If Donald Trump were your ageing father, when would you take away his car keys?

Presidential decisions can mean life or death for millions around the world, that’s why constitutional safeguards exist. But do they work in practice?

President Donald Trump recently interrupted a cabinet meeting in the middle of a war to ramble on at length about a conversation he supposedly had with the head of the Sharpie pen company over supplying bespoke presidential felt-tips, of which the firm said it could find no record. Photo: AP/Mark Schiefelbein

Donald Trump’s cognitive skills are amazing. So amazing! So great! 

So much better than any other dumb presidential contender you could mention, at least according to Trump himself, who bragged once again last week of how he had repeatedly aced what he calls “a very hard test for a lot of people”. (It’s thought he means a screening tool for mild cognitive impairment in elderly people.)