Landed gentry have put their faith in primogeniture for centuries. The eldest inherited the estate, the second joined the Church and the third took a trade. But there is now scientific evidence to prove that they had the right idea.

A 10-year study of nearly 1.5 million Norwegians has concluded that, regardless of family size and income, the eldest child is the most successful academically and at work. Bill Blair, QC, a successful barrister with Tony Blair as a younger brother, might not agree,.

The report shows that a first born is most likely to complete a year more in education, either at school or university, than the fourth or fifth born.

Kjell Salvanes, a professor at the Norwegian School of Economics and Business, said that these differences could be because older children generally take the role of leader among their siblings.

He said: “The eldest child acts as a teacher for the younger children and learns how to organise information and present it to others. Both are valuable skills in education and in the labour market.”

The report may explain why Jimmy Carter’s younger brother Billy became an alcoholic and smoked pot on the roof of the White House and why Bill Clinton’s brother Roger ended up in jail for selling cocaine. It might also explain why Sam Houston Johnson, the younger brother of Lyndon Baines, was kept under surveillance in a bedroom in the White House.

Ms Booth, the wife of the Prime Minister and the eldest of eight sisters, became a QC whereas her siblings are lesser known actresses, journalists and academics.

Professor Salvanes and his colleagues from the University of California examined the population of Norway that were aged 16-74 from 1986 to 2000 to study whether it was the size of a family that made for a successful life or the order in which the child was born.

Although previous studies found that children from larger families did not perform as well academically, the More the Merrier study found that the key factor was birth order.

The report found that on average “adding one child to completed family size reduces average educational attainment of the children by just less than one fifth of a year”.

The professor said that the effect of birth order on the 1,427,100 children studied was similar for the third child in a family of three as for the third child of a family of four or five.

Education is free at school and university in Norway, where schooling is compulsory until the age of 16. But the study found that children born later into a family completed fewer years of education and that the earnings potential for women in particular became worse. Children in large families with an educated mother were worst off.

The report said: “Later-born women have lower earnings, are less likely to work full time and are more likely to have their first birth as a teenager. Later-born men have lower full-time earnings, but are not less likely to work full time.”

Of the 647,035 families — which excluded those with twins and whose children were under 16 in 2000 — about 18 per cent had one child, 41 per cent had two children, 27 per cent had three, 10 per cent had four and 5 per cent five or more.On average lone children were less academic than the average child of two or three-child families.

Professor Salvanes said that if resources were limited parents invested more in the eldest child, but acknowledged there were always exceptions.

As a younger child, he said that he was “more academic” than his older sister.

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