this is a great point. A lot of people will go chasing the best mutual fund or best stock but they end up buying what was great yesterday or last year. As a result the average investor gets less than what the market returns (also you need to account for fees).
I know more than a few people who sold their stocks during the Covid crash or during the great recession when the stocks were way down and they they pop back in after they saw the recovery and saw they had made a huge mistake. Bear in mind these 'nest eggs' were not being used on one case the person was still working so the retirement money was for years down the road. Also why people in retirement usually have a separate spending bucket of cash or cash and bonds that they refill when stocks are high so they can avoid selling during a steep crash.
For the twenty years ending 12/31/2015, the S&P 500 Index averaged 9.85% a year. A pretty attractive historical return. The average equity fund investor earned a market return of only 5.19%.
Why is this?
Investor behavior is illogical and often based on emotion. This does not lead to wise long-term investing decisions. Here's an overview of a few typical money-losing moves that average investors make.
Buying High
Study after study shows that when the stock market goes up, investors put more money in it. And when it goes down, they pull money out. This is akin to running to the mall every time the price of something goes up and then returning the merchandise when it is on sale - but you are returning it to a store that will only give you the sale price back. This irrational behavior causes investor market returns to be substantially less than historical stock market returns
What would cause investors to exhibit such poor judgment? After all, at a 9% return, your money will double every eight years. Rather than chasing performance, you could simply have bought a single index fund, and earned significantly higher returns.
Overreacting
The problem is the human reaction, to good news or bad news, is to overreact. This emotional reaction causes illogical investment decisions. This tendency to overreact can become even greater during times of personal uncertainty; near retirement, for example, or when the economy is bad. There is an entire field of study which researches this tendency to make illogical financial decisions. It is called behavioral finance. The study of behavioral finance documents and labels our money-losing mind tricks with terms like "recency bias" and "overconfidence."
With overconfidence, you naturally think you are above average. For example in one study, 81% of new business owners thought that they had a good chance of succeeding, but that only 39% of their peers did. In another study, 82% of young U.S. drivers considered themselves in the top 30% of their group in terms of safety.
When it comes to investing, overconfidence causes investors to exaggerate their ability to predict future events. They are quick to use past data, and to think they have above average abilities that enable them to predict market movements into the future.