Two researchers, Edward Vul and Harold Pashler, had the idea of asking people to answer this question (and many similar ones) not once but twice. The subjects were not told the first time that they would have to guess again. Vul and Pashler’s hypothesis was that the average of the two answers would be more accurate than either of the answers on its own. The data proved them right. In general, the first guess was closer to the truth than the second, but the best estimate came from averaging the two guesses.
