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Genetics of Sibling Rivalry

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Theme Magazine, Winter 2005
Click here for .pdf of article

A year ago on a snowy night in New York two sisters boarded a plane to India. Each arrived at the airport separately, checked in separately, and caught sight of the other in the waiting lounge but only acknowledged her with a nod. On the plane they sat and read magazines in seats twenty-four rows apart, for the entire seventeen-hour flight. Alexa and Kaili were traveling to India for one of their so-called “bonding trips.” Unfortunately, they had stopped speaking to each other twelve hours earlier. In fact, right up until five hours before the plane departed, Alexa threatened to not come at all. “I tortured Kaili for as long as I could,” said Alexa. “I wanted to punish her.”


This argument may have started thirty-five years earlier, when Kaili, the older sister, at age two, wrapped herself in a blanket and climbed into her baby sister’s empty carriage. Alexa had just arrived home from the hospital and was surrounded by ogling parents. Kaili, enraged with jealousy, wanted the attention back. So into the carriage she went, a return to babyhood, and waited for her parents to find her.

Or maybe the argument started even earlier, when their mother weaned the stubborn and still tugging Kaili off her breast, in order to make room for a new baby sister. Kaili felt gypped, and with little fetal Alexa curled inside the womb the sibling rivalry had begun.

Just before the trip to India Kaili accused Alexa of constantly borrowing money to pay for previous vacations. She thought Alexa was “just using me for my money.” Alexa felt that she was doing Kaili a favor by accompanying her on the trips, “I never wanted to go on the trips in the first place! She bribed me and then held it against me later.” Each said they wanted the sister relationship to end. “We hated each other,” Alexa said. “We thought, ‘Why are we bothering?’”

On their first night in India the sisters were forced to stay together in a dark and foul-smelling hotel room in Bombay. There were bugs the size of a human palm coming out of black holes in the walls. There were no locks on the doors. Something thick oozed from the ceiling. They were so frightened they crawled into bed together and comforted each other through till morning. Each said later, “I was so happy to be with her. I wouldn’t have wanted to be with anyone else. I knew she’d protect me.”

It might seem nature has played a nasty trick in making the bond of flesh and blood one that pulls between poles of hate and love. Or we might be thankful nature has provided endless dramatic material for movies, books, art, and hours of coffee-talk. Aristotle noted that a story about two strangers who fight to the death is nowhere nearly as interesting as a story about two brothers who fight to the death.

But we still wonder: why is the sibling relationship so tumultuous, one week beneficial and loving, the next full of hatred and jealousy? Why are family ties so volatile, yet at the same time so much stronger than any non-kin relationship? Why do we say, “Blood is thicker than water?”

Social scientists and psychologists have created explanations over the years that describe social and environmental effects on family structure, but only one theory has explained the fundamental cause of sibling rivalry. Robert Trivers, thought to be the most influential evolutionary biologist of our time, wrote a paper in 1974 titled “Parent-Offspring Conflict.” Only fifteen pages long, this paper ultimately changed the way we see human nature and evolution. Using hard science to explain the tragedy of the human condition, “Parent-Offspring Conflict” gives us a deceptively simple explanation for the sibling relationship, and subsequently “the many roots of our suffering,” as Trivers explains it. Plainly put, it’s all in the genes.
The theory is essentially this: parents and children, above all else, want to make sure their genes get passed safely to the next generation—but the way in which a child and a parent ensure that safe passage differs. From the parents’ perspective, it is best to lavish attention equally among their offspring. Equal investment in their children’s survival increases the chances that all offspring will successfully pass their genes onward, to their grandchildren.

