Actually looking at data is simple, easy, and straightforward, and yet almost no one actually does it. Here's another one: Adjusted for inflation, what is the average, long-term appreciation of the stock market? Here's the historical Dow Jones index, and here's an inflation calculator. Try it and see!

Be careful here:

1. The DJIA index excludes dividends, which historically produce about half of the total return.

2. The US market was the top performing stock market of the 1900s. So you are looking at a very unrepresentative sample of 1.

3. Several markets went to zero during C20 e.g. Russia post revolution.

4. The market return ignores buy/sell costs and slippage, and taxes. Even buying the index necessarily involves some buying and selling as stocks come and go.

All in all, the ~10% gross nominal return turns into ~3-4% after taking off CPI, using the average market not the top performing one, and taking into account costs and slippage.

In 1900 the stock market was rightly regarded as a crooked casino, so sensible people invested in bonds. So there is potentially a hindsight bias here too - why are we not looking at the investment that was most in favor at the time?

DIY science is harder than it looks. But still worthwhile if you do the homework.