Seasonal variations in secondary sex ratio in man in Perth, Adelaide and Brisbane appear to be influenced by seasonal variations in rainfall. In these cities, the onset of the rainy season is followed about 11 months later by a larger than normal excess of male births. Sex ratio in Sydney, however, where rainfall is more evenly distributed throughout the year, does not vary markedly with season; but a relationship between rainfall and sex ratio again becomes apparent when the data are analysed in terms of wet months and dry months and of wet years and dry years. It is suggested that secondary sex ratio is affected by properties of drinking water that alter with variations in rainfall, and that the property of water concerned is probably its content of trace elements. Support for this hypothesis has been obtained from an examination of sex ratios in two regions of Australia with trace-element deficiencies. It is also suggested that drinking water exerts its effect through the male parent and that the observed variations in secondary sex ratio reflect similar variations in primary sex ratio.
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