I just came across Kevin Rathunde and Mihaly Csikszentmihalyi’s academic study, published American Journal of Education. From the abstract:
“Multivariate analysis showed that the Montessori students reported greater affect, potency (i.e., feeling energetic), intrinsic motivation, flow experience, and undivided interest (i.e., the combination of high intrinsic motivation and high salience or importance) while engaged in academic activities at school. The traditional middle school students reported higher salience while doing academic work; however, such responses were often accompanied by low intrinsic motivation. When engaged in informal, non-academic activities, the students in both school contexts reported similar experiences.”
Kevin Rathunde and Mihaly Csikszentmihalyi. 2005 (May). “Middle School Students’ Motivation and Quality of Experience: A Comparison of Montessori and Traditional School Environments.” American Journal of Education 111, 341-371.