In Merton (1987), idiosyncratic risk is priced in equilibrium as a consequence of incomplete diversification. We modify his model to allow the degree of diversification to vary with average idiosyncratic volatility. This simple recognition results in a state‐dependent idiosyncratic risk premium that is higher when average idiosyncratic volatility is low, and vice versa. The data appear to be consistent a positive state‐dependent premium for idiosyncratic risk both in the US and other developed markets.

Idiosyncratic risk factor models risk premium asset pricing G11 G12