Sex ratio and factors influencing family size in a human population from Spain.

Six hundred and eight students from the Faculty of Biological Sciences of Valencia (Spain) provided information about secondary sex ratio and family size of the parental, present, and projected generation. The average number of children was 3.59, 3.05, and 2.10 and the secondary sex ratio was 113.3, 101.2, and 98.6 for the parental, present, and projected generation respectively. Combination of sexes of the first two children had a significant effect in the parental and the present generations. There was no consistent association between sexes of successive and sexes of nonsuccessive children within families. The most preferred combination of sexes in the projected generation was one male-one female, and male-female was the most preferred permutation. Strong preferences for the firstborn to be male and for an alternation of sexes were also indicated.