Trickey and Topping (2004) detail a large-scale study in Scotland where children participated in a weekly P4C session.
Statistically significant increases in children’s IQ scores over a period of a year were identified, compared with no increase in the scores of the control group.
The same study showed significant gains in verbal and non-verbal reasoning and along with improvements in listening, communication, behaviour, questioning, reasoning, reading, writing and understanding.
A closing of the gap between low- and high-achievement within classes was also found.
Trickey, S. & Topping, K.J. (2004) Philosophy for children: a systematic review. Research Papers in Education 19 (3) p365 – 380