Since the description of SAD the prevalence of this disorder has been of particular interest. The more so, because early studies, indicated the correctness of the on theoretical grounds - expected interaction between latitude and prevalence rate of SAD. More eleborate prevalence studies, mainly in the USA indeed showed a positive correlation between prevalence rate and latitude. In Europe, only few studies have appeared. Although the results of most of these studies are preliminary and research-methods and criteria vary across studies, tentative conclusions are that the prevalence rates in Europe are considerably lower than in the USA, but that there is a similar trend in the direction of an increase of prevalence at higher latitudes.8"10 In the present study, the prevalence of SAD in the Netherlands is investigated.
Cognitive and behavioural processes may constitute a risk for onset and persistence of depression. People who become depressed frequently show enduring negative cognitions which predispose them to depression. In addition, interpersonal processes are supposed to contribute to the etiology and maintenance of depression. Depression-prone persons are presumed to display deficient or problematic social behaviours that elicit negative reactions in others, finally resulting in withdrawal by family and friends. About 60% of human communication is non-verbal. An ethological approach may therefore contribute to reveal behavioural and cognitive vulnerability factors for depression onset or persistence. Various studies support this presumption: High levels of patients' observed behaviour indicating involvement in the interaction between depressives and clinicians at admission are related with persistence of depression.
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