Final answer:
The number of adults internationally is increasing due to historical demographic transitions, the compounding effect of a large global base population, and improvements in nutrition, sanitation, and healthcare which decrease mortality rates. Despite declining fertility rates in many regions, the large number of existing younger individuals reaching adulthood sustains population growth, particularly in less developed regions like sub-Saharan Africa.
Step-by-step explanation:
The increase in the number of adults internationally is influenced by several factors. First, the marked decline in fertility rates has paradoxically combined with a large existing population to still produce a significant number of births. Despite the slower growth rates, this leads to continued population increases. Secondly, during the Industrial Revolution, Europe experienced declining death rates due to better nutrition and sanitation, allowing more children to survive to adulthood. This led to a population surge. As the global economy evolved, however, the demographic transition led to children becoming an economic liability rather than an asset, prompting a decline in birth rates in developed countries.
In contrast, the gap between birth and death rates was once wider, accelerating population growth as more children survived to adulthood. Presently, mortality rates worldwide have decreased substantially while many countries experience an aging population. This demographic shift increases the proportion of the population classified as adults. Additionally, the labor force is impacted as the fertility rate declines and the average age of the population rises, creating significant challenges for the economy and manual labor sectors.
Countries in sub-Saharan Africa currently exhibit the fastest population growth rates, often due to limited access to birth control, minimal female education, and economic structures still reliant on larger family sizes. Moreover, as life expectancies rise and international populations age alongside declining youth populations, the demographics will have profound implications on the standard of living for both young and older adults.