The Federal Reserve recently estimated total household net worth in the U.S. to be about $80 trillion, including real estate and financial assets. And data from the Fed’s Survey of Consumer Finances imply that the top 10% of households by net worth hold about 75%—or $60 trillion—of this total. The bottom 90% of households therefore have a net worth of about $20 trillion.

These data seem to show a country whose wealth is highly concentrated. But the true picture is hardly as stark as critics of inequality claim, because it...