Birth variations in populations which practise family planning

Abstract In the first section it is demonstrated that permanent periodicities may appear in populations, in which fertility occurs at two or more ages, separated by ages, in which there is no fertility at all. It is also shown that where periodicity does disappear, this may take much longer than is usually assumed, before the stable age distribution is attained. The last section of the paper is based on Swedish data relating to female fertility for single-year cohorts and for periods. It is demonstrated that fertility behaviour of cohorts from different decades has varied considerably, not least as regards birth timing. This explains why the period fertility rate has varied in such an irregular fashion, whereas the rate for cohorts has moved much more regularly; after the original decline to an all-time low for the cohorts born around 1905, there has been a slow, and on the whole, regular increase. This increase is largely explained by a rise in nuptiality. If the total fertility rate required for reproduction is computed, it is seen that no cohorts born after about 1885 have reproduced themselves, and that for the cohorts born between 1895 and 1920 the gap was considerable.