The decadelong economic expansion has showered the U.S. with staggering new wealth driven by a booming stock market and rising house prices.

But that windfall has passed by many Americans. The bottom half of all U.S. households, as measured by wealth, have only recently regained the wealth lost in the 2007-2009 recession and still have 32% less wealth, adjusted for inflation, than in 2003, according to recent Federal Reserve figures. The top 1% of households have more than twice as much as they did in 2003.

This...