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. The ratio of male-to-female births has been declining in several countries during the last decades. Little is known about the factors that affect sex ratio in humans, but there is some evidence that the timing of intercourse relative to ovulation (Harlap 1979) and coital frequency (James 1971) affect sex ratio in newborns. Also, demographic factors, such as birth order and race, are associated with sex ratio in populations (Erickson 1976). It is generally agreed that the probability of a male birth decreases with increasing birth order and, to a minor extent, with parental age (James and Rostron 1985). Secular trends of decreasing sex ratio in Latin America (Feitosa and Krieger 1992), Canada (Allan et al. 1997), the United States (Marcus et al. 1998), Denmark (Moller 1996), the Netherlands (Pal-de Bruin et al. 1997), England and Wales (Dickinson and Parker 1996), Germany (Bromen and 1 Departamento de Reproduccion Animal y Conservacion de Recursos Zoogeneticos, INIA, Ctra. de la Coruna Km 5.9, Madrid 28040, Spain. Human Biology , October 2000, v. 72, no. 5, pp. 891-898. Copyright © 2000 Wayne State University Press, Detroit, Michigan 48201-1309