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There are plenty of stupid investments you can make in this world. Stock in Pets.com and Washington Mutual didn't work out so well. Nor did those Las Vegas condos. Some of the social media and Web 2.0 stocks flying high on Wall Street will probably follow suit. But the most foolish investment of all may be right in front of you. And there's a worrying chance you're buying it. The investment? Stock in your own employer.

Late last year the Employee Benefits Research Institute, a think tank, published its most recent...