Here’s a chart that shows the changes in “household composition,” i.e. living alone vs. having a family. As you can see, things have been changing for some time:
In my opinion, this change, which has many roots, is the biggest driver of differences in urban development. Many people no longer need or want the 2,500 sq. ft. single family home for the simple reason that they’re living alone (Or maybe with a cat).
Over at StatChat, they describe the trends:
In 1950, over half of all households consisted of two married parents with children. By 2014 that portion had declined to less than a quarter of U.S. households. The actual number of households with two spouses and children was smaller in 2014 than in 1980, despite the total U.S. population growing by over 40 percent during the period. Given the scale of the change, the decline in family households is arguably one of the most significant demographic trends over the past few decades.
Often demographic trends are interrelated –the decline in family households has certainly been influenced by the aging of the U.S. population. Since 2000, the majority of U.S. population growth has been concentrated in the older-than-60 age group as the baby boomer generation ages. Because the over 60 population accounts for the majority of the population that lives alone, by the 2010 Census, households with only one resident had replaced families with two spouses and children as the most common type of U.S. household
Something to keep in mind. Older people, more single people, fewer families with children. That explains all the new apartment buildings!