"Someone conducted a study: from 1980 to 2015, 'Imagine two people each investing $1,000 in the S&P 500 in 1980. The first person never sells after buying; the second person is more cautious: sells everything whenever the market drops more than 5%; buys everything back after the market rises 3% from its trough. In the end, the first investor's account balance is $18,635, while the second investor has only $10,613.'"