Economics
Why a haircut costs more in Zurich than in Hanoi
Rich countries have higher overall price levels not because everything is pricier, but because highly productive tradable industries push up wages economy-wide, and those wages spill into local services like haircuts, which can't be outsourced. Béla Balassa and Paul Samuelson independently modeled this in 1964, showing why cost-of-living gaps reflect productivity, not just currency values.