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What causes sibling rivalry? An evolutionary perspective

At first glance, sibling rivalry doesn’t seem to make sense from an evolutionary psychological perspective. Siblings, because they share similar genes, should be inclined only to cooperate and help each other achieve reproductive success.

If you help your brother or sister to survive and help them find a suitable mate, and they reproduce successfully, you’re indirectly aiding the reproductive success of your own genes because that sibling of yours shares your genes.

The end result is that some of your genes successfully replicate which is an ideal outcome for you. If this is the case, then why does sibling rivalry exist?

The origins of sibling rivalry

Note that your sibling only shares 50% of your genes. But you have 100% of your own genes. So your own reproductive success comes first before that of your sibling. The math is clear. If you survive and reproduce, more of your genes are passed on to the next generation than when your sibling reproduces.

Evolutionary theory predicts that all else being constant, you should care more about your own survival and reproductive success than that of your sibling even though the reproductive success of your sibling also contributes to some of your own reproductive success.

This is where sibling rivalry stems from. This is the behind-the-scenes logic behind why siblings compete. They want the best for their own selves.

Sibling rivalry in the animal kingdom

Sibling rivalry is observed throughout the animal kingdom- from insects to mammals.

Spotted hyena twins compete aggressively to obtain the nutrient-rich milk from their mother.1 "Some bird siblings jostle for position in their nests. Those with winning moves can sit in the spot where mom is most likely to deliver food", writes Jeana Bryner for Live Science.

Sibling rivalry sometimes even leads to the siblicide behavior i.e. killing of a sibling. The most grotesque example of this is the sand-tiger shark that kills and eats its siblings while in the womb. 

Nazca boobies are seabirds that drag their siblings out of the nest, leaving them to be killed by predators or starvation so that they can have their mom all for themselves.

Cattle egrets, a species of heron, are known to practice siblicide when the parents are away from the nest hunting for insects and fish.

This chilling BBC Earth clip is the darkest example of sibling rivalry in the animal kingdom that I've come across…


The mother didn’t intervene because it’s in her best interests to raise strong chicks. This is the norm of sibling rivalry in the animal kingdom, especially birds. Parents don’t intervene and let the siblings sort it out for themselves.

As cruel as this behavior may seem, it’s geared to better the chances of passing on genes to the next generation. Same is true for humans.

Sibling rivalry in humans

Sibling rivalry in humans starts early, as soon as the second child is born. The first child, who till now was the center of everybody’s love and attention feels dethroned. Things can get worse when the third child is born because, so the parents figure, by this time the other two kids are grown enough to take care of themselves.

So the youngest child ends up being pampered while the oldest feels neglected, resulting in a sibling rivalry. The more the age gap between the siblings the more estranged they’re likely to be in adulthood. A different birth order not only contributes to the development of sibling rivalry but also shapes the behavior of siblings when they become adults.2

While sibling rivalry may manifest in children as fights over having the best toy or the best chair, it can rear its ugly head in adulthood in property and inheritance disputes. Remember, the competition is for resources and resources facilitate reproductive success. So ultimately, the competition is for reproductive success. 

Having an estranged relationship with one’s siblings is distressing because people are wired to rectify relationships with their nearest kin. You may choose to abandon a friend or a lover after having a fight with them but it’s hard to break ties with a sibling who shares half of your genes. Blood is thicker than water.

Role of parents 

Parental action can either weaken or strengthen sibling rivalry. If parents blatantly favor one kid over the other, the feelings of resentment in the unfavored kid will deepen. As researcher Judy Dunny puts it, "The greater the difference in the maternal affection and attention, the more the hostility and conflict between the siblings."

Conversely, parents who don’t practice favoritism and treat all their kids equally and fairly are more likely to raise children less prone to sibling rivalry.



References:

1. Hofer, H., Benhaiem, S., Golla, W., & East, M. L. (2016). Trade-offs in lactation and milk intake by competing siblings in a fluctuating environment. Behavioral Ecology27(5), 1567-1578.

2. Sulloway, F. J. (2001). Birth order, sibling competition, and human behavior. In Conceptual challenges in evolutionary psychology (pp. 39-83). Springer Netherlands.