Are forward exchange rates unbiased predictors of future exchange rates, and if not why?

If there is perfect capital mobility and if exchange rate expectations are unbiased, the forward exchange rate is an unbiased predictor of the future exchange rate. We look at the link between the forward exchange rate and the future exchange rate at different horizons (1 to 5 years) and for two exchange rates (dollar/euro, yen/dollar). We see that the forward exchange rate does not provide any usable information on the future spot exchange rate. The bias between the forward exchange rate and the future exchange rate can be explained by imperfect capital mobility, which introduces the external debt ratio into the relationship between forward exchange rates and future exchange rates.