by Richmond Fed
The standard theory of household-portfolio choice is hard to reconcile with the following facts: (i) Households hold a small amount of equity despite the higher average rate of return. (ii) The share of risky assets increases with the age of the household. (iii) The share of risky assets is disproportionately larger for richer households. Young workers, on average asset poor, face larger labor-market uncertainty because of high unemployment risk and imperfect knowledge about their earnings ability. This labor-market uncertainty prevents them from taking too much risk in the financial market. As the labor-market uncertainty is gradually resolved over time, workers can take more financial risks.
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Source: http://www.richmondfed.org/publications/research/working_papers/2014/pdf/wp14-13.pdf