The word bubble' gets tossed about a lot by the financial media.
It's become quite an emotionally-loaded term. People often use it to dismiss an asset class they disapprove of (bitcoin is a bubble'), or a bull market they're not involved in.
It's also an over-used way to draw attention in headlines, as in Is this dotcom bubble 2.0?'
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If it's to be useful to us as investors, we need something a bit more scientific to let us know whether or not a market is overvalued enough to qualify as a bubble.
Thankfully, US wealth managers GMO are experts in bubbles and they've got the perfect definition.
What is a bubble?
In non-statistical talk, that's when a market gets unusually overvalued compared to the average. It's the kind of thing you'd expect to happen very rarely once every 44 years, to be precise.
The important thing about bubbles, of course, is that they burst (if they didn't, they'd be safe to invest in).
And what GMO found was that in all the major bubbles they identified markets that had hit this two standard deviations level the bubble went on to pop. The markets in question "eventually retreated all the way back to the original trend that had existed prior to each bubble".
Examples include US stocks in 1929, Japanese commercial property in 1991, US housing in 2005/06, and of course, the global bubble of 2008.
The question today of course is: are we at bubble levels again?
Grantham is well-known for being fairly bearish on the market. But he did call the bottom in March 2009, so he's not a stopped clock or a perma-bear' by any means.
While US stocks look overvalued on the cyclically-adjusted price/earnings ratio (CAPE),they are still only 1.4 standard deviations from the mean. In other words, they're expensive but they're not at tech bubble levels yet. To get to bubble' levels, Grantham reckons the S&P 500 would have to hit around 2,250.
How the S&P 500 could hit 2,250
the Federal Reserve tapering' off quantitative easing
But the other issue is that we're not far off entering the third year of the US electoral cycle. Barack Obama was re-elected in November 2012, so we're near the halfway point for his latest term. And that's when the campaigning will start in earnest.
Why does the third year matter? It's all about politics. No serving president wants the economy to go pear-shaped in the run-up to the election. So the Fed becomes particularly willing to bail the market out if things look like turning nasty.
This might sound like a load of market-timing voodoo, and I have to admit I tend to be sceptical of these things. But the evidence since 1964 is quite striking. Since then, on average, the S&P 500 returned 2% in the first year of the cycle, and lost 5.7% in the second year.
In the fourth year, the average gain was 5.6%. For the third year a whopping 20.9%. During the third year, it also pays off to be in the riskiest stocks, which returned 26.2% on average. They typically lost money in every other year of the cycle.
In other words, if you're going to take a punt, do it in the third year of a presidential cycle, because that's when the Fed is most willing to catch you if you fall.
You don't need to invest in the US to profit from American gains
Grantham reckons that, although the market is overvalued, it is likely to have a strong run from around October this year, up until the next election. By that point it "will have rallied past 2,250, perhaps by a decent margin".
Then the bubble will burst, taking the market back down to "around half of its peak or worse, depending on what new ammunition the Fed can dig up".
That's an unpleasant prospect for a value investor. I don't really like the idea of buying an expensive market on the basis that it's likely to get more expensive. For a start, it relies on our best guesses at the behaviour of crowds being accurate. For another, if you want to make money from it, you also have to sell at the right time.
The good news is that, even if you're tempted to time the market, you don't have to invest in the US to benefit from the US presidential cycle. With the US being the most important stock market in the world, the rest of the developed markets tend to do well too during the third year of the cycle. Britain, Europe and Japan all tend to do better than average as well, and they're all cheaper than the US market.
And of course, if you want a really cheap, despised market, you could buy Russia as my colleague Merryn Somerset Webb notes.
Also, for more great investment ideas, if you missed the MoneyWeek conference on Friday,you can get a hold of all the speeches and notes here. Many of the attendees told me it was our best conference yet and I have to say I'd agree with them.
High points included Jim Mellon, with a fascinating and inspiring talk on the incredible advances being made in medical technology; and Terry Smith, with his brutal but spot-on critiques of the fund management industry.
It's really worth listening to the lot find out how to get hold of them here.
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