While a modest fraction of people trade individual stocks, a lot of Americans have some type of 401k, pension, or retirement account that is tied to the stock market.

This pegs it at 50% of private sector workers and 80% of public sector workers. https://www.pensionrights.org/publications/statistic/how-man...

Additionally, in many cases, couples may just have one person with access or contributions to the market - but this still leaves them exposed (for good and bad) to the market.