Matt was sure he loved his brother Jeff and believed himself to be free from sibling rivalry. After all, he had never experienced any sibling rivalry or jealousy.
He could recall experiencing only mild sibling rivalry once or twice in his childhood. That’s it. Both in their late twenties now, Jeff recently got into a relationship while Matt was still single.
When Jeff announced he was in a relationship, Matt was flooded with feelings of jealousy, with an intensity he had never felt before.
Matt’s belief that he was free from sibling rivalry turned out to be wrong, and the whole experience was quite shocking to him.
At first glance, sibling rivalry and doesn’t seem to make sense from an evolutionary perspective. Siblings, because they share similar genes, should be inclined to cooperate and help each other achieve reproductive success, right?
If you help your brother or sister survive and help them find a suitable mate, they can reproduce successfully. By doing this, you’re indirectly aiding your own reproductive success because your sibling shares 50% of your genes.
The end result is that your genes successfully replicate. This is an ideal outcome for you. This is why most people favour their siblings and are willing to make sacrifices for them.
If this is the case, then why does sibling rivalry exist?
Sibling rivalry origins
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 of your sibling. Even though the reproductive success of your sibling also contributes to some of your own reproductive success.
In other words, it’s great that your sibling reproduces but your own reproduction matters more. Your sibling’s reproduction can’t happen at the cost of your reproduction, unless in extreme situations as highlighted in this article.
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. Parental resources are limited, and each sibling wants to maximize the resources they attain for themselves, even if at the cost of their sibling.
Spotted hyena twins compete aggressively to get 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 behaviour 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 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 interest 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 behaviour may seem, it’s geared to better the chances of passing on genes to the next generation.
“But they’re animals”, you might think. “Humans are not like that.”
You’re dead wrong. Extreme jealousy can easily motivate a human to kill their sibling. It’s like suicidal ideation. People don’t choose those thoughts. They arise on their own.
Sibling rivalry in humans
Sibling rivalry and rivalry in humans starts early, as soon as the second child is born. The first child, who till now was the centre of everybody’s love and attention, feels dethroned. Things can get worse when the third child is born. The parents figure by this time the other two kids are old 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 behaviour 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 we’re wired to rectify relationships with our nearest kin. You may 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 in easing sibling rivalry
Parental action can either weaken or strengthen sibling rivalry and jealousy. 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 favouritism and treat all their kids equally and fairly are likely to raise children less prone to sibling rivalry.
- 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 Ecology, 27(5), 1567-1578.
- Sulloway, F. J. (2001). Birth order, sibling competition, and human behavior. In Conceptual challenges in evolutionary psychology (pp. 39-83). Springer Netherlands.
Hi, I’m Hanan Parvez (MBA, MA Psychology), founder and author of PsychMechanics. I’ve written 280+ articles and published one book about human behavior on this blog that has garnered over 3 million views. PsychMechanics has been featured in Forbes, Business Insider, Reader’s Digest, and Entrepreneur.