1.2% were classed as being internet addicted. While small, this figure is larger than the incidence of gambling in the UK, which stands at 0.6%. The research will be published in the journal Psychopathology on 10th February.

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More information: 'The relationship between excessive internet use and depression: a questionnaire-based study of 1,319 young people and adults,' Psychopathology.