The question is more than just a parlour game. It shows how tricky it is to compare living standards over time. Yet such comparisons are not just routinely made, but rely heavily on a single metric: gross domestic product (GDP). This one number has become shorthand for material well-being, even though it is a deeply flawed gauge of prosperity, and getting worse all the time (see article). That may in turn be distorting levels of anxiety in the rich world about everything from stagnant incomes to disappointing productivity growth.