By Guy Monson, chief investment officer at Sarasin & Partners

The peak of the US stock market was as recent as 19 February. The average time to enter a bear market is 136 days.

This time, it took little more than 20 trading days. For investors, the speed of decline is particularly disorientating.

However, three things are worth remembering.

First, the 27 percent fall from peak for the world equity index probably discounts much of the damage and the impact will ultimately be temporary.

Second, the government and central bank response will be massive. The fiscal response could be...