A summary of factors explaining the income of the families in top 20%, in the U.S. are: a. Married-couple families, aged under 35, with less than a high school degree. b. Single-parent family, who does not work full time, and with a college degree or more. c. Married-couple family, aged between 35 and 64 years with a college degree or more. d. Householders aged 65 and older, wife works full time, with less than a high school degree.