A 1999 scientific paper published by Jacobsen reported the sex ratio for 815,891 children born in Denmark between 1980 and 1993.[27] They studied the birth records to identify the effects of multiple birth, birth order, age of parents and the sexes of preceding siblings on the proportion of males using contingency tables, chi-squared tests and regression analysis. The secondary sex ratio decreased with increasing number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors.