How older siblings stunt growth

Ben Affleck is taller than his younger brother Casey

Having an older sibling, particularly a brother, can stunt growth, work suggests.

Experts said the condition of the womb after the first pregnancy may be a factor.

The study of 14,000 families was presented at the BA Festival of Science.

The research, by David Lawson, of University College London, also showed children in larger families were likely to be shorter than average.

Researchers found that children with three siblings were 2.5cm or one inch shorter than the average height for their age.

If you are one of the younger ones, then you can expect to be shorter than your older siblings Dr Lawson

It was suggested siblings may dilute resources - time, money or love - that parents are able to invest in children.

The researchers followed children born in the 1990s and who were enrolled on the Avon Longitudinal Study of Parents and Children, one of the largest public health studies to be set up in Britain.

Each year, the children's height was recorded, along with other details of their development.

While having older siblings of either sex affected a younger child's development, the effect of older sisters was more mild.

Sibling rivalry

One explanation put forward is that boys are more demanding to raise, and so stretch the resources of parents more than daughters.

Dr Lawson said: "It is well known that children from larger families perform less well at school but this study is the first to suggest that this also applies to height. Height is generally a good proxy to health.

"If you are the oldest child, having younger siblings will not affect your development significantly but if you are one of the younger ones, then you can expect to be shorter than your older siblings."

Marjo-Riitta Jarvelin, an expert in epidemiology and public health at Imperial College London, said the environment of the womb might be important.

"We know that when women have had a number of pregnancies, they are likely to put on weight and their blood sugar levels can become poorer, and this may have effects on the foetus growing in the womb.

"It is impossible to know yet what might be causing this effect, though.

"It may be that when families get very large, parents are less able to provide proper nutrition for their children and the youngest are most exposed to that."

Dr Lawson stressed that his work did not look at whether a child is happier as part of a larger family.