Children, on the other hand, see things differently. Each child is made up of genes from their parents, fifty percent from their mother and fifty percent from their father—which, technically speaking, add up to one-hundred percent of their own genes. A child is thus twice as related to herself as she is to her sibling. For instance, Alexa contains one-hundred percent of her own genes, but she also shares fifty percent of her genes with her sister, Kaili. So Alexa has twice as many “Alexa genes” as does Kaili and should therefore, by the logic of maximizing one’s genetic propagation, get twice as much parental attention. From Alexa’s perspective, it is perfectly fair that she receive two-thirds of parental attention, and that Kaili receive the remaining one-third. (Alexa doesn’t give a damn about the other fifty percent of “Kaili genes.”) Of course, it’s the mirror image from Kaili’s perspective. The seeds of rivalry are thus deeply planted.

Consider the cutting of a chocolate cake. The parents want each sister to have equal halves. But each sister wants two-thirds for her and only one-third for her sister. There is no distribution that could please everyone. Conflicts erupt. Parents who painstakingly craft equal presents at Christmas are shocked when they are forced to listen to hours of angry debate over who got the better gift. Parents across cultures plead, “Be nice to your brother!” The classic, “Mom always liked you best!” echoes in the family for years.

Parental investment is a limited resource. There is limited breast milk at the cradle and there is limited inheritance at the grave. Should there be a slight favoring for one child, each child would want to ensure they are the favored one. It’s a pitched battle between siblings that rises in intensity depending on personality, environment, age, and gender difference. But the one-track goal of a gene still runs along the foundation of the sibling relationship: protect and propagate.

Now, this is the point where people get upset. When biologists step into the realm of explaining human choice and passion, most people immediately shoot back defenses about the power of self-will and socialization. “The genes can’t be the overarching explanation!” they say, or “There are too many social, environmental or religious variables! We are not DNA puppets!” And this is true…sort of. Trivers never intended his theory to make us mere machines plodding along a predetermined path of tragedy and happiness. Genes don’t determine the behavior of individuals, but over millennia they express tendencies for specific patterns. By providing subtle instructions for our neural network , genes can become a powerful, if unconscious, influence on our behavior.

A convincing argument for Trivers’ theory comes from the animal kingdom, where sibling conflict is seen in a more naked form. The cuckoo bird is built with a small dip in its back, which it uses immediately following birth to scoop up and toss out all un-hatched eggs in the nest. It kills the competition before it even opens its eyes. The nature of the sibling relationship also crosses history and culture: Cain and Abel, Romulus and Remus, Bart and Lisa. The arts have long understood that sibling rivalry provides the most compelling narratives. The fact that it crosses time, culture, and species tells us there’s something to this genetic story.

But life is not all nasty, brutish and short in the sibling world. There is also the paradox of sibling altruism, love, and loyalty. Trivers’ theory accounts for these as well. Because each sibling shares fifty percent of their genes, each has some interest in protecting those genes and increasing the reproductive success of their brother or sister. Again, it’s all about passing your genes on to the next generation. And if you’re not working on it yourself, then the next best thing is to make sure your sibling has a baby.

Within the first days of their India trip, Alexa fell ill with an infection that covered her body with septic ulcers and rashes. Kaili canceled the itinerary and stayed by Alexa’s side. With Alexa still not well enough to travel by their departure date, Kaili canceled their flight, missed work and stayed until her sister fully recovered. She had completely forgotten she wanted to abandon Alexa only weeks before.

This nepotistic altruism counters sibling competition; it is what makes blood thicker than water. Moral philosophers pose this problem: Think about two buildings on fire. Inside building A is your brother or sister, and inside building B is a group of strangers. You only have time to run into one of the burning buildings. Is there any number of strangers that will cause you to choose building B over A?

Today Kaili can’t remember what made her so angry with Alexa before their trip to India—there had been so many arguments. Yet these days she no longer talks about her “cruel upbringing” and how their parents supposedly neglected her. She doesn’t feel the jealous rages anymore. Now she wants to give more to her sister, without an invisible tit-for-tat contract.

“Maybe it’s the birth of Alexa’s baby, the first in the family,” she says. “All the petty fighting seems so meaningless to me now.”

This entry was written by Christie, posted on December 7, 2005 at 8:03 pm, filed under Home, Print. Bookmark the permalink. Follow any comments here with the RSS feed for this post.

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