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An investigation into the numerical determinants of secondary sex ratioData from the North Wales parishes of Hawarden and Northop were found previously to show seasonality for birth rate. In keeping with values reported in other studies, the annual secondary sex ratio of 105.3% was found. This sex ratio was also found to vary throughout the year in a cyclical way with a peak occuring in late summer. When male and female birth rates were investigated separately, it was found that females showed a more pronounced cyclicity than males with the peaks for both sexes occuring in the spring. A significant negative correlation between sex ratio at birth and mean day lenght (hours between sunrise and sunset) of the putative month of conception was observed. Sex ratio is a useful but derived parameter and has no independant existence upon which natural selection can be said to exert a direct influence. Therefore, the behaviour of the determinants of sex ratio should not be overlooked.