Pedestrians walk along Wall Street near the New York Stock Exchange in New York.

New highs in the U.S. stock market are certainly good news — for those who own stocks.

The bad news is, those who own stocks are only a small part of total U.S. households.

The investment class in the United States is small, and it's not getting bigger.

According to the latest Gallup poll of American households, only 54 percent own stocks, down from 62 percent before the financial crisis.

And many of the households only own small amounts of stock.

New York University economist Edward Wolff conducted a careful study of stock ownership in 2013 and found that 81 percent of the stock market worth is concentrated in the top 10 percent of households by wealth.

I spoke with Wolff and he was kind enough to update the numbers. It's gotten worse: 84 percent of the stock in the United States is owned by the top 10 percent of households by net wealth.