What Causes Sibling Rivalry? An Evolutionary Perspective

Posted on October 14, 2017

If you help your brother or sister to survive and help them find a mate, and they thereby reproduce successfully, you are indirectly making sure of the reproductive success of your own genes, since your sibling shares your genes. As the end result, your genes are passed on to the next generation, which is an ideal outcome for you, from an evolutionary perspective.
This might seem like a logical conclusion, however, if this is the case, then why does sibling rivalry exist?

The origins of sibling rivalry
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.
While your sibling only shares 50% of your genes, you of course have 100% of your own genes. The math is clear: your own reproductive success comes first before that of your sibling, because if you manage to survive and reproduce, more of your own genes are passed on to the next generation than if your sibling reproduces. This is exactly where sibling rivalry stems from.

Sibling rivalries can be found not only in humans, but 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 in 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.
- Cattle egrets, a species of heron, are known to practice siblicide when the parents are away from the nest hunting for insects and fish.
Especially in birds, but also in other species, the parent does not usually intervene in these situations, since it is in their best interest to raise strong children. The siblings are left to sort it out themselves, ensuring that the strongest one comes out on top, which might seem cruel, however it is geared to better the chances of passing on genes to the next generation.

Sibling rivalry can start early in humans, as soon as the second child is born. At that point the first child – who has been the center of everybody’s love and attention – feels dethroned. When a thirst child is born parents often figure that by this time the other two children are old enough to take care of themselves, which makes things worse. The youngest child often ends up being pampered by the parents, while the oldest feels neglected, which ultimately results in sibling rivalries.

It was also found that the larger the age gap between siblings, the more likely they are to end up estranged in adulthood. 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.
However, 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.

The role of parents
Ultimately, 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 who are less prone to sibling rivalry.


Category(s):Child Development, Family Problems, Parenting

Source material from PsychMechanics


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