Shoppers heading to their local mall to buy socks would generally buy fewer of them if they found that prices had, say, doubled. Investors are the opposite of shoppers. They tend to get excited by higher prices for financial assets. If prices of stocks double, it is likely that investors will want them more. And the more they go up, the more investors tend to want them in a phenomenon known as “fear or missing out,” or “FOMO” for short.