In a post titled Why Vegetarians Are More Intelligent than Meat Eaters, evolutionary psychologist Satoshi Kanazawa claims:
Among the British respondents in the National Child Development Study, those who are vegetarian at age 42 have significantly higher childhood general intelligence than those who are not vegetarian at age 42. (Childhood general intelligence was measured with 11 different cognitive tests at three ages before 16.) Vegetarians have the mean childhood IQ of 109.1 whereas meat eaters have the mean childhood IQ of 100.9. The difference is large and highly statistically significant.
The Evening Standard discusses a similar study.
I am skeptical of this claim, so I searched for scientific data investigating this issue, but I couldn't find any solid scientific study conducted over long period or with bigger sample. The most articles I encountered were just speculations made people to support their own claim.
I found the following related research while searching:
Is vegetarianism correlated to intelligence (as measured by IQ or similar methods)?