The situation changes when we directly inspect one daughter cohort. Birth order distribution in a cohort apparently depends on the growth rate. In this case, ratio of older and younger sisters changes with r . This is the other side of the Sisters' Riddle. Deviation of O i ' from O i results from growth in size of mother cohorts.
Positive r causes a bias to young mothers.
If younger mothers tend to bear babies of earlier birth order, which is a natural assumption, the number of younger sisters exceeds older sisters in a growing population. Thus the relationship between ratio of sisters and growth rate depends on the fertility schedules by birth order. Though the condition, which affirms the anticipated relationship, is natural, it is not logically true as is in Lotka's model.