Sibling Rivalry: Will it Ever End?

What is sibling rivalry? Anyone who has brothers or sisters, or more than one child, has a definition for what sibling rivalry is. Put simply, it is when children fight with one another. When I was younger, my brother and I were constantly fighting, over literally everything. We are four years apart so that is to be expected. My sister and I, on the other hand, did not fight very often. She is thirteen years older than I am. This got me thinking: why did my brother and I fight, but my sister and I did not? Does sibling rivalry have limits? Let’s get a better understanding of what sibling rivalry is and why it occurs to try and answer this question.

Sibling rivalry is when siblings are competitive against each other which often leads to fighting with one another. It occurs when siblings demand more resources than they are currently receiving. In humans, resources can include many different things like food, money and attention. Regardless of what it is, they feel that they are not getting enough of it so they must fight with their sibling to gain more than they already have. Sibling rivalry happens in many species, especially K-selected species. K-selected species have small litters, slow maturation, and high parental investment (Workman & Reader, 2004). They are more likely to have sibling rivalry because they are more dependent on their parents for resources. Sibling rivalry can take many forms, with the most extreme form being siblicide.

Siblings share about half of each other’s genes. Looking at this from an evolutionary standpoint, we would assume that siblings would not fight because they are wired to love and look out for one another. However, when resources are low, it is adaptive to be selfish and fight for those resources. This leaves siblings dealing with the lifeboat dilemma. The siblings must ask themselves, should I keep all the resources for myself or should I share them with my sibling? Parents often have more offspring than they have resources for. Keep in mind, that resources do not need to be tangible things, they can be something as simple as time to spend with the child. This means that children have to fight for these limited resources.

An example of sibling rivalry can be found among dogs when they are puppies. The size of a litter can range dramatically between different breeds; however, usually a dog will have more than one puppy. When puppies are born they depend on their mother for a lot of their needs, especially for food. Puppies must nurse from their mother for at least a month after birth. Her milk is full of nutrients that help the puppies grow. Sibling rivalry can happen while puppies are nursing. If the dog has a large litter she may not have enough nipples to feed all the puppies at once. This means that the puppies are competitive against each other to be fed and often push their siblings away so that they can eat first. Sibling rivalry occurs in this situation because there are limited resources.

Sibling rivalry is apparent in many other species and can often lead to siblicide if conditions call for it. Siblicide is when a sibling kills their brother or sister. “If resources turn out to be inadequate for all nursery mates to survive, then sibling rivalry can escalate to lethal extremes” (Mock, 2005).  It has been observed in many bird species (Sulloway, 1997). This occurs with the blue-footed boobie, a small bird. Most chicks within this species have siblings that they are competing for resources with because usually two or three chicks hatch at a time. If the oldest sibling drops 80% below its normal weight because of food shortages, it will peck it’s sibling to death. “In experimental studies in which the necks of booby chicks have been taped to prevent them from ingesting food, aggression increases sharply and is especially pronounced in the elder chick” (Sulloway, 1997). Think back to the lifeboat dilemma, the oldest blue-footed boobie is being selfish and keeping all the resources for himself by killing his sibling. The birds know that if the siblings share the food neither of them will survive because there is not enough for both of them so one takes it upon himself to kill the other. The parents of these birds have little control over this. They must accept it because they need to protect at least one of their investments so that they have reproductive success and pass down their genes. This is a very extreme example of sibling rivalry because it includes siblicide.

So now let’s see if we can answer the question I posed earlier in this discussion. I personally believe that I did not fight with my sister very much because of the age difference. When I was born, she was thirteen years old. She was not in need of the same resources that I was in need of so she did not need to worry so much about me stealing resources from her. She was also becoming more mature as she was getting older and she had more autonomy. She could use different outlets to get what she needed.

My brother and I, on the other hand, are a lot closer in age. He was four years old when I was born. He was still very dependent on my parents. This creates a situation where sibling rivalry could and did occur. My mother told me that at times my brother wanted to send me back. He was not happy that I was taking up so much of my parent’s time and resources and this was one of the ways that he was expressing that. This can be related to siblicide, but on a much less extreme level (thankfully for me!). My brother felt threatened that he was not going to get everything that he needed, so he wanted me gone.

When I think about sibling rivalry on a deeper level, I realize that there probably was sibling rivalry between my sister and I; it was just different than what my brother and I were experiencing so it was not as obvious. She probably was as upset about my needing my parent’s resources as my brother was, but she expressed it in a different way. My brother and I physically fought. My sister could not physically fight with me because she was a lot older and that would not be appropriate. She probably fought with me in a different way. When I was older and able to walk and talk, she may have teased me about certain things. This was her way of fighting with me that was more appropriate for her age. Therefore, sibling rivalry can happen regardless of the age difference between the children.

Luckily, I’ve made it this far and neither of my siblings has tried to take my life. Now that we are older we are all good friends; who would have thought!

Mock, D. (2005). Sibling Rivalry. New York, NY: Oxford University Press, Inc.

Sulloway, F. (1997). Birth Order, Sibling Competition, and Human Behavior. Retrieved from

Workman, L., & Reader, W. (2004). Evolutionary Psychology: An Introduction. Cambridge, NY: Cambridge University Press.

Kaitlyn Andersen

About Kaitlyn Andersen

Kaitlyn Andersen is a senior at the State University of New York at New Paltz. She is currently studying Psychology and plans to go on to graduate school to pursue Mental Health Counseling. She is very active in her community. She has organized a blood drive, been a co-captain of a Relay for Life team and volunteered in an elementary school classroom all in the past year.
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4 Responses to Sibling Rivalry: Will it Ever End?

  1. Rose Rose says:

    Great post Kaitlyn, and welcome! Sibling rivalry research could really profit from an examination of age differences. For birds and dogs, siblings are all the same age, or very close to it (such as with asynchronous hatching). Humans, with slow life histories and small litters, are different. A systematic study of the differences in rivalry for a 10 year or 1 year age gap could be really enlightening!

  2. Chris M says:

    Reminds me of my childhood :)

  3. Kaitlyn Andersen Kaitlyn Andersen says:

    Thanks Maureen! It very well might be :)

  4. Maureen O'Keefe says:

    Good job. Perhaps that is why I am an only child.

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