A study by E. Glenn Schellenberg at the University of Toronto at Mississauga, as published in a 2004 issue of Psychological Science, found a small increase in the IQs of six-year-olds who were given weekly voice and piano lessons. Schellenberg provided nine months of piano and voice lessons to a dozen six-year-olds, drama lessons (to see if exposure to arts in general versus just music had an effect) to a second group of six-year-olds, and no lessons to a third group. The children’s IQs were tested before entering the first grade, then again before entering the second grade. 
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Surprisingly, the children who were given music lessons over the school year tested on average three IQ points higher than the other groups. The drama group didn’t have the same increase in IQ, but did experience increased social behavior benefits not seen in the music-only group.