“I want the bigger half!”: The distorted logic of sibling rivalry

Sibling rivalry – fighting, competition and jealousy between brothers and sisters – has driven parents crazy for generations. For many parents, constant battles between siblings are a daily challenge. The battles typically rage over things like sharing toys (‘but it’s mine!’), who got the most sweets and who is the favourite child. Arguments can escalate into hurling terrible insults and even physical aggression. Sometimes siblings appear to have an intense dislike of one another. To the frustration of parents, this dislike may be resistant to their attempts to apply reason, to impose consequences, or to reward good behaviour. Paradoxically however, parents often comment that their children will instantly jump to the defence of a brother or sister when they are threatened by another child. This love-hate dynamic characterises many sibling relationships.


When faced with daily battles, parents often assume they must be to blame and seek out solutions from parenting books to family therapy. But contrary to what despairing parents may feel, the assumption that sibling rivalry must be caused by inadequate parenting or poor family dynamics can be misguided. Although it is very unpleasant, sibling rivalry is a behaviour which has evolved in humans because it provided benefits to our ancestors. An understanding of the evolutionary origins of the behaviour may go some way towards helping parents to understand and cope with their children’s behaviour.

Sibling rivalry is common in many animal species. Juveniles of a number of bird species, for example, commit siblicide by tipping their siblings out of the nest or pecking them to death. One of the most brutal examples is seen in the giant, prehistoric-looking bird, the shoebill. Shoebills typically have two offspring in each breeding season. The larger sibling will frequently attack the smaller sibling, pecking it viciously until it bleeds. The parents usually do not intervene. Instead, they are complicit with the bully, and may refuse to feed or provide water to the smaller chick, which will eventually die. The 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 chick does not thrive. Watch for yourself in this video – you might need some tissues!

The shoebill

Whilst (fortunately!) sibling rivalry is less extreme than this in humans, it can 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 fifty per cent. From an evolutionary perspective, 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 them successfully so that they reproduce means that the parent’s genes are passed on to grandchildren. Given that siblings share the same proportion of genes as parents and offspring, why do they often seek to do one another harm?


The conflict arises because, despite their shared genes, 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 due to their big brains, and require a huge amount of investment to reach adulthood and successfully reproduce themselves. 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 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. It is therefore 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 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. Incidentally, the solution to this particular conundrum is to let one child chop and the other choose!

Sibling rivalry can be further exacerbated by parents sometimes investing more in one child than another, often without intending to. The arrival of a new baby, for example, can necessitate a lot of time and attention being 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 even hurt the baby. In Siblings Without Rivalry, Adele Faber gives an account of  a parent finding the baby with three long scratches on his back, and his three-year-old brother wearing an evil grin. My husband (one of nine in total) admitted that he used to bite his baby brother until he drew blood! Likewise, babies have evolved their own means of ensuring that their parents attention remains firmly focussed 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 a new baby is conceived. In the ancestral environment, breastfeeding acted as a contraceptive because diet was less rich than it is now. As long as a mother was breastfeeding, she was unlikely to conceive another child. However, after a certain amount of time, her reproductive success would be better served by ceasing to breastfeed a growing child and conceiving again. Yet 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 as children often quite simply refuse to be weaned.

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One researcher has even argued that babies have evolved to keep their mothers awake at night so that increased breastfeeding strengthens its contraceptive effect and prevents conception of a rival sibling. The infant may be abetted by the father in this pursuit; the genes responsible for night-time waking and feeding are suggested to come from him. It is also in the father’s best interests to maximise his partner’s investment in existing offspring since he cannot be sure that any future offspring will be fathered by him. The effect of this is seen even in the womb where genes from the father promote offspring growth to maximise the mother’s investment in the baby, whilst genes from the mother resist this effect, reserving some resources for subsequent pregnancies.

Although many parents strive to avoid favouritism and believe they show no sign of it, there may also be biases that they are not fully aware of. Humans (and non-human animals) have likely evolved to be sensitive to cues from offspring regarding their ability to turn parental investment into reproductive success. Parents may therefore unwittingly favour one child over another, sometimes in only small ways. But because children are so sensitive to their siblings being favoured in any way, they are likely to notice and react to any perceived inequality, and may respond with increased rivalry towards siblings.


In certain circumstances parents may favour children of one sex over another. Because men are able to have many more offspring than women (since they are not constrained by pregnancy) sons can potentially provide their parents with greater reproductive success than daughters. However, sons are only likely to acquire many partners if they are attractive and have a lot of resources. Therefore, in high quality, resource-rich environments, parents may invest more in sons. However, in poorer environments, parents may benefit reproductively from investing more of their limited resources in daughters. This because women are almost guaranteed to be able to reproduce since they are the sex that invests most in offspring, and therefore men compete to mate with them. This is the Trivers-Willard hypothesis. Indeed, some evidence suggests that daughters are more likely to be born in times of famine, and that women living in poor conditions favour daughters by producing breast milk that is more creamy, whereas women living in affluent conditions produce creamier breast milk for sons.

The logic underlying sibling rivalry was summed up in an episode of The Simpsons when Homer said to his squabbling children, “Don’t go easy on each other just because you’re brother and sister. I want to see you both fighting for your parents love!”. Unlike Homer, most parents have the sense to discourage this sort of behaviour because it is in their interests to promote harmony between their offspring and ensure that they are all successful. Parents therefore tend to reward cooperation between siblings and punish conflict. They tend to try to manipulate their children into being nice to one another by teaching them moral codes. However, children are not easily dissuaded from competing with siblings as cooperation is in their parent’s interests far more than their own. This is why eliminating sibling conflict is so difficult for parents.


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. This is why siblings often have a love-hate relationship. It is why they may be engaged in a ferocious battle with each other in one instant, and 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 an issue. However, old tensions can rear their head when resources are at stake in the form of an inheritance. How often do we see family feuds arise when a parent dies and there is conflict over the will?



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