the-BA : Sibling Competition

by Liz Newton

Having brothers and sisters affects physical development later in life, Dr David Lawson of the Department of Anthropology at UCL told the BA Festival of Science on Wednesday.

Older siblings were found to pose a greater threat to development than younger siblings, while brothers were more costly than sisters. These preliminary results were taken from one of the UK's most unrivalled data-gathering projects to date, the ALSPC or 'Children of the 90s' project.

The project used height as a measure of physical development.

The presence of siblings was found to adversely affect growth, with the negative effects being more marked in smaller families.

The presence of siblings represents a dilution of parental resources such as time, money and love, and this is most apparent when the first sibling arrives.

Subsequent siblings dilute parental resources proportionally less than the first one, so the effect on growth is reduced.

However, family size is well correlated with socioeconomic status, and many previous studies looking at the effect of siblings on physical development have not had sufficient control over this factor,

"This is the first study to take into account detailed information on the socioeconomic status of the families over time," explained Dr Lawson. "This way we were able to control for it as a variable".

Over 14,000 families have taken part in the ALSPC project, which has followed children born in 1991 and 1992 until the present day with frequent data collection throughout their childhood.

The effects of sibling competition have been observed in previous studies of traditional societies in Africa. However, the research using the ALSPC data is the first to conclusively show these physical effects in a relatively wealthy population such as the UK.

Early research on sibling competition focused on animals and involved brood manipulation studies, for example by altering the number of eggs in birds' nests, where the physical effects on the offspring were monitored. In larger broods, offspring were consistently found to display slower growth, have a lower chance of survival and lower mating success.

This effect has led to the evolution of siblicide in some species, where one sibling will kill one or more of its other siblings. For example, the Booby of the Galapagos Islands usually lays two eggs, but when the chicks hatch in the nest the larger and stronger bird will peck its smaller sibling to death.

While human sibling competition is rarely this extreme, this study suggests that some of its consequences may reflect those found in nature, such as slower growth. Other effects of siblings on humans are negligible, such as survival and mating success. While past sibling studies have tentatively suggested that the presence of older siblings may affect educational achievement, little research has been carried out into the emotional effects of siblings.

Read more news coverage at the Guardian, the Times and BBC News.