Wealth is not income. Economists sometimes say that "wealth is a stock, income is a flow." They mean that wealth is accumulated over time, and includes assets and debts. Income is what flows in during a give period of time, often measured during a week or a year. The most prominent data source for estimates of US family wealth is the Survey of Consumer Finance, conducted once every three years by the Federal Reserve. Data from the 2013 survey is the most recent available, and the the Congressional Budget Office has released a short report made up of figures and short commentaries showing Trends in Family Wealth, 1989-2013. Here are some snapshots:
The overall wealth of US families totalled about $67 trillion in 2013. As the figure shows, this total has more-or-less doubled since about 1995. Most of increase is attributable to a rising total for the top 10%, which means that the wealth distribution is clearly becoming more unequal over time.
A family in the 90th percentile of the wealth distribution has seen a significant rise over time, although less than a doubling. A family in the 75th percentile has had a more modest rise since the early 1990s. Family wealth levels at the 50th and 25th percentiles haven't changes much in the last 25 years. Given that the rise in total wealth is so much less than a doubling at all of these percentiles, and given that total family wealth doubled from 1995 to 2013, there must be some percentiles where there is more than a doubling. Presumably these are the share of wealth at the very top of the wealth distribution, perhaps the 95th or 99th percentile. At some level, this result is unsurprising. Run-ups in stock prices and housing prices tend to raise the wealth of those who own the most of these assets.