Birth control-sex preference and sex ratio.

Abstract

The results of Goodman (1961) regarding the relationship between sex preference and sex ratio are generalized and the following question is asked: if it is assumed that the sex ratio is under genetic control in which direction can it be expected to evolve in response to the pattern of birth control. This question can be answered by means of the evolutionarily stable strategy (ESS) concept. If a sex ratio r (defined as the frequency of male zygotes) is sought such that if the population sex ratio is in fact r then no mutant allele which produces a sex ratio r different from r is as successful as the typical allele which produces the sex ratio r. Situation A corresponds to family type 1 in Goodmans notation. The preference for male offspring might lead to the decision on the part of a married couple to bear children until a boy is born and then use birth control so as to bear no further children. It is also assumed that the maximum number of children that it is possible to bear is n and that the preferences themselves are fixed and not subject to selection. Results show that alleles reducing the sex ratio from 1:1 will always increase in frequency due to selection for large family size. This implies that male preference increases the number of females if it is assumed that the sex ratio is generally controlled. The obvious must be true if there is female preference. The parents decide that if their 1st child is a boy they will continue to bear children and if their 1st child is a girl they will bear no further children. Situation B corresponds with couples of type 2 in Goodmans notation. In this situation if the 1st child is a boy the parents will continue to bear children until a girl is born. The situations presented are similar to those of reproductive compensation or social selection. It has been observed that illness or death of a child due to a genetic disease can induce the parents to compensate for this child by having more children or may cause genetically normal individuals. Without details of the genetic variance in sex ratio and the ways in which the preferences for male or female offspring affect the decision as to whether or not to have another child it is not possible to make even an approximation of the expected historical change in the sex ratio by these mechanisms.