The Evolutionary Significance of Sibling Conflict


Almost all of the parents when bring in a newborn to a family have to deal with the reactions of the first offspring, who may or may not be too excited to see the new arrival. Thus, begins the lifelong rivalry amongst the siblings. As much as love and care exists between them, they tend to fiddle with anger and competitiveness throughout their lives. Evolutionary psychologists, such as Hamilton and Trivers have produced three famous theories that explain such behavior among the siblings, namely Fitness Theory that explains the biological and genetic structure of the child to produce selfish behavior against the siblings; Parental Investment Theory that explains that parents’ differential investment in offspring result in conflict among them and Parent-Offspring Conflict Theory, which focuses on the lifelong competition among the children for obtaining more resources from the parents then they are willing to offer. Other than these there are also other causes such as parental favoritism and relationships with step and half-sibling.

The Evolutionary Significance of Sibling Conflict


The relationship among siblings shares the same importance in the family relationships like all others. In fact it constitutes the longest duration of all relationships in the family. But when we ponder upon the essence of this sibling relationship, we come across a relationship filled with rivalry and conflict which has been common throughout the human evolutionary history. Almost all of the parents when bring in a newborn to a family have to deal with the reactions of the first offspring, who may or may not be too excited to see the new arrival. Thus, begins the lifelong rivalry amongst the siblings. As much as love and care exists between them, they tend to fiddle with anger and competitiveness throughout their lives. For this reason, parents often prepare mentally their first born to cope with the new additions to the family. There are books and writings available to aid the parents to teach their child to love their siblings. If the sibling rivalry was not so common and not bearing such an evolutionary significance, then perhaps all of the book authors of children’s rivalry books would have gone bankrupt. Parents do know about what’s to come ahead when they decide to reproduce once or twice more and they have their own ways of dealing with their children’s reactions (Dunn, 1982). But as much as it can be done to prevent and bring under control the rivalry and the conflict among the children, the reasons behind such aggression should also be discussed and understood. Evolutionary psychologists have produced three famous theories that explain such behavior, namely Fitness Theory, Parental Investment Theory and Parent-Offspring Conflict Theory. Other than these there are also other causes such as parental favorism and relationships with step and half-sibling, all of which will be discussed in the paper.

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Fitness Theory

The Fitness Theory was presented by Hamilton in 1964 (Mock, 1998) which is very influential in the explanation of the evolutionary sibling conflict. This theory states that if an altruistic pattern of behavior exists in the dominant gene of the parents, then the child being carried would result in having that gene in a larger proportion. He explained the theory in mathematical terms whereby using probability of r to define the share of the child of the gene of the parent. Hamilton developed the theory to explain altruism but his explanation was also used to establish the reality and logic behind the sibling conflict. His theory was used to counter a condition whereby the child is born with a gene that promotes selfish behavior against the siblings. Hamilton’s theory thus, spells situations and circumstances that could lead to a child expressing selfish and aggressive behavior towards the relatives, primarily his own siblings. According to Hamilton and his theory, the sibling rivalry springs out of evolutionary biologically and gene structures inherited from parents which take much stronger depth in the child (Mock, 1998). Children benefit from the greater virtue of fitness in relation to parents but they also intake selfish attributes towards their siblings.

Parental Investment Theory

Another remarkable piece of work in relation to sibling relationships was put forward by Trivers in 1972 in the form of Parental Investment Theory. According to the Parental Investment Theory, parents invested their energy and resources in a child that caters to its future survival at the expense of their energy that they gave in for him. But it so happens that when parents produce another offspring, either the former or the later suffers from lack of sufficient resources on part of the parents, as they can only provide sufficiently for one. Thus, when the parents ensure the sustainable future survival of one offspring, the other is deprived of it. This naturally results in a conflict among the siblings to attain the parents’ resources. Parents’ difference in the investment in the children thus leads to the conflict among the children, who fear having less of the resources to survive in life and feel deprived of the love and care given to another. This usually occurs in children who have a little age difference. In such cases the siblings view themselves as equal and the elderly care does not take strong grounds and thus results in a bigger conflict which lasts for a long time (Mock, 1998).

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Parent-Offspring Conflict Theory

Parent Offspring Conflict Theory was presented by Trivers in 1974, which elaborated why the siblings conflict in relation to the parental resources. He said that the children demand more resources from the parents than they are willing to give and parents on the other hand demand more closeness and love among the children than the children are willing to have among the siblings. The siblings engage in competition for the limited parents’ resources. Parents on the other hand do tend to provide one offspring with a larger pool of their resources than they do to the rest. But there are many reasons behind it such as the birth order, the age difference between the children and the status of the children. One child may be gifted or lacking in health or mental abilities and thus require more resources from parents. But children at a young age do not understand why the resources are not distributed more or less to them. Their conflict for the attainment for the parents’ resources continues throughout their lives, as their personality development at the young age takes place in conflict with the siblings which only grows stronger with age (Michalski, 2008).

Parental Favoritism

Another explanatory element in the conflict-filled relationship among the siblings is the favoritism presented by the parents. Parental favoritism has a lot to do with the birth order of the children. On the other hand, historic works such as that of Trivers suggest that parents tend to provide the maximum of their resources to the child they presume holds potential for greatness in future and take their name to a high position. This is evident where there is an intellectually gifted child present in a family and the parent invest heavily on him to ensure that his future is well maintained and his abilities are not wasted. This is of course a relevant treatment but it so happens on the expense of the stable condition of the other siblings who only see being unfairly treated and in turn suffer from parental favoritism. “Parental favoritism” results in the child having aggressive feelings towards the sibling being preferred over him (Michalski, 2008).

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Conflicts with Step and Half Siblings

Due to the circumstances faced by parents, children have to come across siblings in the form of half and step siblings. Research shows that sibling conflicts exist similarly among full, half and step siblings, but the intensity is different. With full siblings, who share the same genetic structure, the conflict is more as they are born with the perception that these are their direct competitors. Although the step and half-siblings pose as much threat to them for the parents’ resources, the conflict lies more with the full siblings. But then again, the conflict is more with the siblings who spend more time growing up. If the step and the half-siblings arrive during later stages that they do not fall prey to the competition. But they spend enough time together growing up during childhood then conflict exists among them too (Berry, 2005).


Sibling rivalry has immense evolutionary significance as it constitutes reasons and explanations that go beyond the child’s self-learned personality characteristics. Where biological explanations lack ground in explaining the siblings’ lifelong conflict, parental reasons and behaviors provide evidence to the aggressive behavior among their children. Children are bound by the need to be loved and care for and each of the family’s offspring demands maximum attention and investment of efforts of the parents. Not all families have only one offspring and where there are more; the result is only sibling rivalry. Although parents have to do a lot during the early stages to prevent any harsh consequences among the children, only slight deviance of attention from one offspring to another can result in a lifelong conflict among the siblings that only the parents fall accused of.

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