The stock index you invest in isn’t always the most important decision. Here’s what matters even more.

One of the best lessons investors received when the Dow Jones Industrial Average DJIA turned 130 years old on May 26 was a reminder of why time diversification is so important in the stock market.

I’m referring to the portfolio risk reduction from holding stocks for the long term. Most investors think of diversification in terms of holding dozens of individual stocks, and they’re right, up to a point. But what few realize is that holding one stock for many months can provide greater diversification benefits than holding many stocks for one month.

‘I’m unsure of the best approach’: My father, 91, is in hospice care. He left his six children CDs. Can we cash out?

‘I have no preexisting conditions’: I’m 56, earn $198,000 and want to retire early. Can I afford private healthcare?

This investment lesson emerges from the Dow‘s 130-year history because its long-term performance is nearly identical to the S&P 500 SPX, despite major differences in how the two indices are constructed. Since the Dow’s creation in May 1896, it has produced a dividend-adjusted return of 10.4% annualized, according to Finaeon’s Global Financial Database . The S&P 500’s comparable return is 10.2% annualized.

Moreover, as the chart below shows, the paths the two indices have taken over the last 130 years are nearly identical.

Read: This overvalued stock market just flashed a rare buy signal

A brief review of some of the differences between the Dow and the S&P 500 helps us appreciate how remarkable it is that their long-term returns are so similar.