Season of Birth in the General Population and in Patients with Mental Disorder in Norway

¹ Professor Ørnulv Ødegård, Gaustad Sykehus, Gaustad, Oslo, Norway

The monthly distribution of births in 62,190 psychiatric hospital patients is compared with the corresponding pattern in the general population of Norway. The number of births per day varies in a sinusoidal curve between a maximum in January-May and a minimum in October-December. There is an additional, more sharply defined peak in September, which is taken to correspond to a maximum of conceptions during the traditional mid-winter festivities.

It is shown that the January-May maximum varies from time to time in inverse relation to the general birth date: to a birth minimum during the economic crisis of the 1930's there correspond particularly high monthly fluctuations. Any comparison between psychiatric patients and the general population should therefore be made with careful control of the years of birth.

The results of such a controlled comparison are presented, and show that schizophrenic patients have significantly higher monthly birth fluctuations than either the general population or the other diagnostic groups. The September maximum, on the other hand, is the same in all patient groups as in the general population.

These findings are consistent with the existence of some deeper socio-biological difference between schizophrenics and the general population, dependent upon seasonal variations and going back to the time of birth or late pregnancy. Sub-clinical brain damage with consequent personality disorder is suggested as the most likely pathological mechanism.

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