Using Consensus Economics survey data on JPY/USD and GBP/USD exchange rate expectations for the 3- and 12-month horizons over the period November 1989-December 2012, we first show that expectations fail the conventional tests of unbiasedness and do not exhibit a learning process towards rationality. Our approach is consistent with the economically rational expectations theory (Feige and Pearce, 1976), which states that information costs and agents' aversion to misestimating future exchange rates determine the optimal amounts of information on which they base their expectations. The time variability of the cost/aversion ratios justifies at the aggregate level a representation of expectations as a linear combination of the traditional extrapolative, adaptive and regressive processes augmented by a forward market component, whose parameters are allowed to change over time. This mixed expectation model with unstable heterogeneity is validated by our Kalman filter estimation results for the two currencies and the two horizons considered. Although the relative importance of the fundamentalists' (chartists') is found to increase (decrease) with the time-horizon, chartist behaviour appears to dominate fundamentalist behaviour for both horizons. Central bank intervention is then effective in stabilizing the foreign exchange markets if it encourages fundamentalist activity.