Before London taxi drivers are allowed to convey paying customers in their renowned black cabs, they must be ‘of good character’, in a reasonable state of physical health, and have spent, on average, about three years studying for a gruelling examination that tests their spatial awareness of all the city’s streets, shortcuts, and famous and not-so-famous landmarks and locations. This process extends well beyond remembering the basic street layout; candidates must be able to determine the shortest route between any two locations in the city, and may be asked to pinpoint one of around 25,000 different points of interest, or ‘points’, pieced together in a jigsaw-like series of 320 set routes, or ‘runs’, that crisscross the ancient capital. This body of learning is known as “The Knowledge of London”, or simply “The Knowledge”. Of those starting to learn the Knowledge, only 30% will eventually succeed in gaining It and passing the examination to become bona fide London cabbies.

Inevitably, the existence of over 20,000 heads so thoroughly stuffed with such a distinct pattern of learning proved irresistible to neuroscientists. In 2011, they used MRI imaging to inspect the brains of trainee London taxi drivers before and after Knowledge acquisition. They showed that one particular structure, the hippocampus⁠⁠— which is concerned with spatial memory and navigational ability⁠⁠— became measurably larger in individuals who had successfully attained the Knowledge and gained cabbie status. For many years, scientists believed such changes were impossible in adults, and that only foetuses and children could manage such brain-changing feats.