New Delhi, Nov. 19: An elder brother can slightly dampen the chances of younger siblings having children, according to new research on how the sex of an older child influences the life history of younger ones.

Researchers at the University of Sheffield in the UK, who examined population data from Finland, have found that both men and women have a reduced probability of reproducing when their older sibling is a boy than when it is a girl.

Earlier studies based on populations in Sweden and Kenya had shown that the presence of elder brothers appears to decrease the marriage prospects, resources and reproductive success of younger male siblings.

Some scientists had attributed these observations to competition resulting from social influences such as parents investing more resources in their elder sons.

The new study has shown that compared to people with elder sisters, people with elder brothers have lower probability of reproducing, higher age when they have their first child, and larger intervals between births of children.

The study, published today in the journal Evolution and Human Behaviour, was based on church records of farming and fishing communities in pre-industrial Finland from the mid-18th century, spanning nearly 150 years.

The differences in probability of reproducing were small but significant — 68 per cent men and 66 per cent women born after an elder sister reproduced, but only 64 per cent of men and 60 per cent women born after an elder brother did.

“We can’t generalise this to other populations yet — we’ll need more studies,” said Ian Rickard, a zoologist studying how early developmental (prenatal) conditions may influence life history — growth, development, survival and reproduction.

But the findings suggest that the observed differences in reproductive success of younger siblings may not always be influenced by social factors, but may also emerge from underlying biological mechanisms.

The precise mechanisms remain unclear, Rickard told The Telegraph. But it is possible that physiological changes in the mother associated with bearing a male foetus may influence “resources available” to her subsequent offspring.

“We would be speculating, but it’s possible that male foetal testosterone may enter the maternal bloodstream and influence maternal physiology that could lower the maternal resources available for subsequent offspring,” Rickard said.

The researchers said one way to explore the mechanism would be to study and compare the physiological states of pregnant women before and after they have delivered male and female babies.