The evils of futures markets:

In the 1950s, onion growers were often shocked at the low prices they were getting. Casting around for a villain to blame, they alighted on derivatives traders, and they persuaded Congress to ban any futures trading in onions.

Today onions are the only commodity for which futures trading is banned. Not coincidentally, onion prices remain extremely volatile: they doubled in 2008, and then fell by 25 percent in 2009.