 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.imagesfigure 1figure 2figure 3 
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