For every 15-point increase in childhood IQ score at age 10, kids were 38% more likely to be vegetarians at age 30, reported Catherine R. Gale, Ph.D., of the University of Southampton in Southampton, England.

The association remained statistically significant after adjusting for potential confounders including gender, socioeconomic status & academic achievement, Dr. Gale & colleagues said online in BMJ.

The study included 8,170 participants in the 1970 British cohort study. Mental ability was assessed at age 10. Diet was assessed at age 30. Socioeconomic variables were gathered at both times.

A total of 366 participants reported being vegetarian at age 30 (4.5%).

Vegetarians were likely to be female (74% vs. 26%). They were also likely to come from the highest social class (34% vs. about 11% for the lowest) & to have remained in the highest social class as adults (46% vs. about 11% from the lowest).

Vegetarians were also likely to be academic achievers. Nearly 1/2 (44.5%) had a degree or diploma, compared with 2.7% with no academic qualifications.

However, even after adjusting for these factors, a 15-point increase in childhood IQ score (one standard deviation) was significantly associated with the odds of becoming a vegetarian (odds ratio=1.20; 95% confidence interval=1.06 to 1.36.) The unadjusted odds ratio was 1.38 (95% CI=1.24 to 1.53).

On average, children who grew up to be vegetarians scored about 5 points higher on the IQ test than whose who did not (P<0.001).

The only class of vegetarian for whom the results didn't hold true was vegan. Vegans consume no animal products whatsoever, not even eggs or milk. The study found that vegans had an average childhood IQ score that was nearly 10 points lower than other vegetarians (95.1 for vegans vs 104.8 for other vegetarians; P=0.04).

However, this result could be unreliable because of the small sample size: only nine study participants were vegan, the authors noted.

When strict vegetarians (no meat or fish) were compared with those who called themselves vegetarians but sometimes ate chicken or fish, no significant differences were found.

Though vegetarians were on average more intelligent, better educated & higher academic achievers, these factors didn't translate into significantly higher income for vegetarians. "It may be that ethical considerations determined not just their diet but also their choice of employment," the authors speculated.

"Compared with non-vegetarians, vegetarians were less likely to be working in the private sector & more likely to be working in charitable organizations, local government, or education," they said.

One reason behind the findings may be that smart kids tend to make smarter choices about their health as adults, the authors speculated. Just as individuals of higher intelligence are less likely to smoke because of the known health risks, they may be more likely to embrace vegetarianism because of the touted health benefits, which include lower cholesterol, lower blood pressure & reduced risk of obesity & heart disease, the authors suggested.

On the other hand, "the association between IQ & vegetarianism may be merely an example of many other lifestyle preferences that might be expected to vary with intelligence but which may or may not have implications for health," the authors concluded.