Sibling Rivalry - it's in the Genes

Petty fights in the back of the car, temper tantrums over the possession of a favourite toy, irrational jealousy over who is the family favourite - squabbling brothers and sisters have infuriated parents for generations.

Petty fights in the back of the car, temper tantrums over the possession of a favourite toy, irrational jealousy over who is the family favourite - squabbling brothers and sisters have infuriated parents for generations.

But just what makes siblings fight? Contrary to what some despairing mums and dads may feel, the fact their offspring go at each other like cat and dog has often got little to do with their parenting skills.

In fact sibling rivalry is part of human nature and has a genetic basis. We may not like it but sibling rivalry has evolved in humans because it provided benefits to our ancestors.

We might actually think ourselves lucky that we don't take sibling rivalry to the extremes sometimes seen in the animal world. In some bird species, chicks will tip their siblings out of the nest or even peck them to death.

And their parents do not always intervene. Instead, they are sometimes complicit and may refuse to feed or provide water to the smaller chick.

This often happens with Shoebill birds, as parents are not able to provide for two large offspring, and the second chick appears to be nothing more than an insurance policy in case the first does not thrive.

Whilst sibling rivalry is less extreme in humans, it can still be intense and problematic, and is also driven by conflict about parental investment.

Such behaviour is initially puzzling. Siblings share the same proportion of genes with one another as parents share with their children, approximately 50 per cent.

Human behaviour has been shaped to help us to pass on our genes. Because we share so many genes with our relatives, we have evolved to want to help them. This is called kin selection.

Ultimately, this is why parents invest so much in their children. Raising children successfully so that they reproduce means the parents' genes are then passed on to grandchildren. So why all the conflict between siblings?

It is because with the exception of identical twins, siblings do not share all of their genes. This means that each child would prefer their parents to invest more in them than in their siblings.

Human children are born relatively early and require a huge amount of investment to reach adulthood and successfully reproduce.

Each child tries to secure as much investment as possible from their parents. However, most resources - time, food, money - are finite, and what parents give to one child they cannot always give to another.

Parents therefore have to make decisions about how to allocate resources to maximise the reproductive success of their group of offspring, although this is not necessarily done consciously.

Most parents do seek to treat their children equally, sometimes obsessively so. So it is puzzling and frustrating to parents when this seems to backfire.

Children are quite likely to respond to the precise halving of a chocolate bar with claims of unfairness, and shouts of "he's got more than me".

The reason is that each child has evolved to want more than their fair share, at the expense of their brothers and sisters. Treating children equally is not what they want.

Sibling rivalry can be exacerbated by parents sometimes investing more in one child than another, often unintentionally. The arrival of a new baby, for example, can mean time and attention is diverted away from an older and more independent toddler.

But this is not in the best interests of the toddler, who may throw tantrums, demand attention and in extreme cases even hurt the baby.

Likewise, babies have evolved their own means of ensuring attention remains firmly focused on them. Their cries can signal their health and vigour, ensuring parents know they are valuable and alerting them forcefully to their need for attention.

Very young children can engage in a form of sibling rivalry even before the arrival of another child. In the ancestral environment, breastfeeding acted as a contraceptive because our diet was less rich than it is now.

As long as a mother was breastfeeding, she was unlikely to conceive again. However, after a certain amount of time, her reproductive success would be better served by stopping the feeding and having another child.

It was in the child's interests to delay this for as long as possible and maintain their mother's investment. This is why weaning can be such a difficult time with children often quite simply refusing to let it happen.

Sibling relationships are characterised by an inherent tension between the close genetic relatedness of brothers and sisters, which favours cooperation, and their desire to compete for more parental investment.

It is why they often have a love-hate relationship - engaged in a ferocious battle with each other in one instant, then jumping to each other's defence in the face of an external threat in another instant.

The good news is that in most cases sibling rivalry largely resolves when children grow up and parental investment is less of a factor.

However, childhood tensions can rear their head when resources are at stake. You only have to look at the rows a parent's will can spark among their offspring.


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