Many studies have been conducted that link music education to learning development and success in other areas.  Here are some interesting facts:

Students who were exposed to music-based lessons scored a full 100 percent higher on fractions tests than those who learned in the conventional manner. Second-grade and third-grade students were taught fractions in an untraditional manner ‹ by teaching them basic music rhythm notation. The group was taught about the relationships between eighth, quarter, half and whole notes. Their peers received traditional fraction instruction.
Source: Neurological Research, March 15, 1999

Young children with developed rhythm skills perform better academically in early school years. Findings of a recent study showed that there was a significant difference in the academic achievement levels of students classified according to rhythmic competency. Students who were achieving at academic expectation scored high on all rhythmic tasks, while many of those who scored lower on the rhythmic test achieved below academic expectation.
Source: "The Relationship between Rhythmic Competency and Academic Performance in First Grade Children," University of Central Florida, Debby Mitchell

A ten-year study, tracking more than 25,000 students, shows that music-making improves test scores. Regardless of socioeconomic background, music-making students get higher marks in standardized tests than those who had no music involvement. The test scores studied were not only standardized tests, such as the SAT, but also in reading proficiency exams.
Source: Dr. James Catterall, UCLA, 1997

"Orchestra at the Opera" by Edgar Degas

Music training helps under-achievers. In Rhode Island, researchers studied eight public school first grade classes. Half of the classes became "test arts" groups, receiving ongoing music and visual arts training. In kindergarten, this group had lagged behind in scholastic performance. After seven months, the students were given a standardized test. The "test arts" group had caught up to their fellow students in reading and surpassed their classmates in math by 22 percent. In the second year of the project, the arts students widened this margin even further. Students were also evaluated on attitude and behavior. Classroom teachers noted improvement in these areas also.
Source: Nature May 23, 1996

The world's top academic countries place a high value on music education. Hungary, Netherlands and Japan stand atop worldwide science achievement and have strong commitment to music education. All three countries have required music training at the elementary and middle school levels, both instrumental and vocal, for several decades. The centrality of music education to learning in the top-ranked countries seems to contradict the United States' focus on math, science, vocabulary, and technology.
Source: 1988 International Association for the Evaluation of Educational Achievement (IAEEA) Test

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