Older siblings both help and hinder their younger brothers and sisters

Big brothers and sisters have long been said to torment their younger counterparts but new research by the University of Sheffield shows they can actually increase the chances of their siblings living to adulthood.

However, they soon show themselves to be a nuisance as the more older same-sex siblings you have in young adulthood, the less likely you are to marry and have children, the research reveals.

THUMBThe study aimed at investigating the effects of sibling interactions at different life stages for one’s longevity and reproductive success, and was carried out by experts from the University’s Department of Animal and Plant Sciences by analysing church records of about 20,000 Finnish people born between 1750-1900.

During childhood, a boy with no elder siblings had only on average a 68 per cent chance of surviving to age 15, whereas a boy with four elder sisters or brothers had a probability of surviving of 75 per cent. On the contrary during adulthood, each living same-sex sibling decreased the probability of ever having children so that, on average, men with no elder brothers had five children whereas men with elder brothers only had four children.

Principal investigator Aïda Nitsch, of the University of Sheffield, said: "These results demonstrate that family members often both cooperate and compete and that benefits and costs of having elder siblings may vary across life and between sisters and brothers. Elder siblings have a key role in the family and understanding their effect is pivotal to understanding the evolution of the human family.”

Project leader Dr Virpi Lummaa, also of the University of Sheffield, added: “Raising, simultaneously, several offspring is a challenge for mothers and the help provided by elder siblings is crucial in humans. Having elder siblings was good during childhood in terms of reduced mortality risk, in line with suggestions that elder siblings often help parents to raise their younger siblings and provide care in various human societies.

“However, siblings’ interactions are not limited to childhood and they often continue into adulthood. At least in the historical context, in young adulthood, the more same sex siblings you had alive, the less likely you were going to marry, and less children you raised yourself, suggesting that in adulthood it was actually a nuisance, probably because they competed over the same inheritance and marriage prospects.”

The new study, published in Proceedings of the Royal Society of London, shows that the nature of the effects of siblings interactions could change across life thereby extending the understanding of sibling relationships when several young co-reside in a family. Overall, siblings have been documented to have developmental, psychological, morphological as well as behavioural consequences on each other.

The experts used detailed church records of births, deaths, marriages and wealth status from Finland which were kept for tax purposes to track the effects of sibling presence over the entire life cycle of individuals: survival to adulthood, marriages, and fertility of individuals, given the composition of the family and how it changed over time.

The project was funded by the European Research Council (ERC) and the French National Centre for Scientific Research (CNRS) and was carried out in collaboration between the University of Sheffield and Dr Charlotte Faurie at the Institute of Evolutionary Sciences of Montpellier, France.

Additional information

To view the paper online please visit: http://rspb.royalsocietypublishing.org/content/280/1750/20122313.full.pdf+html

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Paul Mannion
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The University of Sheffield
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