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Birth Order--Evolution at its Best?

ekim's picture

Nearly 170 years ago, Darwin introduced the story of evolution. Darwinian evolution explains the small differences within the same type of individuals and how this variation creates competition for the best to survive and pass on the best genes (1). Now, 170 years later, Darwinian evolution still prevails, even in the household of every family.

It is no surprise to see siblings in argument, especially because of clashing personalities. But why is the fighting so common? Is it only due to differences in personalities? Or could this fight actually be Darwinian evolution in the works? Could this fight be a form of competition to seek superiority?

In families with more than one child, there are temperaments and personalities that are more frequently seen in first-borns than later-borns and vice versa. Frank Sulloway's theory on birth order may explain this type of variation between siblings. First-borns tend to be conforming, ambitious, academically oriented, and respectful. On the contrary, later-borns show more unconventional, flexible, almost unpredictable, and rebellious personality traits (2). The reasons for these tendencies are in the roots of sibling rivalry, where siblings vie for parental attention and resources. The one who achieves the most would be considered "the fittest." For instance, in the world of sand sharks, instead of showing the subtleties of human sibling rivalry, "siblicide" is exemplified. The sand shark siblings eat each other to death to gain the mother's affections, and thus the survivor becomes "the fittest." Similarly in the world of birds, bird siblings peck each other to death to prevail as the best (2). In both cases of "siblicide," the first-born is the survivor due to its advantage in physical size. This advantage is a matter of "sibling selection": the better sibling will survive. In the family household, "sibling selection" exists in the same way that it does among sand shark siblings and bird siblings.

Although in the human world, a sibling is not killed to determine who emerges as "the fittest," a sibling is outshined upon. If biological selection is the explanation for birth order tendencies, then are later-borns always doomed to be "the unfit?" The first-borns not only have physical size to their advantage, but also get a head start. With first-borns taking the lead in living up to parents' expectations, the later-borns must find a new "niche" to impress and outshine the first-borns. This new "niche" concept harks back to Darwinian evolution and "the principle of divergence," where competition forced the finches to search for new resources on the Galapagos Islands. The "principle of divergence" is replayed in the family household, when competition forces later-borns to find new ways to show-off, hence the more rebellious and unconventional personality traits are seen in later-borns(2). Thus, if biological birth order is the explanation for sibling rivalry, and if sibling rivalry is a form of Darwinian evolution, then where is the "change over time" that evolution typically accounts for? It seems as though sibling rivalry is a repeated form of Darwinian evolution: every generation goes through a cycle of sibling rivalries that are always the same type of sibling rivalries. So what explains the consistently similar competition that does not change over time?

The social birth order theory could explain the consistency in sibling rivalry. Alfred Adler explains that the variation within siblings and sibling rivalry is due to social interaction within family dynamics. Adler suggests that each position in the family elicits different traits (3). For instance, the oldest child becomes self-righteous and authoritative because as the eldest, they have to share parental affection with the later-born and live up to parental expectations to set an example; the only child becomes spoiled because they become the center of parental attention, and they have a hard time mingling with others due to their inexperience in sharing (with a sibling) (4). Although Adler's theory differs from Sulloway's theory-not only in terms of biological versus social perspectives, but also-in what first-borns' traits elicit, the same concept lies behind both theories-birth order affects varying tendencies of different siblings. Thus, family dynamics could explain the consistency in competition between siblings because family conventions (how children are raised and whatnot) have not changed drastically over the years, in turn maintaining the relatively consistent sibling rivalry. However, if family dynamics affect sibling rivalry, does that mean change in family dynamics could change the processes and outcomes of sibling rivalry? Could later-borns take on the characteristics of first-borns in times of change? Furthermore, which comes first in the birth order theory: nature (Sulloway's biological perspective) or nurture (Adler's social perspective)?

As far as I am concerned, more than social aspects, biological aspects seem to play a bigger role in accounting for birth order traits and sibling rivalry. And therefore, the biological perspective is first, the social perspective second. This is because the core of birth order traits and sibling rivalry is a matter of diversity. The social aspects of sibling rivalry and birth order traits cannot fully account for all forms of diversity, which is the root thereof. It can only explain the intensity thereof. Furthermore, even the variation within the tendencies of first-borns and later-borns can be explained through the biological perspective in terms of genes, which determine inherent temperaments and such. Social interactions would not determine variation, but simply foster them. Social interactions are not the roots of variation, and hence not the roots of birth order traits and sibling rivalry. Social interactions are simply the honing of processes after the biology behind sibling variations take place. Thus, the biology behind birth order traits and sibling rivalry start Darwinian evolution within the household through variations within siblings. And in turn, whether or not family dynamics change, the change in outcome of birth order traits and sibling rivalry depends on the person's self, and that person's self is at the question of biology.


(1) "The Idea Takes Shape." American Museum of Natural History. 2007.
American Museum of Natural History: Darwin. 6 Nov. 2007.

(2) Simonton, Dean Keith. Origins of Genius: Darwinian Perspectives on
Creativity. 1999. U.S.A.: Oxford University Press. 135-138.

(3) Eischens, Alissa D. "Birth Order and Its Repercussions."
Northwestern University. August 1998. The Dilemma of the Only
Child. 7 Nov. 2007.

(4) Ph.D., Stein, Henry T. "Adlerian Overview of Birth Order
Characteristics." Alfred Adler Institutes of San Francisco and
Northwestern Washington. 2007. Alfred Adler Institutes. 7 Nov.


DrStein's picture

New Classical Adlerian Web Site Address

The Classical Adlerian Psychology web site address has been changed to Please correct the older link ( as it is no longer active.

Also, please change "The Alfred Adler Institute of San Francisco" to "The Alfred Adler Institutes of Northwestern Washington and San Francisco."