Mark:

While I don't depend (God no!) on Wikipedia, this is a decent summary:

The Household income in the United States is a measure of current private income commonly used by the United States government and private institutions. To measure the income of a household, the pre-tax money receipts of all residents over the age of 15 are combined. Most of these receipts are in the form of wages and salaries (before withholding and other taxes), but many other forms of income, such as unemployment insurance, disability, child support, etc., are included as well. The residents of the household do not have to be related to the householder for their earnings to be considered part of the household's income.[1] While the use of household income remains among the most widely accepted as households tend to share a common economic fate, the size of a household which is commonly not considered may off-set gains in household income.[2]

In 2006, the median annual household income according to the US Census Bureau was determined to be $48,201.[3] The median income per household member (including all working and non-working members above the age of 14) in the year 2006 was $26,036.[4] In the year 2005, there were approximately 113,146,000 households in the United States. 19.01% of all households had annual incomes exceeding $100,000,[5] 12.7% fell below the federal poverty threshold[6] and the bottom 20% earned less than $23,202.[7] The aggregate income distribution is highly concentrated towards the top, with the top 6.37% earning roughly one third of all income, and those with upper-middle incomes control a large, though declining, share of the total earned income.[8][2] Income inequality in the United States, which had decreased slowly after World War II until 1970, began to increase slowly in the 1970's, and has increased more quickly since then.[9] Households in the top quintile, 77% of which had two income earners, had incomes exceeding $91,705. Households in the mid quintile, with a mean of one income earner per household had incomes between $36,000 and 57,657.[10]

The 2006 economic survey also found that households in the top two income quintiles, those with an annual household income exceeding $60,000, had a median of two income earners while those in the lower quintiles (2nd and middle quintile) had median of only one income earner per household. Due to high unemployment among those in the lowest quintile the median number of income earners for this particular group was determined to be zero.[5] Overall the United States followed the trend of other developed nations with a relatively large population of relatively affluent households outnumbering the poor. Among those in-between the relative extremes of the income strata a large and quite powerful section of households with moderately high middle class incomes[8] and an even larger number of households with moderately low incomes.[5] While the median household income has increased 44% since 1990 it has increased only slightly when considering inflation. In 1990, the median household income was determined to be $30,056; $44,603 in 2003 dollars. While personal income has remained relatively stagnant over the past few decades, household income has risen due to the rising percentage of households with two or more income earners. Between 1999 and 2004 household income stagnated showing a slight increase since 2004.[11][12]

(You can look up the original yourself if you're interested.)

However...the study was done before the "sub-prime" crisis. You might want to peek at one of reports from Bloomberg.

The need for two or more income earners for household income to rise is, or should be, troubling. Even more troubling should be the last line in the quote above.

If I had both the time and inclination I'd do a study of personal and household incomes in a constant dollar scenario (using, say, 1980 dollars as the base) and see what happens. I don't have either, sorry.