Fertility, infertility and generation replacement.

Throughout most of human history 'natural fertility' and life expectations of about 35 years kept population numbers reasonably balanced; but the achievement of life expectations of 75 or more years destroyed this 'natural' system. As longevity increased amongst Western peoples, explosive growth rates were only prevented by increasingly efficient checks on fertility. Today, these checks are so universal and so efficient in Australia and in many other Western countries that 'intrinsic' levels of fertility are below generation replacement level. Changes in fecundity were not a significant factor in this demographic revolution; at least 10-12% of couples remain without issue, but the prime cause of the change is the evolution of the two-child family, a process which now involves only about a sixth of a couple's fecund married life. For the rest of their years most couples are now voluntarily infertile. Correction of involuntary sterility is important to individual couples but has little significance in terms of generation replacement.