Longitudinal studies are studies in which large groups of people are tracked over a long period of time. One such study in New Zealand followed 1,055 children over 25 years and looked into the links between heavy drinking and depression. By the time the children had got to 17-18 19.4% of them were either abusing or dependent on alcohol and 18.2% were diagnosed with depression. People who fulfilled the criteria for alcohol abuse or dependency were nearly twice as likely to have depression as well, a relationship that held true even when other risk factors such as the use of cannabis and other illegal drugs, falling in with a bad lot (or 'affiliation with deviant peers,' in psychology-speak), unemployment and having a criminal partner were taken into account. The researchers thought the link could either be because alcohol triggers off a genetic weakness for depression or because alcohol itself is a depressant.
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