Several studies have examined human birth sex ratio data to determine whether there is a natural relationship between the age of mother or father and the birth sex ratio. For example, Ruder has studied 1.67 million births in 33 states in the United States to investigate the effect of parents' ages on birth sex ratios.[48] Similarly, Jacobsen et al. have studied 0.82 million births in Denmark with the same goal.[49] These scientists find that maternal age has no statistically significant role on the human birth sex ratio. However, they report a significant effect of paternal age.