But the link between quantitative easing and stock markets is sufficiently indirect to warrant closer attention. Just why is it good for shares if central banks are buying bonds?
Fundamentally, quantitative easing is supposed to be a monetary stimulus, giving the same economic boost as interest-rate cuts. Cheaper mortgage debt puts money into consumers' pockets, while cheaper corporate debt boosts profits. Cuts in bank deposit rates make spending more popular relative to saving, both among consumers and companies. And lower interest rates encourage investors to move capital abroad, devaluing the currency and boosting exports. Assuming consumers and companies do not have reasons to hoard cash - as John Maynard Keynes famously theorised they did in the 1930s - higher profits and disposable incomes should boost economic growth. So far, so familiar.
But whether this has actually happened as a result of quantitative easing is unclear. As ever with economics, there is no test case: we do not know what would have happened without unconventional monetary policy. But the economic response to the central banks' experiments has been at best underwhelming. In the UK, the economy eventually picked up in 2013, led by the housing market and consumer spending. But it is easier to trace that back to the Bank's Funding for Lending Scheme or the government's Help to Buy programme - or just the passage of time - than to quantitative easing. In Japan, the clearest effect of the radical monetary stimulus initiated by the Bank of Japan in April 2013 has been a plummeting yen. That has indeed boosted corporate profits, thanks to the currency translation effect, but not yet market share. Output and prices - the key targets of monetary policy - remain unmoved.
Of course, the Japanese stock market has rallied strongly. Likewise, investors can note the seeming correlation between the size of the US Federal Reserve balance sheet and the path of the S&P 500 since the financial crisis (see graph). Such empirical evidence is presumably why investors have greeted the ECB's move with such enthusiasm. They have concluded that bond-buying works for stock markets – never mind why, or whether the wider economy benefits.
There is one theory - much emphasised by the Bank of England back in 2009-10 - why quantitative easing should boost stock prices. The story goes that purchases of bonds by the central bank encourage portfolio managers to move into riskier assets. So bond fund managers sell sovereign debt to the Bank and buy corporate debt instead; corporate debt funds in turn buy high-yield bonds; high-yield bond funds buy equities.
But this theory remains controversial. A Bank of England paper published in September found evidence of UK insurance companies and pension funds moving into corporate debt, but none that they bought equities. Moreover, as James McCann, European economist at Standard Life Investments, points out, it might work less well in Europe than it did in the UK or US. That's because banks dominate the European financial landscape, and given regulatory pressures these may be less willing to move up the risk-curve than portfolio managers.
To my mind, the key reason why equities remain attractive is as a 'least worst' provider of income in a low-rate world. The FTSE All-Share index is expected to yield 3.7 per cent this year, compared to 1.4 per cent for 10-year gilts (a multi-decade low). Insofar as that so-called 'reverse yield gap' is the product of quantitative easing - which it may be a little - it is a valid reason to buy shares. But it's also vital to remember that share-price increases are not sustainable without an economic recovery that buoys corporate profits. On that score, the ECB's new bond-buying programme may prove yet another disappointment.
