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Why would investors choose a large weighting in the stock of their employer? Holding employer company stock is very risky. It is optimal only for employees with very low risk aversion or with very high return expectations for company stock. Rational one-period models cannot explain high observed weightings in company stock in defined-contribution pension plans. Nor can a behavioral interpretation. Some factors must be missing from rational choice models to explain employer company stock holdings.
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Douglass et al. (2004) studied this question.
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