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Updated at 10:40 a.m. ET on January 24, 2019.

The fight might be over the last fruit strip or the TV or the best chair in front of the TV; it doesn’t really matter. My children’s conflict has many causes but only one true one: They are siblings, and that’s what siblings do. The war between brothers and sisters is eternal, each generation renewing the hostilities that have defined sibling relations since humanity began.

Although it seems as if my children never give it a rest, in fact they fight far less than the average. Statistically, they should be arguing more than three times an hour, a number researchers landed on not by interviewing children or parents but by installing microphones in the subjects’ homes. Younger children fight even more—six times each hour. This means they have a fight—a real fight, not just cross words—every 10 minutes.

It is very disturbing when the people you love most in the world turn savagely on one another, and from the parents’ perspective, it makes no sense. They’re fighting for the affection, attention, and material goods that their parents supply, all of which said parents are in no mood to hand over after a few hours of constant bickering.

From the combatants’ point of view, however, the conflict is unavoidable. Children fight because they’re wired to. Sibling rivalry is an evolutionary imperative, an innate impulse. We’re programmed to turn on the usurpers who compete with us for precious resources like food and parental attention, and we begin early. By six months, infants get upset when their mother pays attention to a baby doll. By 16 months, they know what bothers their siblings and will annoy them on purpose.

For many of us, our relationships with our siblings are the most profound relationships in our lives, more important and influential than the ones we have with our parents. They are in fact the only relationship many of us have for life, with someone who’s around from the beginning until the end. Humans generally maintain lifelong sibling relationships; we’re one of the few species that does.* Which gives us a long, long time to hold a grudge.

Sibling conflict is not unique to humans, and humans are nowhere near as bad as some other animals are. Many animal siblings actually kill each other, often while the parents look on blithely. In certain bird species, sibling murder is so common it’s known as obligate siblicide. Black eagles are particularly vicious. In one of the few observed accounts, the slightly older chick attacked its slightly younger sibling 38 times over the younger’s three-day life span, delivering 1,569 blows with its sharply hooked beak. There was, by the way, more than enough food for both.

Sand tiger sharks commit sibling murder on a far greater scale, beginning before they’re even born. They play an in utero version of the Hunger Games, using their nascent teeth to chomp up all the sibling embryos they can. The shark that’s eventually born is just the last one standing. How did researchers figure this out? A biologist dissecting a pregnant shark was bitten by an embryo, still swimming around in the uterus, still looking for siblings to eat. Pigs are vicious, too, born with teeth that are angled to gash littermates while they nurse.

Sibling rivalry is common to all living things, even plants, which will chemically poison competing offspring to divert resources to themselves. Even bacteria fight with their bacterial siblings, resorting, like sharks, to cannibalism and fratricide.

Human siblings rarely resort to murder, and even more rarely to cannibalism, but they certainly scrap. For most of history, however, sibling conflict was subject to little examination and even less concern. Given how incredibly annoying it is (it is the part of parenting I hate most, which is saying something, given that parenting is a job that also requires cleaning diarrhea out of neck folds), it seems surprising that there’s so little complaining in the historical record. I can only assume that parents either didn’t see it as a problem, or didn’t see it as their problem.

While there’s little recorded evidence of parents trying to stop sibling conflict, there’s plenty of evidence that conflict occurred. Both myth and history are full of examples, with the Bible alone providing a good half-dozen case studies. Sibling conflict shows up in about 20 of the 50 chapters in Genesis. The very first homicide occurs between the very first brothers, Cain and Abel. Esau and Jacob, like sand tiger sharks, begin fighting while still in the womb. Later, Jacob favors his own son Joseph so blatantly that Joseph’s jealous siblings throw him into a well and sell him into slavery.

Sibling rivalry occurs in a lot of religious traditions and ancient mythologies. It informs both the Book of Mormon (the scripture) and The Book of Mormon (the musical). In the Hindu epic the Mahabharata, Arjuna kills his brother Karna, and in the Norse sagas, brothers are forever fighting and killing one another off. Romulus whacks Remus after they bicker over a wall. Zeus gets along with his siblings a bit better, marrying two of them (gross) and teaming up with the rest to fight his father in the War of the Titans. Once the war is over, however, the siblings go back to intra-familial turf wars and squabbling.

