Background Month of birth as a suicide risk factorhasnotbeen adequately explored.The findings of published studies

Declaration of interest None. Differential risk effects of season of birth have been reported for several diseases, raising the possibility that early environmental exposure in utero and during infancy may influence the risk in adulthood. Seasonal trends of birth have been established for acute lymphoblastic leukaemia in children (Sorensen et al, 2001a), early-onset non-Hodgkin’s lymphoma (Langagergaard et al, 2003), breast cancer (Kristoffersen & Hartveit, 2000), testicular cancer (Prener & Carstensen, 1990), Crohn’s disease (Sorensen et al, 2001b), coronary heart disease (Lawlor et al, 2004) and brain tumours (Brenner et al, 2004). In their reviews, Castrogiovanni et al (1998) and Torrey et al (2000) found that more patients with schizophrenia, Alzheimer’s disease, epilepsy and narcolepsy are born in December and January, whereas affective disorders, alcohol dependence, autism, dyslexia, and multiple sclerosis are reported more frequently in those born during spring and summer months. Month of birth has also been linked to human longevity (Doblhammer & Vaupel, 2001). However, few studies have investigated the effect of month of birth on suicide rates. Huntington (1938) reported that the distribution of birth month for those who died by suicide was different from those who did not. Pokorny (1960) found an overrepresentation of suicide cases in which the person was born in the month of July. Kettl et al (1997) reported variations in season of birth for suicide among Alaskan natives, with more suicide deaths in those born in the summer. Chotai & Asberg (1999) reported that people born in winter in Sweden were significantly more likely than those with other birth seasons to have used hanging as a suicide method. Using 12-year suicide data from Cheshire, UK, an increased risk of violent suicide, particularly hanging, among people born in the summer was found by Salib (2002), quite the opposite of the Swedish study. However, the above studies share some major methodological limitations, including small sample size, lack of controls and the use of inappropriate statistical methods. This study examines the association between suicide and season of birth using routinely collected suicide data over a 22-year period in England and Wales. The size of the database used in this study provides a powerful opportunity to assess this issue, with nearly 27 000 suicides from over 11 million births, and greatly exceeds the size of all previous studies. Our hypotheses were that the risk of suicide would vary according to month of birth and that this association would remain after adjusting for the effects of the total number of births per month in the population. We also predicted that this differential in risk of suicide by month of birth would be present when looking at suicides by gender and method of suicide (violent and non-violent).