ahhh. that's household income data, not family income data. households are composed of one or more unrelated individuals. Median household income, then, counts things like four college students living together or my friend, M, who has five kids and a 3 br/2ba apt. She's helping out a woman who got evicted, is pregnant, and out of work from her job as a stripper. M's house appears to be a one income, 7 person household.
A family is composed of two or more biologically or legally related individuals. (marriage, adoption, birth)
I used to get annoyed with folks who'd say that two parent families were some tiny silver of all households in the US. Well, like duh, when a household included single individuals, then it wasn't surprising that demographic shifts such as the baby boom, a growing elderly population, and lengthening of time to first marriage resulted in an increase of single households making it appear that families were in this HYYYYOOOOOOGE decline.
kelley