Should-Read: Sarah Perry: The History of Fertility Transitions and the New Memeplex: "European cultures have historically prevented people from restricting family size within marriage...
...The European marriage pattern allowed for the control of fertility only through delaying and restricting nuptiality. A new pattern, allowing for controlled fertility within marriage, simultaneously originated in New England and France in the late eighteenth century. The new pattern traveled with a new set of values, including suffrage, democracy, equality, women’s rights, and social mobility. Its main mechanism of spread was education, the availability of which also incentivized the new fertility pattern’s adoption by providing a clear way for parents to compete for the future status of their children by having fewer children. The new pattern spread across Europe, North America, and Australia during the late nineteenth and early twentieth century, encountering temporary, partial resistance from some groups. Even Catholics and Mormons worldwide adopted controlled fertility by the early twentieth century or earlier.
As the new pattern grew to dominate the western world in the twentieth century, Asia and Latin America transitioned to the new pattern. Sub-Saharan Africa entered a fertility transition beginning in the 1980s that is ongoing. In each of these transitions, when controlled fertility was adopted, the pre-transition positive (eugenic) relationship between fertility and wealth became a negative (dysgenic) relationship. Only tiny pockets of culture that maintain extreme separation from the new pattern – especially through refusing outside education and preventing women from contact with the outside world – have fertility patterns plausibly consistent with uncontrolled fertility. These may include the Amish and Hassidim in the United States.
Once the fertility transition to controlled fertility occurs in a population, its fertility generally continues to decline until it is below replacement. The benefits of the new pattern are increased material wealth per person, a reduction in disease, starvation, and genocide, and upward social mobility. The main drawback is the onset of a dysgenic phase that may end civilization as we know it.