It's official - vegetarians really are smarter. But it is not because of what they eat. Bright children are more likely to reject meat and opt to become vegetarians when they grow up, a study has shown. (Read full study here.) Clever veggies are born not made.

The finding helps explain how a team of vegetarians won the BBC Test the Nation competition in September, when they beat off competition from six other teams including butchers, public school pupils and footballers' wives to achieve the highest overall IQ score.

The top scoring individual in the contest, Marie Bidmead, 68, a mother of five from Churcham, Gloucester, was also a vegetarian. "I think it shows we veggies are good thinkers. We think about what we eat for a start," she said.

Researchers from the University of Southampton who conducted the study agree. They suggest that vegetarians are more thoughtful about what they eat. But they say it is unclear whether bright children choose to become vegetarians for the health benefits or for other reasons, such as a concern for animals, or as a lifestyle choice.

The scientists began investigating the link between IQ and vegetarianism because people with higher intelligence have a lower risk of heart disease, which has long puzzled doctors.

A vegetarian diet is associated with a lower cholesterol level, lower blood pressure and less obesity - all risk factors for heart disease. The researchers wondered if this could explain the health advantage of having a high IQ. They cite Benjamin Franklin, the 18th-century statesman and scientist, who said that a vegetarian diet results in a "greater clearness of head and quicker comprehension". He may not have realised that this was because of whom was eating rather than what was eaten.

However, early last century doctors were less enamoured of the practice. Robert Hutchison told the British Medical Association in 1930: "Vegetarianism is harmless enough though it is apt to fill a man with wind and self-righteousness."

The study, published in the British Medical Journal, was based on more than 8,000 people born in 1970 whose IQ was measured at age 10. Now aged 36, the researchers found 366, just under one in 20, said they were vegetarians (a third of these ate chicken or fish but none touched red meat).

As well as being brighter, the vegetarians were better educated and of higher social class but the link with intelligence remained statistically significant even after adjusting for these factors. Despite their intelligence they were not wealthier and more likely to be working for charities or in education. "It may be that ethical considerations determined not just their diet but also their choice of employment," the report said.

It concludes: "Our finding that children with greater intelligence are more likely to report being vegetarian as adults, coupled with the evidence on the potential health benefits of a vegetarian diet, may help to explain why higher IQ in childhood or adolescence is linked with a reduced risk of coronary heart disease in adult life."

The benefits of forsaking meat

* A vegetarian diet tends to be lower in fat, higher in fibre and vitamins

* Vegetarian diets are associated with lower cholesterol, lower blood pressure, and less obesity

* Vegetarians have lower rates of heart disease, less diabetes and may have less risk of cancer and dementia

* The Vegetarian Society, claimed to be the oldest in the world, was founded in Ramsgate, Kent, in 1847. Mahatma Ghandi, George Bernard Shaw and Linda McCartney were members

* 'Vegetarian' is derived from the Latin vegetus, meaning 'lively' and was intended to be suggestive of the English 'vegetable'