The study of 8,179 was reported in the British Medical Journal and researchers said it could explain why people with higher IQ were healthier because a vegetarian diet was linked to lower heart disease and obesity rates.
Twenty years after the IQ tests were carried out in 1970, 366 of the subjects said they were vegetarian - although more than 100 were eating either fish or chicken.
Men who were vegetarian had an IQ score of 106, compared with 101 for non-vegetarians and female vegetarians averaged 104, compared with 99 for non-vegetarians.
There wasn't any difference in IQ score between the strict vegetarians and those who said they were mainly vegetarian but who admitted to eating fish or chicken.
The researchers found that vegetarians were more likely to be female, to be of higher occupational social class and to have higher academic or vocational qualifications than the non-vegetarians, although, these differences were not reflected in their annual income, which was similar to that of the meat eaters.
