Siblings bring very unique adaptive problems that have been repeated throughout human evolutionary history.
First, siblings are a major social alliance, after all, you and your fellow citizens are genetically correlated by 50%. But unlike other relatives, siblings are also major contenders for parental resources, and parents have evolved a preference for some of their children. The theory of parent-child conflict tells us that it is difficult for parents and children to agree on a view of best interests. This has left children with the constant problem of adaptability: competing with each other for resources from their parents. Given such conflicts, it is not surprising why the emotions of our fellow citizens are always love-hate and contradictory.
In a compelling analysis of research, it is proposed that the problem of adaptation imposed by parents creates different "niches" for children, the differences of which depend mainly on the order of birth. Specifically, because parents often prefer the eldest son (daughter), the eldest son (daughter) is generally more conservative and more likely to maintain the status quo. However, the second son (daughter) has difficulty obtaining resources by maintaining the existing structure, so they need to rely on rebellion to achieve their goals. According to Sulloway, the children born next, especially the middle ones, will develop a more rebellious personality, because if the existing order is maintained, they will have the least resources. However, young children (girls) tend to receive more investment from their parents, because parents usually stop paying attention to other children and invest most of their energy and resources in their last direct reproductive carrier.
Evolutionary psychologists have found some evidence to support these predictions. They found that middle-born children scored lower on family stability and family identity compared to the eldest and youngest sons. For example, intermediate-born children rarely see genetic relatives as the people they feel closest to, and they are unlikely to become the roles in the genealogy.
The above results and other theories found in other studies support the fact that birth order affects an individual's choice of ecological niche, that the eldest son (daughter) is more likely to develop a strong relationship with her parents and believe that their parents are objects to lean on, while children born in the middle are more likely to seek alliances from people outside the family. Interestingly, even if parents treat each child equally, children born in the middle still receive the least amount of total investment. The reason for this is that the eldest son (daughter) receives the full investment of the parents early in life, because other children have not yet been born at that time; After the older siblings have established their own families, the younger son (daughter) can fully enjoy all the resources of the parents. Instead, children born in the middle always share their parents' resources with others, because they have always had their siblings at their disposal throughout their upbringing. So, even if parents try to treat each child fairly, the middle-born child will still suffer, which may be the reason why the middle-born child lacks a sense of identification with their family.
(Bass's Evolutionary Psychology reading notes)