How many people in the US benefit from stock market rises?

Paul Krugman provides a helpful pie chart to illustrate how many people in the US benefit from increases in the stock market, including those 401(k)'s the Pod Pundits and Republicans like to talk about in connection with this tax bill:

I've been getting some mail from people insisting that a corporate tax cut helps ordinary families, because half the population owns stock. And it's true that many people own stock indirectly via their retirement accounts. But how much? Here's the distribution of ownership: pic.twitter.com/WWUoNJ8Ace

— Paul Krugman (@paulkrugman) December 19, 2017