A consensus is emerging that returns to the currency carry trade are driven by two factors. One of these is clearly consumption risk but there is disagreement about the identity of the remaining factor. This paper bolsters the case for volatility being the unknown factor. A structural model that specifies that monetary volatility is the second factor is tested for 42 monetary regimes using the artificial economy methodology. The negative slope in the Fama regression arises when monetary volatility is low and the precautionary savings motive dominates the intertemporal substitution motive. When monetary volatility is high, the Fama slope is positive in line with uncovered interest parity. We conclude that, given the predominance of precautionary savings, the degree of monetary volatility explains whether uncovered interest parity holds.