Here's a little something I want you keep in mind as you read the rest of this column. Seventy years ago a single Bic Cristal pen cost the equivalent of one week's wages for the average secretary. Today you can buy a pack of 20 for a tenner. That works out to about 0.03% of the weekly earnings of a secretary. The result of this stunning fall in price or rise in productivity is that Bic sells 7.4 billion writing instruments and other stationery products every year. Brilliant, isn't it? Hang on to that for a few minutes while we move on to something rather more miserable: the state of mind of the people in charge of managing the ridiculously large portfolios of assets held by central banks. They are responsible for $5.5trn worth of wealth. But, according to a new poll reported in the Financial Times last week, they strongly suspect that as the very loose monetary policy of the last decade is reversed and interest rates are allowed to rise, there is a chance they might lose rather a lot of it. This makes them very nervous. As it should. After all, they surely know best what is likely to happen to markets next.

I have often bored you about the September 2011 quarterly report from the Bank of England that predicted that, while quantitative easing (QE) would work on the economy in various ways, the one to which it attached "particular importance" was the "portfolio balance channel". This was code for "bubble creation channel" or more simply, a very sharp rise in asset prices. The purchase of assets under QE, said the Bank at the time, would push up both the price of the assets bought (mostly bonds) but also the prices of other assets. "When the central bank purchases assets, the money holdings of the sellers are increased," it said. "Unless money is a perfect substitute for the assets sold, the sellers may attempt to rebalance their portfolios by buying other assets that are better substitutes. This shifts the excess money balances to the sellers of those assets who may, in turn, attempt to rebalance their portfolios by buying further assets and so on." Think of the money poured into the markets by QE as a mountain of hot potatoes being chucked from one asset manager to another and you get the picture. Potatoes in, prices up. The US Fed has also made no secret of the fact that one of the main points of QE was to shove up bond, house and stock prices and make everyone feel better. The then chairman Ben Bernanke wrote as much in an article in the Wall Street Journal in November 2010. The good news is that this bit of the plan worked. In a paper out a few weeks ago (staff working paper No 720) bank analysts suggested that without QE house prices in the UK would have been 22% lower by 2014 than they actually were. Equity prices would have been 25% lower.