I also wonder about that stat (no links provided?).
At any rate, even if the 90/10 statement is true, I'm not sure that supports the conclusion that they
"really couldn't care less what happens on Wall Street.". That 90% of the population may only hold 10% of the stocks, but if it is a considerable amount of their savings, I'm sure they do care. For many people, their only invested savings might be their 401K/IRA (or similar). And I'd expect those are typically 50% or more in stocks. So those people surely
will care if they see their life savings drop in half!
That 90/10 view sounds more like class-warfare fuel than any explanation about the little guy not caring about the markets. And as
MuirWannabe points out, you could still be concerned, even with no direct ownership, since a falling market often means layoffs. For someone with little/no investments, losing their job could certainly be more devastating than seeing a portfolio drop (and eventually recover).
edit/add: This link backs up the 56% number. More interesting to me is, 24% of those with household income below $40,000 own stocks.
https://news.gallup.com/poll/266807/percentage-americans-owns-stock.aspx
-ERD50