It's not just George Osborne who has got some thinking to do after the Lords' vote against his plans to cut tax credits. So too has the media, including the BBC.

What I mean is that their immediate reaction to July's Budget was generally positive. Not many journalists flagged up the looming trouble over tax credits. This poses the question: why did they fail?

Part of the answer is simple pressures of time, exacerbated in some cases by spin doctors not going out of their way to paint a clear picture. But there were also three other things going on.

One was a failure of maths. Pretty much the first thing you should do after a Budget is to look at the Red Book's costings of the policy measures. These tell you which of the Chancellor's announcements involve serious money and which don't.An immediate look at these in the summer (table 2.1) would have told you that the cuts in tax credits were big - a cumulative £20bn between 2016 and 2020. That should have set some bells ringing. But what of Osborne's claim that the higher living wage would offset these losses? The natural reaction should have been: "what does the OBR say about this?" A quick look would reveal that they reckon this would add £4bn to wages by 2020; par 8.10 of this pdf. You don't need to be a maths genius to work out that that there's a big gap here. But this was beyond many political journalists.

A second problem, I suspect, was a framing effect. We have, said Adam Smith, "a disposition to admire, and almost to worship, the rich and the powerful." This predisposes even "neutral" journalists towards the Chancellor. And the fact that the election victory loomed larger in their memories than the "omnishambles" Budget of 2012 exacerbated this tendency to exaggerate Mr Osborne's talents.

Thirdly, there's the heuristic of social proof: if other people seem to believe something, we tend to at least sympathize with that view. It was perfectly natural to look at Tory MPs cheering the Budget - remember IDS's reaction? - and infer that it was, from their point of view, a good one. This too would have made it hard to see that there was a big problem with it.

My point here is that the favourable coverage of July's Budget was not merely because journalists were Tories - though of course many were. It's also because there were psychological mechanisms at work which led them to miss what, in retrospect, was the big story. I too - in the day job but not the blog - was guilty here. We should, therefore, question whether even non-partisan journalists are doing a sufficiently good job of scrutinizing the government.