Tim Worstall demonstrates that the myth of speculators causing food price spikes, i.e. increase market volatility, is just that: A myth. Banning speculation will not decrease volatility, it will increase it. In other words: Banning speculation will decrease market stability.
To proof his case, he takes real world data. He shows a chart (reproduced below) comparing monthly price changes in crude oil, where speculation is allowed, with price changes in onions, where speculation has been banned for 50 years now. Spot the difference?