At the early stages of the recovery, the wealthy rebounded while the rest floundered. One study found that in 2010, 93 percent of the economic gains in the United States went to the top one percent of earners.
Some chalked this up to political policy. Others blamed the Fed, or tax rates on the wealthy, or a rigged financial system.
But there's a more simple answer: stock investments.
A new research paper from the Pew Research Center, using census data, found that from 2009 to 20911, the top 7 percent of Americans by wealth owned 63 percent of the nation's total wealth – up from 56 percent in 2009. The mean wealth of the top 7 percent (about 8 million households) jumped to $3.17 million from $2.48 million over the two years.
(Read more: Booming Market Adds 300,000 Millionaires)
In contrast, the mean wealth of the bottom 93 percent fell to $133,817 from $139,896.
The reason, according to the paper, is that stocks recovered and housing didn't. The wealthy have their wealth concentrated in stocks, while the less affluent have their wealth concentrated in their homes.