Epidemiology of schizophrenia

Schizophrenia is diagnosed 1.4 times more frequently in males than females, and typically appears earlier in men[7]—the peak ages of onset are 20–28 years for males and 26–32 years for females.[8] Onset in childhood, before the age of 13 can sometimes occur.[9][10] A later onset can occur between the ages of 40 and 60, known as late onset, and also after 60 known as very late onset.[11]

Generally, the mean age of first hospital admission for treatment of schizophrenia is between 25 and 35. Studies have suggested that lower income individuals tend to have their disorder diagnosed later after the onset of symptoms, relative to those of better economic standings. As a result, the lower social classes are more likely to be living with their illness untreated.[3]

It is generally accepted that women tend to present with schizophrenia anywhere between 4–10 years after their male counterparts.[12] However, using broad criteria for diagnosing schizophrenia shows that males have a bimodal age of onset, with peaks at 21.4 years and 39.2 years old, while females have a trimodal age of onset with peaks at 22.4, 36.6, and 61.5 years old.[13]

This additional post-menopausal peak of late-onset schizophrenia in women calls into question the etiology of the disease and raises a debate about "subtypes" of schizophrenia, with men and women being susceptible to different types (see causes of schizophrenia). This is further supported by the variability in presentation of the disease between the genders.[14]

Other theories that may explain this difference include protective or predisposing factors in men or women that may render them more (or less) susceptible to the disease at different points in life. For example, estrogen may be a protective factor for women, as estradiol has been found to be effective in treating schizophrenia when added to antipsychotic therapy.[14]

In 2000, the World Health Organization found the prevalence and incidence of schizophrenia to be roughly similar around the world, with age-standardized prevalence per 100,000 ranging from 343 in Africa to 544 in Japan and Oceania for men and from 378 in Africa to 527 in Southeastern Europe for women.[15]

However, the impact of schizophrenia tends to be highest in Oceania, the Middle East, and East Asia, while the nations of Australia, Japan, the United States, and most of Europe typically have low impact. Despite relative geographical proximity, the DALY rate of schizophrenia in Indonesia is nearly double that of Australia (the nations with the highest and lowest respective DALY rates). Discrepancies between DALY rates and prevalence may arise from differences in availability of medical treatment: years lived with mental disorders carry significantly higher DALY values when unmedicated than when medicated.[15]

In Western Europe it has been documented that immigrant groups are more likely to be diagnosed with schizophrenia. The immigrant groups that are predominate in the increased schizophrenia diagnosis are of black origin.[17] The highest rates of schizophrenia diagnosis come from those of Afro-Caribbean ethnicity and those with black African descent.[17] In the US, African Americans have been found to be three times more likely to be diagnosed with schizophrenia, and when taking socioeconomic status into account they are two times more likely than their white counterparts.[17] However, those diagnosed with schizophrenia in developing countries have been found to have better course and outcome than their counterparts in industrialized countries.[18] These improved outcomes may be because these countries place emphasis on harmonious interpersonal relationships.[18]

In two natural experiments conducted on populations that experienced famine, the rates of schizophrenia diagnosed were observed. During both the Chinese Famine (1950s) and the Dutch Hunger Winter (1944-1945) the cohorts of the exposed group were twice as likely to develop schizophrenia as compared to the unexposed cohorts.[19] It is possible that prenatal nutritional deficiency plays a role in the development of schizophrenia, in particular the lack of micronutrients. Countries with poor prenatal care, low food supply, or developing countries could have a higher incidence of schizophrenia, but more research is needed to confirm this hypothesis.

This disease has been found to be correlated with the season of birth. It's known that individuals with the disease are more likely to born in the winter months. Though the risk is small, persons born in the winter months are about 10% more likely to develop schizophrenia. [20]

There have been several studies done that show a connection between schizophrenia and cannabis. The genetic liability for people with cannabis use disorder was strongly correlated to schizophrenia. It was also found to show mixed evidence that pointed to an occasional connection between cannabis use disorder and schizophrenia.[21]

• Prevalence of mental disorders
• Sex differences in schizophrenia

• Saha S, Chant D, Welham J, McGrath J (May 2005). "A systematic review of the prevalence of schizophrenia". PLOS Medicine. 2 (5): e141. doi:10.1371/journal.pmed.0020141. PMC 1140952. PMID 15916472.