Amid the thin trading volumes of a typically sluggish August, investors have been caught off balance as the normal relationships between US equities, government bonds and commodity prices have broken down.

At the end of July, the S&P 500 equity composite was below 1,000 points and the price of crude oil loitered below $70 a barrel. During August, the S&P and oil have risen to their highest levels in 10 months, amid hopes of a sustained recovery in the economy.