Season of Birth Effects on Psychiatric Disorders

The influence of season of birth on the risk and manifestation of psychiatric disorders has been a subject of research for over half a century. Consistent epidemiological evidence indicates a modest excess (around 5–8%) of individuals with schizophrenia, bipolar disorder and, to a lesser extent, schizoaffective disorder being born in winter and spring in both hemispheres. Similar, albeit weaker, patterns have been observed for major depressive disorder and autism. The seasonality effect is thought to arise from environmental factors acting during gestation or in early postnatal life, including maternal exposure to infectious agents, variations in sunlight and vitamin D synthesis, fluctuations in temperature and nutritional status, and changes in toxin exposure. These factors may influence neurodevelopmental processes via maternal immune activation, circadian gene regulation and epigenetic mechanisms. While the season of birth signal remains relatively small at the population level, it provides a valuable window into the complex interplay between genetics, prenatal environment and brain maturation. Understanding these associations holds promise for targeted preventive strategies, such as maternal vaccination, nutritional supplementation and public health planning, and continues to inform models of the neurodevelopmental origins of psychiatric illness.

Research from Nature Portfolio

Recent large-scale analysis in older adults found that neither month nor season of birth reliably predicts symptoms of depression or anxiety, explaining less than 0.1% of variance after accounting for age and sex. The study used harmonised psychometric assessments across multiple European regions and applied multilevel modelling to adjust for demographic factors. The findings challenge the notion of a lasting season-of-birth effect on mood and anxiety outcomes in later life, emphasising the need to consider developmental stage and cohort effects.

Season of Birth Effects on Psychiatric Disorders publication trend

The graph below shows the total number of articles in season of birth effects on psychiatric disorders across all publications each year (not limited to Nature Index journals).

Technical terms

Neurodevelopmental hypothesis: A model proposing that early life environmental factors disrupt brain development, increasing vulnerability to psychiatric disorders.

Photoperiod: The duration of daylight within a 24-hour period, which influences hormonal and circadian processes.

Maternal immune activation: The maternal inflammatory response to infection or inflammation during pregnancy, which can affect fetal brain development.

Global Assessment of Functioning (GAF): A clinician-rated scale assessing overall psychological, social and occupational functioning.

Positive and Negative Syndrome Scale (PANSS): A structured clinical instrument for measuring symptom severity in schizophrenia, distinguishing positive (e.g. delusions) and negative (e.g. social withdrawal) symptoms.

References

1. Season of birth has no effect on symptoms of depression and anxiety in older adults. Scientific Reports (2022).
2. Seasonality of births in schizophrenia and bipolar disorder: a review of the literature. Schizophrenia Research (1997).
3. Being Born in Winter–Spring and at Around the Time of an Influenza Pandemic Are Risk Factors for the Development of Schizophrenia: The Apna Study in Navarre, Spain. Journal of Clinical Medicine (2021).
4. Winter birth: A factor of poor functional outcome in a Swiss early psychosis cohort. Schizophrenia Research (2024).