It is one of the most hotly debated topics in pregnancy and early motherhood. Does breastfeeding really boost a baby's intelligence?

Now the largest scientific study yet carried out has settled the issue. Breastfed babies are indeed smarter - because their mothers are. Mothers who breastfeed tend to be more intelligent, more highly educated and to provide more stimulation at home. The higher IQ of their babies is therefore mostly inherited, accounting for 75 per cent of the difference between them and bottle-fed babies, the researchers found.

The rest of the difference is down to the environment in which they are raised. Breastfed babies have mothers who are older and better educated, and live in nicer homes where they get more attention.

When all these factors were taken into account, breastfeeding made less than half a point's difference in the intelligence scores - laying to rest a myth that has held sway for almost 80 years.

Geoff Der, a statistician from the Medical Research Council's social and public health sciences unit at the University of Edinburgh, said: "This question has been debated ever since a link between the two was first discovered in 1929. We found 73 articles which dealt with the link."

He added: "Breastfed children do tend to score higher on intelligence tests, but they also tend to come from more advantaged backgrounds."

The study, published online by the British Medical Journal today, is based on US data on the breastfeeding history and IQs of 5,000 children and 3,000 mothers, which was not available in the UK. Mr Der concluded: "There is no reason why the same findings would not apply here."

The researchers also looked at families where one child was breastfed and the other wasn't. This confirmed the findings that breastfeeding made no difference to IQ.

Mr Der said: "Intelligence is determined by factors other than breastfeeding. But breastfeeding has many benefits for both mother and child. It is definitely the smart thing to do."

In England and Wales, 77 per cent of babies are breastfed but more than a third of mothers stop within the first six weeks. Nine out of 10 mothers in the professional and managerial class start breastfeeding, compared with just over six out of 10 among manual workers.

Breastfeeding boosts the baby's immune system and protects against infections, and reduces the risk of asthma and eczema in childhood. It also reduces the risk of diabetes, high blood pressure and obesity in adulthood. But Rosie Dodds, policy researcher at the National Childbirth Trust, said evidence from parts of the world where breastfeeding is more common among poorer women cast doubt on the claim that it had no link with intelligence.

In the Philippines, where bottle feeding is a sign of status preferred by working mothers, a study published in the Journal of Nutrition in 2005 found that babies who were breastfed had higher intelligence, despite their more deprived backgrounds.

Ms Dodds said: "We cannot rule out an influence of breastfeeding on intelligence especially in babies born prematurely who may have missed out on what their biological growth would have been. Breastfeeding is more likely to provide the nutrients they need to grow and develop."

Another study of 14,600 babies, half of whom were breastfed, conducted by University College hospital, London, and published in the US journal Pediatrics this year, found there were more developmental delays among the children who were bottle fed.

Nature vs the bottle

* Breast milk provides all of the baby's nutritional needs for the first six months.

* Breastfeeding rates in the UK, where 77 per cent of women take it up, are among the lowest in Europe.

* Two thirds of the mothers who start breast-feeding in the UK stop within six months.

* Breastfeeding boosts the baby's immune system, protecting it against infection.

* Bottle-fed babies have a five times higher risk of being admitted to hospital with diarrhoea and vomiting.

* Breastfed babies have a lower risk of chest, ear and urinary infections.

* Allergies such as asthma and eczema are commoner in bottle-fed babies.

* Breastfed babies have a lower risk of developing diabetes, high blood pressure and obesity.

* Mothers who breast-feed have a lower risk of breast cancer.