The evidence tilts in Montessori's favor. A 2006 study of 112 students in a Montessori school and conventional public schools in Milwaukee found that the Montessori students performed significantly better on both cognitive and social measures.
Half of the students in the study were 5 years old, and half were 12. The Montessori 5-year-olds performed better than those their age at other schools when it came to identifying letters and words, solving basic math problems, and ordering and categorizing. The young Montessori students interacted more positively on the playground and were more likely to deploy reasoning in social negotiations, often with appeals to abstract values such as justice and fairness. The researchers found no differences between the spatial reasoning, vocabulary, and concept formation skills between the two groups of 5-year-olds.
The differences between the two groups of 12-year-olds were less pronounced, but still present. Essays written by Montessori students used more complex sentence structures and were rated as more creative, but the students in the conventional public schools appeared to have "caught up" on many of the researchers' other measures. The Montessori students tended to select more constructive responses to hypothetical social problems, and they reported feeling a stronger sense of community at their school.
In a 2006 interview with Scientific American, University of Virginia psychologist Angeline Lillard, who led the study, speculated that the less-conspicuous differences in academic performance between the Montessori and non-Montessori 12-year-olds could have been a result of the school being only three years old when the 12-year-olds enrolled back in 1997. Lillard noted that it takes time for a school to put Maria Montessori's method into practice.