Having a brother or sister can stunt growth, but it is the youngest siblings that are the worst affected, research suggests.

Analysis of children born in the 1990s showed that children with three siblings were 2.5 centimetres (one inch) shorter than the average height for their age. The lost height for older siblings was shown to be temporary but there were long-lasting impacts for the youngest children.

Growth rates for children are considered to be a good indicator of future health. On the basis of the study, the researchers said that the health of the youngest siblings could be compromised in adulthood.

Presenting the preliminary findings of the study to the Festival of Science, David Lawson, of University College London, said yesterday that having brothers or sisters could have a lifelong impact on health.

“Siblings may represent a threat to development outcomes because they dilute resources – be that time, money or love – that parents invest in children,” Dr Lawson told the conference. “We show that, all else being equal, growth is significantly retarded by the presence of siblings.”

Dr Lawson emphasised that the conclusions of the study had yet to be confirmed.

The researchers studied children born to 14,000 mothers. The growth rates of the children were measured and then heights were compared with average heights for the corresponding age. They found that to have older siblings had a a bigger impact on the younger siblings than vice versa.

The most damaging combination was to be the younger sibling of older brothers. There was a penalty for having older sisters but the consequences were less than for having older brothers.

The researchers said: “While sibship size effects on height amongst modern societies have been suggested elsewhere, to our knowledge we are the first to demonstrate differential consequences of siblings in regard to relative age and sex.”

Although the precise cause of the stunted growth was unclear, the researchers suggested that a factor could be the condition of the womb after the first pregnancy.

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