Curious if you have a source for that?

> What does "adjusted for reinvested dividends" mean?

Dividends are payed out in cash and this means the analysis assumes they're used to buy more shares.

There's an argument that the American economy in the 20th century is itself a selection bias. It's the most successful economic period in history. Other countries in the same period or the world economy in other periods do not have such spectacular returns.

Some people think that we are facing secular stagnation, and the economic growth is slowing down. The avg federal reserve interest rates have been falling at 2% per decade for 5 decades, and signifies less growth in the economy. Sure, this low interest rates have actually fueled the stock market boom as the value of future earnings is high, but the growth is not sustainable. For one, we are going to end up with some stable p/e ratio, and the growth of stock market because of low interest rates will stop. And two, the interest rates will also stop going down as fast, as a rate of say -10% sounds pretty bad in what it means for the economy (negative growth of that magnitude), so people do not think we will have such a situation.

To be honest, the belief that the market is overvalued is a big part of my motivation for buying individual stocks despite the overwhelming evidence that it's a fool's errand.

[1] Using the S&P 500 as a proxy. There's non-linkable buttons to see the last 10, 20, 30 years https://www.macrotrends.net/1324/s-p-500-earnings-history

Adjusting for reinvested dividends refers to an assumption that the cash dividends paid out by a company are re-invested to buy more shares of the stock. The concept being that a company can pay out a cash dividend or use that money to invest / grow their business. Therefore, to calculate a "total return" we need to factor in the price increases of a stock + the dividends that were paid by the company.