Demographic and behavioral determinants of the reduction of male-to-female birth ratio in Spain from 1981 to 1997.

The ratio of male-to-female births has been declining in Spain since 1981. In the last few decades, the proportion of male newborns has also been decreasing in other industrialized countries. It has been hypothesized that these declines are due to environmental factors such as a longer exposure to environmental pollutants, hormonal levels, or sexual behavior. The objective of this study was to examine the evidence of decline in the male proportion of births in Spain and to correlate it to demographic causes (mother's age, marriage age, and age difference between husband and wife). The analysis was based on birth records in Spain from 1945 to 1997. The data show a significant correlation (p < 0.01) between the reduction in the ratio of male-to-female births observed and two variables, the variations in mean age at marriage and the older age at which women give birth. More women are delaying childbearing until their thirties and, as a consequence, there has been a decrease in the proportion of male newborns.