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Being the firstborn child can certainly have its advantages – no hand-me-downs, a few extra years of devoted attention from parents, and the ability to always pretend that you know best.

Studies have shown that being the eldest also has its financial perks: firstborn children are more likely to score higher on IQ tests, get better exam results and earn more later in life.

And a new economics study has now drawn on decades of Danish data, and suggested childhood illnesses could be one of the reasons for the difference.

Researchers at the University of Copenhagen found illnesses during the early years could account for the 1.9 per cent wage gap that is commonly seen between siblings.

The findings are similar to those made by the University of Edinburgh, which found that firstborns scored higher in IQ tests and the gap usually appears within the first year of life.

open image in gallery Firstborns are more likely to score higher on IQ tests and perform better in exams ( Ben Birchall/PA Wire )

The researchers, collaborating with a team from Sydney University, examined data from 5,000 children who were given reading and picture vocabulary tests every two years.

It found younger siblings were two to three times more likely to suffer from respiratory illnesses, which could result in them being hospitalised.

Children who suffered from illnesses during their early years were found to achieve less on the academic front, with potential reasons being that infections could have diverted energy from the development of the brain.

Given that nurseries are often blighted with bugs, common colds and illnesses, it is often the case that firstborn children pick up these viruses and pass them on to their baby siblings.

Another explanation is the difference in parental attention that is afforded to firstborn children during their first years. Younger siblings often receive less one-to-one attention, given that their parents are already juggling childcare responsibilities.

Parents are encouraged to ensure that vaccinations are kept up to date and to keep sick children at home, instead of sending them to nursery.