The British Journal of Psychiatry 141: 186-190 (1982)
© 1982 The Royal College of Psychiatrists
The season of birth of schizophrenic, neurotic and psychiatrically normal twins
KS Kendler
The quarter of birth of 536 schizophrenic, 1,991 neurotic and 12,085
psychiatrically normal twin pairs from the National Academy of Sciences-
National Research Council Twin Registry was compared. When either all twin
pairs, or only monozygotic or dizygotic twin pairs, were considered, no
significant differences were found in the quarter of birth between the 3
groups. No difference in quarter of birth was found for twin pairs
concordant versus discordant for schizophrenia. Despite the increased risk
for intrauterine and perinatal complications in twin births, schizophrenic
twins appear (at least) to be no more vulnerable than schizophrenic
singletons to the seasonal gestation and birth- related insults which
probably underlie the seasonality in schizophrenic births. Controlling for
season of birth probably has little effect on the results of twin studies
of schizophrenia.
