...there is the key to demographic growth.
A while ago I came across this rather interesting study from Australia about why people have three or more children. A couple of very interesting statistics and trends (remembering that while fairly similar in cultural background, Australia is somewhat more liberal and less fertile than the US):
It cites another study with the following breakdown of who was having children from 1986 to 1996 based on a survey of women who were 30 in 1986.
By 1996 22% of these women (now 40 and so unlikely to reproduce further) had no children.
16% had one child.
35% had two children.
20% had three children.
7% had more than three children.
Now, of the children born to this group of women, more than 50% were born to the 27% of women who had three or more children.
Clearly, while the percentage of women having three or more children is small, their effect on the next generation is huge. Think about it. 27% of the population will be the parents of the majority of the next generation. While 73% of the population will have left behind a minority of descendants. Even accounting for children not following the cultural/moral/political paths of their parents, it looks like fifty years could create a huge change in a population base soley on who is reproducing and who is not.
Based on how important the women with three or more children are to the propagation of the species in Australia, this study sought to look at what factors were predictive of whether a woman would have three or more children. The study identified several factors that predicted whether a woman was likely to have three or more children:
1) Women who had their second child by age 27 were 3.8 times more likely to have three or more children.
2) Women who had not been in an 'other de facto relationship' (i.e. non married, live-in relationship) were 2.6 times more likely to have three or more children.
3) Women who said their first birth was unplanned were 1.6 times more likely to have three or more children.
4) Women who were Catholic were 1.6 time more likely to have three or more children.
It seems to me this gives us a pretty clear picture of the sort of women who are mothering more than half of the next generation in Australia:
Rather than going through a series of live-in relationships your typical mother of three or more married fairly young and had her children in her mid twenties through early thirties. Whether or not she uses artificial birth control, she has a less rigid approach to "planning" than the average woman, and her first child "just happened" not long after she got married. She's probably religious and quite possibly Catholic.
Now, there are two ways to look at this: The study essentially concludes that since early marrying religious women who have not had a previous live-in relationship are currently becoming less common in society, that Australia can expect to see fewer and fewer women have three or more children. In the short term, that seems to be pretty clearly the case. However, in the mid to long term, it may well be that what we are seeing here is the mechanism behind a social mores pendulum at work. As we work down to where a solid majority of children are being born to the minority of families with a more conservative religious and family ethic, the next two generations may see society in turn swinging back to a more generally conservative religious and family climate.