They 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 natural factors such as paternal age, maternal age, age difference of parents, birth order could explain the time trends. The scientists found that the peak ratio of male proportion precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely.