Everything prior to Section 2.9 is in terms of aggregate births of the several years or other periods, considered as a time series. If each family made its decisions on having or not having children in relation to the conditions of the year, without reference to the number of children it has had in the past, nothing more need be said. But suppose now that couples aim at a certain number of children; good or bad times cause them to defer or to advance their childbearing, but not to change the total number. Then the fluctuations in the time series of births are less consequential; the drop of the birth rate in a depression would be made up in the subsequent business upswing, and the rate of increase of the population would be lower only insofar as older parents imply a greater length of generation. Constancy in the total number of children per family results in constancy in the average number of children per cohort, and the Bureau of the Census and the Scripps Foundation have taken advantage of this constancy in their projections.