The Science of Siblings

As an only child, I have always been fascinated by siblings. Just like an exclusive club, the complex interactions between my friends and their siblings seemed foreign and complicated to me. Simple things, like the way they argued, or shopped for each other, or complained together, made me yearn for that level of easy companionship.

My dad (left) and his four siblings. Even through this image, you begin to get a sense of their differences.

When I was in ninth grade, I became good friends with a girl who was the third child in her family. She lived far away, so I would spend weeks at time staying with her family. And although I really did love her, I had an ulterior motive! Each night, I would excitedly call my parents, recalling stories about fights between the assertive and intelligent eldest brother, tactful and anxious middle sister, and my creative and confident friend. And as I was being drawn into the family fray, I quickly became engrossed with the effect sibling order had on their very different characters. Could the differences between my friend's siblings be explained by adaptive radiation? Was it possible that, like the anoles we had learned about in AP biology, it was the need to crush competition that drove siblings to occupy different roles, or niches, in the family?

The family who fueled my interest in siblings!

 

Looking for answers, I turned to the psychologist Frank J. Sulloway. It was Sulloway who popularized the Family Niche Theory in the1990s, arguing that it was a sibling's need to eliminate competition for parental attention that shaped personality. According to him, there are distinctive factors in every family that cause similar traits to develop in most people born in the same family position.

Take the first born, for example. During the first few years of development, eldest children have zero competition for their parents' time and attention. Score! The lack of sibling rivalry means that the only pressure exerted on their personality comes from their parents. This results in eldest children who are high achievers and respectful of authority, as these are traits that most parents esteem. The excess of attention also results in slightly higher IQs, and an over representation of first borns at both Harvard and Yale. In fact, eldest children make up 41% of Harvard's class of 2021! 

Moreover, once competition is introduced into the family in the form of siblings, first borns have a clear physical advantage from the get-go. This makes them more likely to use physical power then their younger siblings, who have to resort to social manipulation. (Side note!!! Some famous firsts include Hillary Clinton, Beyoncé, Winston Churchill, and all the actors who played James Bond. Fits the mold if you ask me!)

James Bond: the self assured prototype of the eldest child. (source)

Next up, and the most interesting in my mind, is the middle sibling. In the animal kingdom, parents are more likely to favor first borns, who are closer to their reproductive years, and last borns, who are more susceptible to disease and predation. Although this trend is much less distinct in the human world, the stereotype of the forgotten middle child certainly rings true. According to Sulloway's theory, middle children are more self-sufficient than their siblings, and generally removed from the family group. They're also usually lower academic achievers than their older and younger siblings. In that same Harvard class that was 41% oldest child, only 14.5% were middle children compared to about 20% of the general population.

Although a joke, the popularity of shirts like this one demonstrates just how deeply sibling stereotypes resonate. (source)

Finally, comes the youngest child. Stereotyped as the baby from birth, last borns come into families that have a more limited number of available niches. As a result, youngest children turn towards experimentation that allows them to find their own unique, uninhabited place in the family group. This often creates a youngest child who is more comfortable with risk and unpredictability than their older siblings. An interesting youngest child fact: it was youngest children that were the first financial backers of the Enlightenment. How's that for an exploratory spirit?

Famous siblings I have always been fascinated by! (source)

Although I found Sulloway's theory fascinating, I was skeptical about its ability to be proven empirically. However, after some research, I was pleasantly surprised that I was able to put my skepticism to rest. First borns, for example, are statistically less likely to participate in dangerous sports than their siblings. They are also much more likely to report self-assuredness at work and demonstrate a higher ego than their younger counterparts. Both of these statistics are consistent with the description of oldest children as responsible and high achieving. 

And, although the following is purely anecdotal, I was also interested to note that dictators like Stalin and Mussolini were eldest children, while groundbreaking thinkers like Marx and Darwin were youngest children. 

In fact, it was Darwin who is responsible for much of our modern day understanding of evolution and ecology. The Family Niche Theory itself would not have been possible without his observation of finches. While on the Galapagos, Darwin realized that the fourteen distinct species of finches he was watching had all evolved from a shared common ancestor. In order to decrease competition for resources, the populations of finches had evolved over time to fill different ecological niches within the environment. This process, called adaptive radiation, increased the fitness of all the newly speciated birds. 

Adaptive radiation as observed by Charles Darwin. (source)

Much like Darwin's finches, siblings are able to survive and thrive when they have the least possible competition for their parents' time and energy. This leads children to go to great lengths (subconsciously of course) to differentiate themselves. Think of it like this: if you and your brother both play guitar, you only have access to half of your parents nurturing love and attention at the end of year concert. But, if you take up painting while your brother plays guitar, you won't have any competition for their time at the art show. 

Although this is a simplistic example, it is effective in proving the point at hand. By separating themselves from their siblings, children are finding unoccupied niches, and performing their own form of adaptive radiation, within the family unit. 

Now, it is important to know that the data supporting this Family Niche Theory is limited at best. Every family is different, which makes it nearly impossible to observe consistent trends between a large sample of family groups. Additionally, most of the statistical data about personality is self-reported, making it both biased and precise. However, the ecological premise of natural selection influencing family structure is a strong one. So next time you get into a fight with your siblings, take a moment to think about why. Perhaps it's because you've been competing for the same ecological niche!