On a scale of one to seven, how well do you understand how a flush lavatory works?

This was a question asked by two Yale psychologists, Leonid Rozenblit and Frank Keil, almost two decades ago. Before I explain why, here’s a follow-up exercise: write down your lavatory explanation in as much detail as you can. You may wish to draw a diagram, or explain it to a friend. Or not.

You may then reflect that you knew a little less than you realised. That was the experience of many of the study’s subjects — and not just for lavatories (why does all the water disappear down the U-bend?) but also for zips, quartz watches, helicopters, speedometers, cylinder locks, piano keys and sewing machines. People felt they understood the mechanisms that surrounded them, but their confidence was severely dented by the simple act of giving them pencil and paper and saying: “Show me.”

The same exercise can be performed with politics. In 2013, Steven Sloman and Philip Fernbach, authors of The Knowledge Illusion, were members of a research team that did just that, inviting people resident in the US to rate their understanding of American policy proposals such as introducing unilateral sanctions on Iran, a cap-and-trade system for carbon emissions and a national flat tax. They also asked people to rate their approval of each policy, which would have been unnecessary for lavatories and zips. (Lavatories are useful, zips self-evidently malevolent.)

Professors Sloman and Fernbach and their colleagues found that — just as with locks and speedometers — people tended to overrate their knowledge at first, and then discover some humility when asked to be more specific.

Perhaps British voters could use a dose of the same medicine when it comes to our understanding of Brexit. Leave or Remain, many of us came late to the realisation that there was a difference between the single market and the customs union. I am still not sure most people can explain what that difference is. Many people have strong views about Prime Minister Theresa May’s withdrawal agreement; rather fewer could give a convincing account of how it differs from the political declaration that accompanies it.

When Mrs May began her premiership with the statement that “Brexit means Brexit”, it dawned on most of us that the details of the whole project might need a little more work. But she wasn’t the only one who was vague.

I’d love to see the contenders for the Conservative party leadership quizzed a little less about their cocaine habits and instead forced to sit down and write a detailed explanation of what a no-deal Brexit actually is. While we wait, perhaps the same exercise could be given to the 160,000 Conservative party members who are about to select the country’s next prime minister.

How long, for example, will HM Revenue & Customs wave through imports without inspections? Will the French reciprocate? What are the implications of “trading under World Trade Organization rules” for the UK’s banking and insurance industries? How large are those industries?

How many other developed countries are content to rely solely on WTO arrangements in their trade with the EU? Is the WTO capable of enforcing the rules anyway, given the current crisis in its appellate body? How likely is the EU to grant permission to British farmers to sell meat, milk or cheese? Would any of these decisions be different if the UK refused to pay the “divorce bill” it had negotiated?

I don’t think it is especially shameful that we ordinary voters are incurious about the ins and outs of Brexit, any more than we should be obliged to understand the workings of a quartz watch. An ability to read the time is generally sufficient. But I am stunned by just how little we seem to demand of our political leaders.

We want tailors to understand sewing machines, locksmiths to understand locks and plumbers to know that a lavatory is basically a siphon. But our standards for politicians seem far lower. The next prime minister is likely to be a person who believes that if we demanded it with enough gusto, sewage would remove itself from our homes in some scatological remix of Mary Poppins — and that anyone who tells you otherwise is clearly a shill for Big Porcelain.

We should expect more of anyone who wants to lead the country. And since our politicians have grown so fond of punting the hard questions back to us, perhaps we should also demand more of ourselves.

Profs Sloman and Fernbach found that asking people to explain the workings of the policies they so fervently supported or opposed had a humbling effect. When people realised that they knew less than they had once believed, they quite reasonably wound their necks in as a result. It seems strange to die in a ditch for something we can’t clearly explain, even to ourselves.

Next time you find yourself in some heated political debate, perhaps you should suggest that both sides pause to explain the policy in question. You may find you understand less — and agree more — than you realised.

Written for and first published in the Financial Times on 21 June 2019.

My book “Fifty Things That Made the Modern Economy” (UK) / “Fifty Inventions That Shaped The Modern Economy” (US) is out now in paperback – feel free to order online or through your local bookshop.