Planning for retirement takes a lifetime of hard work. But most people make a crucial mistake that can cost add up to hundreds of thousands of dollars less in their retirement nest eggs after they leave their careers.

In the following segment from their video guide to investment planning, Motley Fool director of investment planning Dan Caplinger talks with Fool markets/IP bureau chief Mike Klesta about this crucial mistake. Dan notes that most people are too conservative with their retirement investments, especially as they get closer to retirement. Although the impulse to avoid major losses of investment capital is natural, Dan notes that the long lifespans after people retire makes it necessary to consider putting more money into growth assets like stocks, relying less on bonds and cash. Combined with Social Security to help address the risk of outliving your money, Dan concludes that being willing to invest more aggressively -- even if it's in relatively conservative stocks -- can add up to huge amounts of extra money over the course of your lifetime.

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