As many of the world's central bankers and some of the great economists gather in Jackson Hole to compare notes about the state of their own and the global economy, it is enlightening to consider the backdrop to the meeting.

Countries that account for about 60% of global GDP -- that is, the US, UK, Canada, the eurozone and Japan -- have had their policy interest rates at 1% or less for close to four years. In the case of Japan, it is nearly two decades of zero interest rates.