Have you noticed that two siblings raised in the same home — even twins — often develop personalities that are completely opposite rather than similar? One may become responsible, rule-abiding, and disciplined, while the other grows into an independent, rebellious person who defies authority. This phenomenon has long been studied not only by parents but also by psychologists and sociologists. What’s happening here is more than mere “difference”; very often personalities swing to opposite poles.
Genetic Lottery and Biology
Siblings share on average 50% of their parents’ genes. However, how that 50% is distributed is random. For that reason, one sibling may be prone to anxiety while the other is more resilient. Even in identical (monozygotic) twins, completely opposite temperaments can be observed; this demonstrates that genetic similarity alone does not determine personality (Bouchard, 2004). Biology provides only a groundwork; the full picture is painted by family dynamics and psychological processes.
Psychology of Differentiation: “Deidentification”
Another concept used to explain siblings’ divergence is “deidentification” in psychology (Schachter et al., 1976). Two children in a family cannot occupy the same role. If the older sibling stands out through academic success, the younger sibling feels the need to express themselves in a different domain: humor, sports, art, or a rebellious attitude. This differentiation reduces competition and gives each child a unique place within the family. However, the process often operates through contrast: one child becomes the “one who makes the family proud,” while the other takes on the role of “the one who challenges the family.”
Birth Order
Alfred Adler’s birth order theory provides an important framework for understanding sibling oppositions (Adler, 1928/2019). Firstborns tend to be responsible, orderly, and closer to authority; they experienced the family’s initial focus of attention. Middle children, compared with their older and younger siblings, may feel the need to prove themselves and therefore become more creative and adaptable; sometimes they develop opposite tendencies. Youngest children, on the other hand, often build a more independent, free, and rebellious identity; they choose a different path in reaction to the roles already taken in the family. Within this framework, it is not a coincidence that one of two sons is disciplined and hardworking while the other is rebellious and independent—this is a natural result of psychological and social balances (Plomin & Daniels, 1987).
When Contrast Turns into Conflict
Contrast can add variety to a family, but if handled poorly it can lead to serious conflicts. One sibling may become withdrawn while the other constantly seeks attention. A child who strictly follows rules and a “freedom fighter” sibling frequently come into conflict. Parents are caught in the middle here: the freedom granted to one child may be perceived as unfair by the other, and the discipline applied to one may create jealousy in the other (McHale et al., 2012).
What Parents Can Do
Contrast is natural and inevitable; preventing conflict is possible.
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Avoid Comparisons: Instead of saying “Your older brother works so hard, why don’t you?”, appreciate each child’s strengths within their own context.
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Create Shared Spaces: Find activities that bring opposite characters together. For example, if one likes sports and the other likes art, organize family nature walks or trips to the theater that can engage everyone’s interests.
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Teach Conflict: Rather than telling them to “stop” when fights occur, teach skills for expressing different viewpoints and reaching agreement. When managed properly, conflict can become a problem-solving ability.
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Be Fair: Equality and fairness are not the same. One child may need more boundaries while another needs more freedom; approaching them according to those needs is perceived as fairer by the children.
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Validate Their Identities: If one child excels academically while the other makes the household laugh with their humor, show that both identities are valuable. This increases siblings’ tolerance of one another.
Conclusion: Crisis or Enrichment?
That siblings do not resemble each other—or, on the contrary, drift to opposite poles—is an interesting aspect of human nature. These oppositions can sometimes cause conflict; however, when managed correctly they provide the family with diversity and the community with resilience. From the point of view of a psychologist and sociologist, the contrast between siblings is not a crisis, but an enrichment.
References
Adler, A. (2019). The practice and theory of individual psychology. Martino Publishing. (Original work published 1928).
Bouchard, T. J. (2004). Genetic influence on human psychological traits. Current Directions in Psychological Science, 13(4), 148–151.
McHale, S. M., Updegraff, K. A., & Whiteman, S. D. (2012). Sibling relationships and influences in childhood and adolescence. Journal of Marriage and Family, 74(5), 913–930.
Plomin, R., & Daniels, D. (1987). Why are children in the same family so different from one another? Behavioral and Brain Sciences, 10(1), 1–16.
Schachter, F. F., Shore, E., Feldman-Rotman, S., Marquis, R. E., & Campbell, S. (1976). Sibling deidentification. Developmental Psychology, 12(5), 418–427.