To understand technophysio evolution's impact on the global population, we can look at what demographers call the Demographic Transition Model (DTM). The DTM is a model reflecting the experience of Western Europe, extrapolated to interpret anticipated global demographic trends. There are four phases to the DTM:
Stage 1: Death rates (deaths per 1,000 population) and birth rates (births per 1,000 population) vary but tend to balance each other out. Prior to the eighteenth century, this was the near universal story for humanity (although as we have seen, very slow steady growth had been the norm for many centuries before.)
Stage 2: Death rates begin to decline significantly. Birth rates stay constant for a while and then began to drop in a pattern similar to death rates. Though both are declining, the birth rate stays significantly above the death rate for decades (see the dashed line in the chart.) This leads to a significant increase in the population.
Stage 3: Death rate decline begins to slow and level out. Meanwhile fertility rates continue to decline leveling out at a later date. The population continues to experience growth but at an ever slowing rate.
Stage 4: Death rates and birth rates stabilize at or below replacement levels.
Wikipedia provides a chart for 250 years of Sweden's history that illustrates this. Notice the parallel death and birth rates until the beginning of the 1800s. The stages are demarcated for you at the top of the chart.
Stage 2 emerged in Europe because of technophysio evolution. Increased agricultural productivity led to a surplus of food and an improved diet. Improved health and vigor meant more productive people. It reduced the death rate as people lived longer and more productive lives. (Robert Fogel notes that, in England, agriculture improved the caloric intake, but the rapid urbanization created health problems that tended to retard advances in life expectancy until the late nineteenth century.) With improved health, and enough resources to meet basic subsistence needs, people began to address environmental problems like waste elimination and personal hygiene. This lessened the amount of disease and allowed more people to live longer and more vigorous lives.
Stage 3 was when the infant mortality rate (children dying before their first birthday) was noticeably lower. Whereas only four or five children out of eight might have lived to adulthood in the past, now seven or eight children might reach maturity. Fewer children were needed to perpetuate the family, and birth rates declined. Increased industrialization also meant urbanization and a departure from traditional values about family size and the role of women. Women no longer gained as much status from family. They become more educated and literate, often entering the workforce. This led to greater affluence and greater sophistication, creating a better quality of life. People became evermore empowered, and greater demands were made for environmental improvements, safety, and health care.
Stage 4 has seen a stabilization of death and birth rates. Concerns about quality-of-life issues, including environment and health, have risen in importance.
Some believe there may be a stage 5. Eventually, our ability to extend life expectancy will hit a wall, and the death rate will rise a little before stabilizing. With affluence, death rates may begin to increase because of decreased physical activity and increasingly unhealthy lifestyles like overeating. How birth rates will respond isn't known. Whether this is true and to what degree is a subject of debate.
We can extrapolate the DTM to represent global changes with one significant change. The distinguishing feature of the global model is that instead of a relatively smooth transition into declining death rates over decades, death rates plummet quickly. Most developing nations experienced population explosions in the years after World War II when large amounts of aid, vaccines, and technological knowledge were made available. Traditions and customs about family size can't adapt as quickly. Therefore, the gap between birth and death rates (see the dashed line in the graph below) is considerably larger than with the DTM for Western nations. This leads to very rapid population growth.
Here is a chart showing the global birth and death rate from 1950 with projections until 2050:
Another distinguishing feature of the global DTM is the linkage between increases in per capita income and improvements in living conditions. In the Western DTM, growth in per capita income seemed to draw living conditions to higher levels. The global experience has been more a matter of improved living conditions and health, drawing per capita income to higher levels. Aid and technology transfers from the West have essentially shortened the learning curve for other nations following the West through the demographic transition.
It is important to note that the DTM is not without controversy. Looking nation by nation, there is considerable variation in how demographic transitions occur. Still, the DTM is a good standard that helps us frame questions about specific nations and regions. It does seem to capture a sense of the overall global trend.
Next, we will look at the economic growth machine that made the recent rapid population growth while expanding prosperity possible.
(For a scholarly presentation of technophysiological evolution, see The Escape from Hunger and Premature Death, 1700-2100: Europe, America, and the Third World. Wikipedia has a pretty good article on the Demographic Transition Model.)
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