Do futures markets have a stabilizing or destabilizing eect on commodity prices? Empirical evidence suggests that both effects are possible. We investigate this question further using a learning-to-forecast experiment with two coupled markets: a spot market and a futures market. The first exhibits negative feedback between forecasts and prices, associated with stable prices, while the second market is of the positive feedback type, which makes it susceptible to bubbles and crashes. The results show that the effect of a futures market on spot price stability changes non-monotonically with the coupling strength. When the coupling is weak the futures market has a stabilizing effect on spot prices. Under those circumstances increasing the coupling strength reduces spot price volatility. However, at larger coupling strengths this trend reverses and the effect of the futures market on spot prices becomes destabilizing.