Annual variation in birth rate of people who subsequently develop schizophrenia

Background Previous attempts to decide whether there is significant year-to -year variation in the birth rate of people who subsequently develop schizo phrenia have given conflicting results, probably because of differences in the statistical methods employed. Aims To determine whether there is significant year-to-year variation in th e birth rate. Method Variation in the birth rate for the period 1921 - 1960 was studied i n three separate national data sets - English, Danish and Scottish - using cubic splines to smooth the distribution curve before calculating residuals from a Poisson distribution. Results Over-dispersion was found in all three data sets, particularly in t he Danish and Scottish data. However, the correlation between the sets of s tandardised residuals derived from the three data sets was only statistical ly significant for Denmark v. England. Conclusions There was statistically significant year-to-year variation in t he birth rate of people who subsequently developed schizophrenia in three c ountries in north-west Europe in the years 1921 - 1960. This is potentially a clue to the nature of the environmental determinants of schizophrenia, b ut better data will be needed before useful explanatory hypotheses can be g enerated and tested. Declaration of interest No external Funding; no confli ct of interest.