Research hasn't tended to endorse the ideas of coffee as detrimental to your health — sometimes it suggests quite the opposite — but this one scientific endeavor should warm the hearts of any dedicated java fanatic.
This experiment, the story goes, was conducted on identical twins sentenced to death in Sweden in the second half of the 18th century. King Gustav III, who ruled from 1771 to 1792, wanted to prove his conviction that coffee was a sort of poison for humans, so he offered to commute the sentences to life in prison if one brother would drink three pots of coffee a day and the other three pots of tea. Two doctors were assigned to monitor the experiment. Here I'll hand the story off to Malcolm Gladwell, in a July 2001 New Yorker article:
Unfortunately, the two doctors in charge of the study died before anyone else did; then Gustav was murdered; and finally the tea drinker died, at 83, of old age — leaving the original murderer alone with his espresso, and leaving coffee's supposed toxicity in some doubt. [Gladwell]
The story may not be true, or it may be exaggerated. But Gladwell isn't the only reputable writer to relay the tale. Mark Pendergrast also includes it in his authoritative tome, Uncommon Grounds: The History of Coffee and How It Transformed Our World. Pendergrast notes that the health claims of coffee purveyors has also sometimes pushed the bounds of credibility. But this story Pendergrast passes on is probably close to the truth:
A centenarian Frenchman was told that coffee, which he drank to excess, was a poison. "If it is poison," he said, "I am a fine example of the fact that it is a very slow poison." [Uncommon Grounds]