It used to be that most US corporate stock was held by taxable US investors. Now, most corporate stock is owned by a mixture of tax-deferred retirement accounts and foreign investors. Steven M. Rosenthal and Lydia S. Austin describe the transition in "The Dwindling Taxable Share
Of U.S. Corporate Stock," which appeared in Tax Notes (May 16, 2016, pp. 923-934), and is available here at website of the ever-useful Tax Policy Center.
The gray area in the figure below shows the share of total US corporate equity owned by taxable accounts. A half-century ago in the late 1960s, more than 80% of all corporate stock was held in taxable accounts; now, it's around 25% The blue area shows the share of US corporate stock held by retirement plans,which is now about 35% of the total. The area above the blue line at the top of the figure shows the share of US corporate stock owned by foreign investors, which has now risen to 25%.
A few quick thoughts here:
1) These kinds of statistics require doing some analysis and extrapolation from various Federal Reserve data sources. Those who want details on methods should head for the article. But the results here are reasonably consistent with previous analysis.
2) The figures here are all about ownership of US corporate stock; that is, they don't have anything to say about US ownership of foreign stock.
3) One dimension of the shift described here is the ownership of US stock is shifting from taxable to less-taxable forms. Stock returns accumulate untaxed in retirement accounts until the fund are actually withdrawn and spent, which can happen decades later and (because post-retirement income is lower) at a lower tax rate. Foreigners who own US stock pay very little in US income tax--instead, they are responsible for taxes back in their home country.
4) There is an ongoing dispute about how to tax corporations. Economists are fond of pointing out that a corporation is just an organization, so when it pays taxes the money must come from some actual person, and the usual belief is that it comes from investors in the firm. If this is true, then cutting corporate taxes a half-century ago would have tended to raise the returns for taxable investors. However, cutting corporate taxes now would tend to raise returns for untaxed or lightly-taxes retirement funds and foreign investors. The tradeoffs of raising or lower corporate taxes have shifted.