Stage 4 is characterized by the achievement of low mortality and the rapid emergence of low fertility levels, usually near those of mortality.Population growth may become quite high, as levels of fertility and mortality increasingly diverge. Stage 3 involves further declines in mortality, usually to low levels, and initial sustained declines in fertility.The fertility rate remains high, leading to at least moderate population growth. Stage 2 is characterized by a declining death rate, especially concentrated in the years of infancy and childhood.Stage 1, presumably characterizing most of human history, involves high and relatively equal birth and death rates and little resulting population growth.Most societies in the world have passed through the first two stages, at different dates and speeds, and the contemporary world is primarily characterized by societies in the last two stages, although a few are still in the second stage. There are many versions of demographic transition theory (Mason 1997), but there is some consensus that each society has the potential to proceed sequentially through four general stages of variation in death and birth rates and population growth. Some societies have eventually had fertility declines and emerged with a very gradual rate of growth as low levels of births matched low levels of mortality.
Since about 1800, however, this situation has changed dramatically, as most societies have undergone major declines in mortality, setting off high growth rates due to the imbalance between deaths and births. The human population has maintained relatively gradual growth throughout most of history by high, and nearly equal, rates of deaths and births.
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