Brothers, sisters... and a sting in the evolutionary tale

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FOR as long as any kind of animal life has inhabited the Earth and families were created, siblings have existed with all their joys and woes. Name a pair or group of siblings who say they have never had fights, or at least regular tiffs over toys, food, friendships or even parental affection and they are almost certainly being economical with the truth.

One newspaper correspondent recently described her early relationship with her sister: “Like most kids, I liked to tease my younger sister. I would tell her she was not related to me and was, in fact, the milkman’s daughter. One day I hatched a plan: I told my mum I wanted to learn how to write a signature and asked her to write hers (on a folded piece of paper).

“I walked away with a sly grin, and stuck the piece of paper on my sister’s bedroom door. On the other side of the paper, I had written a note that said: ‘Dear Lucy, I have been keeping this a secret but have to tell you that you are the milkman’s daughter, signed …’ Needless to say, Lucy cried her eyes out, and then I got caught and seriously smacked!”

One five-year-old girl who I won’t name was so mortified at the arrival of a new baby sister in the house that she plotted a campaign of green-eyed crime, which involved attempting to lob heavy objects into the infant’s cot at every possible opportunity.

She was never left alone with the baby after her parents heard a shriek from the newborn and ran to find an encyclopedia lying in the Moses basket not far from the child’s head and her sister cowering in the bathroom.

Today they are loving young adults who laugh when reminded of those early days...but they are still prone to count their respective Christmas presents obsessively and the older woman still has a tendency to perceive slight if anyone is too effusive in their praise of her sibling.

These stories happen to be about girls, but sibling rivalries seem to be just as common, if not more so, among boys.

It may also continue long into adulthood, with issues such educational or professional achievements, material possessions and inheritance commonly bringing old childhood emotions flooding back to the surface.

New research by the University of Sheffield shows that although older siblings have often been known to torment younger brothers and sisters they can actually increase the chances of those siblings living into adulthood.

However, while older siblings are useful in this way at one stage of life, they soon show themselves to be a nuisance, and the more older same-sex siblings you have in young adulthood the less likely you are to marry and have children yourself, the research reveals.

The study aimed to investigate the effects of sibling relationships at different stages in terms of longevity and reproductive success.

Researchers examined population data from church records kept in Finland between 1750 and 1900, and found that during childhood a boy with no older siblings had only on average 68 per cent chance of surviving to the age of 15. But a boy with four older sisters or brothers had a probability of surviving of 75 per cent.

Moving on into adulthood, each same-sex sibling decreased the probability of ever having children so that, on average, men with no older brothers had five children whereas men with older brothers only had four children.

The data shows that siblings both co-operate and compete, and the upside and downside of having siblings vary both across life and also between sisters and brothers.

Project leader, evolutionary biologist Dr Virpi Lummaa, said: “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, in line with suggestions that elder siblings often help parents to raise their younger siblings.”

However, in young adulthood, “...the more same sex older siblings you had alive, the less likely you were to marry and the fewer children you raised yourself, suggesting that having these older siblings was actually a nuisance – probably because they competed over the same inheritance and marriage prospects – with the eldest brother, for example, inheriting the farm while the youngest only worked as a labourer”.

It seems there can be very good evolutionary reasons for siblings sometimes to regard each other with mixed emotions...