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It has been proposed that environmental chemicalization is responsible for the recent decline in male ratio, but these speculations are based on statistics going back only a few decades. The objective of this study was to evaluate whether Finnish long-term data are compatible with the hypothesis that the decrease in the ratio of male to female births in industrial countries is caused by environmental factors. We analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from 1751 to 1997. Running averages of 9 years (1751-1904) or 5 years (1905-1997) were analyzed for sex ratios. Additionally, to identify potential explanations for the findings, births from 1990 to 1997 were correlated with various family parameters. We found an increase in the proportion of males from 1751 to 1920; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II. None of the family parameters (paternal age, maternal age, age difference of parents, birth order) could explain the time trends. The turning point of male proportion precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association unlikely. Moreover the trends are similar to those observed in other countries with worse pollution and much greater pesticide use.