The U.S. Census Bureau is full of data. I was digging into the problem of the long term evolution of the household and family mean and median income and found a precious piece of measurements. Since 1950, the CB has been publishing the average size of family. For households, this data series is limited by 1975. Figure 1 shows the evolution of both variables. The family mean size was falling between ~1965 and 1990. (I would guess that the household size experienced a similar drop since 1965.) Since ~1990, there was no change beyond the measurement accuracy: 3.16 in 1988 to 3.16 in 2009 and 3.13 in 2011.
What is the reason behind the falling mean size? Figure 2 depicts the evolution of the number of families of a given size (from 3 to 7+ ) normalized to the total number of families (from 2 people to 7+) in the USA. Figure 2 demonstrates that bigger families started to break into smaller pieces around 1965. The five people families still on a long term decline, but the share of 6 and 7+ people families stabilized around 1990. An immediate result of the split was the increasing share of 2, 3, and 4 people families between 1963 and 1990. However, the 3 and 4 people families started to break more intensively in 1990 and their shares have been on a negative trend ever since.
Figure 3 shows the only winner of the breakage process – the two people families. By 2025, a half of American families will consist of 2 people!
Figure 1. The mean size of family and households in the U.S. as reported by the Census Bureau.
Figure 2. The shares of families with different sizes.
Figure 3. The share of 2 people families.