Original ArticlesThe prevalence of seasonal affective disorder in the netherlands: a prospective and retrospective study of seasonal mood variation in the general population
Introduction
Data on the prevalence of seasonal affective disorder (SAD) have accumulated over the past 5 years. The use of the same instrument in nearly all studies, the Seasonal Pattern Assessment Questionnaire (SPAQ; Rosenthal et al 1987) and the application of (more or less) the same criteria [formulated by Kasper et al (1989)] facilitates the comparison of studies (Rosen et al 1990). Results from studies on samples from the general population in the USA Kasper et al 1989, Rosen et al 1990, Terman 1988, Booker and Hellekson 1992 show prevalence rates of 2.6–9.2%. This large range can partly be explained by latitude differences in samples studied (27° N–64° N) Potkin et al 1986, Lingjaerde et al 1986.
Differences in prevalence rates in European studies are smaller than in the USA. In Switzerland (47° N; Wirz-Justice et al 1992), 2.2% of the population was estimated to suffer from SAD, while in Iceland (62° N–67° N; Magnusson and Stefansson 1993), the SAD criteria of the SPAQ were met by 3.8% of the responders. One study was performed at the same time in Sweden and Finland. In both countries the SPAQ was sent to a random sample of the population (Hagfors et al 1995). The prevalence rate in Sweden (55° N–69° N) was 3.9%, whereas this figure was almost twice as high in Finland (60° N–70° N). This high SAD rate in Finland (7.1%) not only deviates from the figures in Sweden and Iceland, but also from an earlier Finnish study (Hagfors et al 1992), which revealed a prevalence rate of 3.4%. In Italy (39° N–46° N; Muscettola et al 1995). The prevalence rate for winter SAD was 4.4% and for summer SAD 2.1%. The reliability of these figures is, however, doubtful, since the response rate was extremely low (13.6%). In most studies, more women than men met SAD criteria, and younger women more often than older women.
This study estimated the prevalence of SAD in The Netherlands. In addition, the role of demographic and social variables in SAD is studied. A special aspect of the study is that approximately 400 questionnaires were sent each month over a period of 13 months. This way, mood was both retrospectively as well as directly assessed at different points in time. The purpose of this design was twofold: 1) to study the influence of month of completion of the questionnaire on seasonality and on the prevalence rate of SAD; and 2) to study the variation of mood over the seasons.
Section snippets
Sample
A total number of 5356 subjects was selected randomly from the community registers of 15 communities in the three northern provinces of The Netherlands. The communities and the numbers drawn from each community were chosen in such a way that the sample was representative for the urbanization level of The Netherlands. The percentage of men and women was equal. The age range was chosen between 18 and 65 years.
Instruments
The questionnaire package sent out contained three groups of variables:
Responses
The response rate per month varied from 49.0% to 56.8%. The total response was 2819 questionnaires (52.6%). Significantly more women than men responded (54.0% versus 45.8%; 0.2% of the data on gender were missing) (χ2 = 18.64, p < .0001).
Seasonal affective disorder
SAD criteria were met by 3.1% of the respondents (n = 88), with 3% (n = 85) showing a winter pattern. Using a 95% confidence interval (2 SD), 2.4–3.6% of the Dutch population between 18 and 65 years of age suffers from winter SAD. Criteria for S-SAD were met by
Discussion
In a number of respects the prevalence rate of SAD (3.1%) in the present study is in line with the expectations. In the first place, this figure strengthens the positive relationship between prevalence and latitude. In the USA this correlation (Spearman rs, one tailed) is r = .90, p = .003 (Mersch et al in press). When we exclude the unreliable Italian data (Muscettola et al 1995), the prevalence rate of the present study raises the correlation in Europe from r = .45, p = .225 to r = .70, p =
Acknowledgements
The authors wish to thank Prof. Dr. Hans Ormel for his advise on this research project.
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