From Mr Tim Murphy.

Sir, As one reads “Farmers doomed to pay price for export restrictions” (April 18) and sees the unexpected consequences of market intervention, one might recall Ray Bradbury’s chilling short story A Sound of Thunder, where characters pay the price for travelling back in time and disrupting the course of natural events. The familiar flaw in these two tales? Meddling in infinitely complex systems such as the global market or the space-time continuum leads inexorably to unforeseen and often unmanageable consequences.