Here's a quote taken from a report written by geographer Joel Kotkin ("What is a City for?"):
In developing countries, where the megacities of the future are being formed, as the price of space rises, the quality of life declines, and city services become less accessible and efficient. Average household size and fertility rates in cities have begun to decline. For example, while The World Bank data puts fertility for China and Japan at 1.6 and 1.4 respectively, Beijing and Shanghai are experiencing much lower rates than the national average. In Tokyo, fertility rate is about 1.2. In Shanghai, according to National University of Singapore demographer, Gavin Jones (2009), it has dropped to a remarkably low 0.7.These phenomena can be seen in virtually every part of the world, from developing countries such as Iran, China, Mexico and across Northern Africa, birth rates have plunged towards those of higher income countries as they have urbanised. Birth rates among Muslims in Europe, as well, have dropped (Pearce, 2010, pp.114–116). Divorce over the past decade has grown by 135% in Iran, where women now constitute 60% of college graduates. Meanwhile, household size has declined to less than 3.5, according to the most recent national census. In Tehran, another city of largely apartment dwellers with forbidding cost of living, especially for housing, the latest average household size in 2011 was reported to be 3.1 (Erdbrink, 2012).
On the one hand urban growth drives economic growth, innovation and development but at the same time the reality for individuals and families is that costs rise to the point where raising children is forfeited because the couple simply can't afford to have a family. And because there are no families, no children in our cities the development of the urban environment does not provide for children. Our economic development strategies focus on attracting the young, single and highly educated to creating what Terry Clark from the University of Chicago called the
"...new city, built around the needs of what he calls “the slimmer family” of childless couples and often single professionals, focuses primarily on recreation, arts, culture and restaurants; a system built around the newly liberated individual."
In a world where few people have families and, where they do those families are small, we build up problems for our future. So far the gap left by the absence of children is made up by migration mostly from rural areas. The question we need to ask is what happens when the migrants run out?