The income of the typical American family—long the envy of much of the world—has dropped for the third year in a row and is now roughly where it was in 1996 when adjusted for inflation.

The income of a household considered to be at the statistical middle fell 2.3% to an inflation-adjusted $49,445 in 2010, which is 7.1% below its 1999 peak, the Census Bureau said.

The...