Retirees shouldn't be afraid to put money into the stock market, says Fisher Investments' founder Ken Fisher. The securities have the potential to provide a much higher return than what's available elsewhere.

Seniors are often put off by what they see as a complicated investment opportunity, however, preferring instead to put their money in long-term bonds, which they see as less risky. That strategy is a mistake, Fisher told Jim Cramer during TheStreet's "Investing for Your Future" webcast on Wednesday, June 22.

"You've got 25 years," he said, using a 70-year-old who planned to live to 95 as an example. "What does well in 25-year periods? Well, it's not bonds with 2.1% yields."

Making the right choices for retirement investments is becoming more important than ever, as years of low interest rates curb portfolio returns while medical advances allow people to live longer. A study by McKinsey & Co. last year found millennials will have to work seven years longer than their parents or double the percentage of income they save to achieve the same wealth at retirement.

Currently, about 46 million U.S. residents are 65, the traditional retirement age, or older, according to recent U.S. Census Bureau data. That figure will more than double by 2060, the bureau projects.

Fisher noted that many seniors may view the stock market as too complicated to invest in efficiently. But he said that the most important aspects of investing are simple.

"It's very easy for people to focus on the complexity instead of the simplicity and forget about what the most basic, simple things are," he said. 

To Fisher, the simplest question is: Do you believe earnings will increase? That is a question even "normal" people, without any particular expertise in financial markets, can answer, and it's the key to determining the value of a company's stock.

Cramer, TheStreet's founder, has criticized bond-buying himself in the past, saying it's not a good strategy for building wealth.

He stuck to his criticism Wednesday, asking Fisher: "Do you think people understand how bad -- I'm not going to say dangerous -- but how useless bonds are in terms of trying to be able to create wealth?"

Fisher didn't say, but noted that bonds may be useful for other purposes, such as stabilizing volatility in the short term. 

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