Household income can be a misleading metric. In 2010, the top 5% of households had more full-time, all-year-round workers than the bottom 20%. This value says little about an individual’s income. We are at a point where those who earn more also, on average, work more hours.
As the average size of families and households decline, the average household income will experience a concomitant decline relative to per capita income. Between 1967 and 2007, the household income rose by 30%, while per capita income rose by 100% over the same time.

We see in the data that the creation of households was increasing faster than population growth. The US Bureau of the Census concluded:
“The main reason for the more rapid rate of household formation is the increased tendency, particularly among unrelated individuals, to maintain their own homes or apartments rather than live with relatives or move into existing households as roomers, lodgers, and so forth.”