Researchers have observed the existence of high and low prevalence (total number of cases) and incidence (number of new cases during a specified time frame) pockets for some psychiatric disorders, with rates varying depending on season of birth. These may be related to variances in temperature, precipitation, and sun exposure, however, the underlying mechanisms involved are largely unknown.

What is the evidence for winter birth as a risk factor for bipolar disorder?

Moderate quality evidence finds a small, increased risk of affective psychosis in people born in late winter in the northern hemisphere (January to April). Affective psychoses is a broad diagnostic category that includes bipolar disorder.

October 2021

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For some time, researchers have observed variations in population rates of schizophrenia, with rates changing depending on the time and place of birth. For example, the prevalence rate in a given population may be increased if birth was at a higher latitude with a cooler climate. These variables are also related to variances in diet, precipitation, sun exposure, socioeconomic status and genetic factors, as well as age and gender differences. Therefore the observed relationships between rates of schizophrenia and higher latitude and cooler climates may have several related explanations.

What is the evidence for latitude, climate and winter birth?

Moderate to high quality evidence finds a small relationship between winter or winter/spring births and increased risk for schizophrenia in the Northern Hemisphere. High quality evidence also finds a small relationship between winter/spring births and subclinical psychotic symptoms in children in Japan and the U.K.

Moderate quality evidence finds a small effect of increased prevalence of schizophrenia with increased latitude and decreased annual mean daily temperature in the Northern Hemisphere. Moderate to low quality evidence finds this association is greatest in people with older fathers at birth (over 45 years old), and in disadvantaged ethnic minority groups.

Moderate to high quality evidence finds a small effect of increased rates of deficit schizophrenia (predominately negative symptoms) in offspring born in the summer months of June and July in the Northern Hemisphere.

April 2022

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