In my previous post, the evolution of mean family size was presented. In the same post, I also presented the long-term decrease in the mean household size from 2.89 in 1975 to 2.65 in 2011. The households break into smaller pieces as well as families. Here, we address the question of the most efficient size for a household. I propose to measure the efficiency in terms of income per person for a given household size. This measure might be not conventional but definitely explains the fall in the household size as the consequence of maximum income.
Figure 1 depicts seven curves of income per person (the average income for a given household size divided by the household size) for households of different sizes (10 people for the category 7+) normalized to the average household income (for all households) in a given year. The two people household is as efficient as one person household since 1998. (Figure 2 depicts the same pattern for American families.) Not surprisingly, the portion of one person and two people households increases rapidly since the start of measurement in 1975. These are two most efficient (in terms of personal income) household sizes. The portion of smallest households will rise in the future along their long term trends. Income rules!
Figure 2. Income per person for families of different sizes (10 people for the category 7+) normalized to the average family income in a given year.
Figure 3. The portion of households of a given size since 1975.