Ability, willingness and need to take risk
Author
Larry Swedroe defines asset allocation as "the process of investing assets in a manner reflecting one’s unique ability, willingness and need to take risk," with willingness referring to risk tolerance and whether investors have the “fortitude and discipline” to stay with an allocation during market downturns.
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Ability to take risk involves the investment time horizon, need for liquidity, stability of earned income, and the flexibility to adapt if there is a need for a plan B.
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Willingness to take risk is characterized as the eat well/sleep well trade-off. Taking more risk is required to enable the possibility of higher expected returns (eat well). However, if investors take more risk than they are emotionally able to handle, then it is likely that they will abandon their investment plans if their portfolios suffer sufficiently severe losses. So it is unwise for investors to take so much risk that they will be unable to sleep well during the inevitable stock market downturns.
Need to take risk is determined by the rate of return required to achieve financial objectives. Any investor deciding to take more risk because of perceived "need" should do so keeping in mind that taking extra risk could well backfire and lead to lower returns. Perhaps more importantly, once the investor has "won the game" by accumulating sufficient wealth, it is unwise to take more risk than is needed, since the value of additional gains is much less important than the consequences of severe losses.