Lifetime estimates for depressive and bipolar disorders with a seasonal pattern average between 0.4% and 2.9% in US, Canadian, and UK community studies. Some estimates may be as high 9.7%. However, this is likely to be due to differences in the sampling and diagnostic criteria used. Rates of SAD may be slightly higher among people living in more northern latitudes. Although the latitude-SAD association has been demonstrated in North American samples, this finding has not been reliably replicated in European cohorts. This suggests the influence of other factors, such as genetic variability, cultural differences, and climate. Autumn or winter onset of major depressive episodes is much more common than other seasonal-mood fluctuations. Approximately 20% of people with SAD have a bipolar I or II disorder, with SAD being more likely in bipolar II disorder. The incidence of SAD may be higher in some populations with anxiety, ADHD, and premenstrual dysphoric disorders. Alcohol use may also increase as a means of self-medicating SAD symptoms in some populations. The average age of onset is between 20 and 30 years, with declining rates in older populations. SAD is approximately 3 to 5 times more likely among women, which is a greater gender difference than that observed in non-seasonal depression. The prevalence in children and adolescents ranges from 3.3% to 4.2%, with the incidence increasing among girls during puberty. Other studies have noted that parental ratings of depression are more severe among 16- to 18-year-olds than among 6- to 15-year-olds when assessed during the fall and winter months.
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