Coffee was wildly popular in Sweden in the 17th century — and also illegal. King Gustav III believed that it was a slow poison and devised a clever experiment to prove it. He commuted the sentences of murderous twin brothers who were waiting to be beheaded, on one condition: One brother had to drink three pots of coffee every day while the other drank three pots of tea. The early death of the coffee-drinker would prove that coffee was poison.
It turned out that the coffee-drinking twin outlived the tea drinker, but it wasn’t until the 1820s that Swedes were finally legally permitted to do what they had been doing all along — drink coffee, lots of coffee.
The cornerstone of the scientific revolution is the insistence that claims be tested with data, ideally in a randomly controlled trial. Gustav’s experiment was noteworthy for his use of identical male twins, which eliminated the confounding effects of sex, age and genes. The most glaring weakness was that nothing statistically persuasive can come from such a small sample.
