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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.
Seasonality in birth rate in two 19th century North Wales parishesAn understanding of the basic patterns in the seasonality of birth rate may be useful in certain fertility techniques. The use of artificial birth control, however, has had the effect of masking the influence of underlying biometeriological factors. To elucidate trends in birth seasonality in the absence of current social factors, a study useing nineteenth-century parish records was undertaken. Analysis of 5,905 births recorded in the baptismal records of the parishes of Hawarden and Northop between the years 1837 and 1886 revealed significant seasonal trend with a peak occuring in the spring. Further analysis showed a significant positive correlation occuring between the (standardized) numbers of births in each month and the mean day length (hours between sunrise and sunset) of the pulative month of conception.