Investing in stocks risks just like many other asset classes. Stocks can go up or down for any number of reasons. In a bull market, equities go high and higher for many months or years in a row. The same thing occurs in the other direction in bear market. The duration of a bull or a bear market varies every time. If a bear market drags on for years then an investor that needs to take out their capital from the market may be in a difficult situation. Selling stocks when markets are down can lead to substantial losses. I came across the following infographic that shows:

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Source: Charles Schwab