America's consumer-confidence index has fallen to a 28-year low, which suggests that consumer spending will fall. This, in turn, will spur firms to cut costs and limit pay rises.

Just a 10 percent increase in body mass index, a measure of weight relative to height, can cut a man's real earnings by 3.3 percent and a woman's by 1.8 percent, according to economists.

The BPP index declined significantly on the day that Lehman Brothers collapsed in September 2008 as online retailers in America immediately cut prices.