By Jack Hartigan
In 1859, Charles Darwin illuminated the people of the world with the truth behind the behavioral and reproductive tendencies of organisms. On the Origen of Species delves into how organisms tend to react to improved circumstances, whether it is a disappearance of a predator, an abundance of food, a change in season, etc., by producing more offspring and expanding their population. This is done because organisms are naturally wired to do whatever is in their power to prevent the extinction of their species, and what better way to accomplish this than to reproduce as much as possible. While this trend is arguably the most biologically pragmatic truth established by scientists, it is not a completely universal truth. Homo sapiens, the genus that includes all humans, react to improved circumstances by curtailing reproductive rate rather than raising it.
Figure 1 from http://freakonomics.com/2011/06/10/the-rich-vs-poor-debate-are-kids-normal-or-inferior-goods/
This behavioral pattern of humans is shown in Figure 1, where the inversely proportional relationship between reproductive rate (TFR) and occupational income is displayed by year, with lower TFRs in high-income families and higher TFRs in low-income families. This trend is essentially the reason behind the shape of the demographic transition, in which humans are shown to lower their reproductive rates as their respective country becomes more and more developed. Dr. Anna Goodman of the London School of Hygiene and Tropical Medicine offers an explanation to this phenomenon by categorizing the reproductive tendencies of humans into two categories; r-selection and K-selection. R-selection applies to humans who produce many offspring but do not invest themselves fully in each child. This is used in conditions that cause high infant mortality rates. The other strategy, K-selection, applies to humans who produce very few offspring, but dedicate themselves fully to their nurturing and well-rounded development. This is used in conditions where contraceptives, medical attention, and food is handily available.
The demographic transition essentially shows that, as a country advances through the chronological stages of the transition, the reproductive rates of people in the country begin to spiral. This is because as countries advance through the stages of the demographic transition, they become more developed and the economical situation of the countries improves.