A season of birth tendency has been shown for psychiatric disorders and suicidal behaviour. This study aimed to examine the association between season of birth and self-mutilative behaviour (SMB).

The study sample consisted of 508 (40.9% males) 12- to 17-year-old adolescents consecutively admitted to the Department of Psychiatry of Oulu University Hospital, Finland. The birth month of each adolescent was categorized into one of the four seasons: spring (March-May), summer (June-August), autumn (September-November) or winter (December-February). The information of SMB was based on K-SADS-PL-interview, which included an item on non-suicidal physical self-damaging acts without intent to die. A total of 142 adolescents (27 males, 115 females) met the criteria for SMB. The association between season of birth and SMB was assessed with a logistic regression analysis after controlling for adolescent's age, previous suicide attempts and DSM-IV diagnosed psychiatric disorders.

The monthly distribution of births of adolescents with SMB differed statistically significantly from that observed in the general population of the same age (c2=8.29, df=3, p=0.043). An association between season and birth with SMB was seen in girls (Wald=8.46, df=3, p=0.037), but not in boys. Among girls born in autumn, the likelihood for SMB was significantly increased (adj. OR 2.9; 95% CI 1.4-6.2) as compared to girls born in winter.

Birth during autumn may predispose girls to SMB via dysfunctional neurotransmitter systems. These findings may also be related to seasonal rhythms in parental mood and poor early caregiving of the offspring.