The findings are based on changes in the social class of two groups of individuals born in 1958 (17,419 people) and in 1970 (16,771 people).

The researchers asked each of the children's mums, when their child was five or seven years old, whether they had breastfed him/her.

They then compared people's social class as children -- based on the social class of their father when they were 10 or 11 -- with their social class as adults, measured when they were 33 or 34.

Social class was categorised on a four-point scale ranging from unskilled/semi-skilled manual to professional/managerial.

The research also took account of a wide range of other potentially influential factors, derived from regular follow-ups every few years. These included children's brain (cognitive) development and stress scores, which were assessed using validated tests at the ages of 10-11.

Significantly fewer children were breastfed in 1970 than in 1958. More than two-thirds (68%) of mothers breastfed their children in 1958, compared with just over one in three (36%) in 1970.

Social mobility also changed over time, with those born in 1970 more likely to be upwardly mobile, and less likely to be downwardly mobile, than those born in 1958.

None the less, when background factors were accounted for, children who had been breastfed were consistently more likely to have climbed the social ladder than those who had not been breastfed. This was true of those born in both 1958 and 1970.

What's more, the size of the "breastfeeding effect" was the same in both time periods. Breastfeeding increased the odds of upwards mobility by 24% and reduced the odds of downward mobility by around 20% for both groups.

Intellect and stress accounted for around a third (36%) of the total impact of breastfeeding: breastfeeding enhances brain development, which boosts intellect, which in turn increases upwards social mobility. Breastfed children also showed fewer signs of stress.

The evidence suggests that breastfeeding confers a range of long-term health, developmental, and behavioural advantages to children, which persist into adulthood, say the authors.

They note that it is difficult to pinpoint which affords the greatest benefit to the child -- the nutrients found in breast milk or the skin to skin contact and associated bonding during breastfeeding.

"Perhaps the combination of physical contact and the most appropriate nutrients required for growth and brain development is implicated in the better neurocognitive and adult outcomes of breastfed infants," they suggest.