Phil recently tweeted that one problem with Starmer is that he doesn't realize that "politics isn't the same as running the DPP." This hints at a problem many of us have.

Phil's right. Leading the Labour party isn't like running the DPP. For one thing, the party isn't as hierarchical as the DPP: people don't simply do as they are told. And for another, a Labour leader needs to be a salesman, whereas the boss of the DPP does not. Moving from the DPP to the Labour leadership is like a boss moving from a monopoly utility to a growth company needing to catch customers' attention in a competitive environment. The two jobs require different skill sets.

What's more, the forensic skills Starmer acquired as a lawyer aren't necessarily very useful to him now. He needs to intuit what voters will want in 2024. There's not much hard evidence here, so an ability to handle a mountain of it isn't much value.

Starmer's failure to fully appreciate these shortcomings are, though, forgiveable because it's a widespread error. The French call it deformation professionelle. Our professional training and experience don't just give us particular skills. They also inculcate into us a mindset that whilst useful in some contexts is a menace in others. If your only tool is a hammer, everything looks like a nail.

We see the same thing in Johnson. A newspaper columnist must tell his readers what they want to hear and be superficially persuasive, but need not bother with detail, implementation or administration. He expresses a view and moves on without suffering any penalty for being wrong. Johnson is merely importing the characteristics he displayed at the Telegraph into Downing Street. He is proof of the fact that sometimes the skills you need to get a job aren't the same as those you need to do it well.

Some lawyers are prone to professional deformation in that they are inclined to over-estimate the role of the law in social change. The "commitment" to spend 0.7% of GDP was an act of parliament. But as David Allen Green (a lawyer too smart to fall into this trap) says, this was "useless, useless, useless." Useless it might have been, but unprecedented it was not. The 2010 Child Poverty Act tried to legislate away child poverty. But of course it failed. Abolishing poverty requires much more than posh people's words.

Some engineers make the same error. They are apt to see design where there is in reality emergence and to seek more control than is possible. Diego Gambetta and Steffen Hertog show (pdf) that engineers tend to "take more extreme conservative and religious positions" - even to the point of being disproportionately Islamist terrorists:

Engineering as a degree might be relatively more attractive to individuals seeking cognitive ''closure'' and clear-cut answers as opposed to more open-ended sciences - a disposition which has been empirically linked to conservative political attitudes...Engineering is a subject in which individuals with a dislike for ambiguity might feel comfortable.

Similarly, doctors (like other professionals) can over-estimate their own expertise and be arrogant and patronizing towards laypeople.

And of course even good journalists are prone to biases such as overweighting individual agency to the neglect of emergence, focusing on salient events rather than slow-moving change, prioritizing scoops over improving public understanding or being too deferential to the (wrong?) experts. Such biases are important as they colour public attitudes to politics.

Now, when I write about cognitive biases it is usually a memo to myself. And this is true here. Economists are also prone to professional deformation. Our fancy models which told us that globalization was a net good led us to under-estimate the prevalence and anger of losers from the process. Our emphasis on incentives can blind us to the fact that people's motivations are often more complex. And my education and formative years led me to think that politics should be about economic policy which leaves me befuddled now that this is no longer the case.

None of this is to say that engineers, lawyers, doctors or economists have no place in policy-making. They emphatically do: heck, if we stretch the point there might even be a place for journalists. Instead, we must recognize that the perspectives of even the smartest of us are partial and biased.

What matters, therefore, is cognitive diversity. The lack of it can be dangerous. It's possible perhaps that if Blair and Bush had been more influenced by good military minds the Iraq war wouldn't have been so disastrous. And one reason why Brexit has been such a mess is that the decline of trades unions meant that the Tory party had lost people who had experience of tough negotiations and bargaining, to be replaced by those who thought that shouting like a toddler could get you want you wanted.

It's too much to ask of anyone that they overcome the biases and partialities inculcated by years of training. We should, however, ask of decision-making processes in any organization that they harness diversity as a counterweight to professional deformation. It's not obvious, however, that our political institutions do this.