One of the most egregious of cognitive biases is the confirmation bias - our tendency to see what we want. We're seeing examples of this today, with Brexiteers cheering FTSE's rise whilst Remainers point to sterling's fall.

In truth, these are two sides of the same coin. One reason why the Footsie has risen might well be that sterling's fall is expected to be permanent, and so will raise overseas earnings - which account for around three-quarters of the FTSE 100's total earnings. It's no accident that today's biggest risers include a lot of overseas earners, such as Pearson, Standard Chartered and Royal Dutch.

We can put this in more sophisticated terms. Uncovered return parity (pdf) tells us that, in the absence of shocks, equity returns should be equalized across countries so that a relatively strong stock market should be accompanied by a weaker currency and vice versa.

But of course, that phrase "in the absence of shocks" is doing a lot of work. If a country were to enjoy a positive growth shock, its stock market should rise relative to others in common currency terms. This is because investors would buy the equities in the hope of rising dividends, and perhaps the currency too in anticipation of higher interest rates as a consequence of that faster growth.

So, is this what the UK has enjoyed? No. Quite the opposite. Since June 23, MSCI's index of UK stocks has fallen by 3.3 per cent in US dollar terms* whilst the MSCI world index has risen 2.7 per cent.

This underperformance of six per cent is consistent with the sort of long-term hit to GDP which NIESR and the CEP (pdf) expect from Brexit. Is that a coincidence?

Granted, Brexiteers are right to say that the lower pound will stimulate exports and output. But Remainers are also right to say that the pound has fallen precisely because investors expect weaker growth as a result of Brexit: it's fall in the last few days might well be due to the perceived increased prospect of a "hard Brexit".

The fact that UK equities have under-performed in common currency terms since June 23 tells us that the stock market believes that, on balance, the latter effect outweighs the former. This is consistent with empirical evidence which tells us that past falls in sterling haven't had a huge impact on net exports**.

If stock markets are saying anything, it is that they agree with mainstream economists that Brexit will harm the UK.

Let's put this in context, though. The UK's underperformance since June is actually quite small in the context of its under-performance since mid-2014. That's due in part to the UK's bigger weighting in mining stocks. But it might also be because expectations for relative growth were falling even before this June: small caps and the FTSE 250 are well down in dollar terms since mid-2014.

I must, however, caveat all this massively. Drawing strong inferences from noisy and complex phenomena is dangerous, and downright daft when those inferences are politically motivated. And paying attention to short-term market fluctuations is almost to invite error - in particular, the mistake of myopic (pdf) loss aversion (pdf). Charles Kindleberger used to say that "Anyone who spends too much time thinking about international money goes mad." The same applies to stock market moves.

* I've updated this to include today's rise in UK shares and fall in sterling.

** Yes, the UK economy did nicely after we left the ERM in 1992. But this was probably due more to the monetary and fiscal easing than to the pure exchange rate effect.