In the Bible, many of the fights are over parental affection, which is what psychology traditionally blamed for sibling rivalry, when it considered the topic at all. But recent studies indicate siblings are actually fighting over something simpler than that: toys. Eighty percent of sibling fights are over possessions. Parental affection comes in last as something worth fighting over, at a dismal 9 percent.

My children share little besides genetic material, and they don’t share very much of that. Siblings are not, in fact, that similar. My husband and I produced one cautious and thoughtful girl, and two years later rolled out our second model, a whirling tornado of a boy. I really don’t know what we were thinking putting them in the same bedroom. Although they agreed to the arrangement, it’s a little bit like co-housing Alan Dershowitz and Torquemada.

Parents are always surprised when their children turn out to be nothing alike, I suppose because in every other instance, when we put the same ingredients together, we end up with the same product. Though we could not believe how different our children are, we should have seen it coming. Siblings share about 50 percent of their genes, but those genes don’t manifest in the same ways. In genetic terms, this is called recombination. The geneticist David Lykken compares the process to scrambling a telephone number: Arrange the digits in a different order, and you get someone else entirely.

Physically, siblings share few characteristics. Only 20 percent share an eye color; only 10 percent have the same complexion or hair. They’re dissimilar in temperament, too. Siblings share, at least by one study’s accounting, only 15 percent of their personality traits, making them only slightly more alike than two unrelated people raised in two completely different homes. Weirdly, siblings grow less alike the longer they live together. Even identical twins don’t share all their genes. Although they come from a single genome, they have different individual mutations: 359, on average.

Being raised in the same home, it turns out, doesn’t count for much. Siblings can experience it differently, each growing up in what researchers describe as their unique microenvironment. No wonder siblings can remember the same event in entirely different ways. Even concrete facts become subjective. One study found that 53 percent disagree on their father’s level of education, 46 percent on their mother’s. They don’t even agree on their parents’ ages, differing 25 percent of the time.

It’s as if they had different parents, because essentially, they did. Parents almost invariably treat their children differently, even if they try not to. Previous generations were more obvious about it, but we still do it in a thousand ways, sometimes consciously, sometimes not, responding to each child’s gender, age, mood, and—even though we shouldn’t—likability.

Modern parents haven’t stopped playing favorites; they’ve just stopped doing it openly. Though few parents today will admit they have a favorite child, studies indicate that about two-thirds of parents do. In one small but astounding survey, 80 percent of mothers acknowledged favoring one child over the others. This was no secret to their children, 80 percent of whom agreed. Interestingly, however, when they were asked which child their mother loved most, they almost always got it wrong. Similar results are borne out in larger studies: Two-thirds of children accurately perceive that their parents have a favorite, but less than half get the favorite right.

The idea that you’re supposed to treat your children equally is recent, and it’s still not the norm in much of the world, where different siblings might have different roles and even different titles. In English, we refer to both younger and older siblings as sister or brother, but Chinese has separate terms for each. A gege (older brother) has different rights and responsibilities than a younger one (didi), as do a jiejie (big sister) and meimei (little sister). In Japan, an old slang term for the second son was “Master Cold Rice,” because historically he ate only after the firstborn got his food.

Treating all your children the same is certainly not the norm historically, either. Playing favorites is called “parental differential treatment,” and it was standard practice until fairly recently. Treating all your children the same would be as ridiculous as, say, treating your husband and the doorman the same because they’re both men, greeting them both with kisses and giving both tips for bringing up the mail. The two just play different roles, and there are different expectations for each.

As the 17th-century poet Anne Bradstreet insisted: “Diverse children have their different natures; some are like flesh which nothing but salt will keep from putrefaction; some again like tender fruits that are best preserved with sugar; those parents are wise that can fit their nurture according to their Nature.” This was especially true in the large families of previous generations, where you might have a dozen children spaced out over 25 years. With so many kids, favoring one wasn’t such a big deal; if you weren’t the darling, you were in excellent company. And even if parents wanted to treat all their children equally, they just couldn’t. It’s one thing to divide a cupcake into perfect halves, quite another into perfect twelfths.

A few experts thought parents should at least try. In 1600, the Welsh writer William Vaughan urged, “Parents must ordinarily use equality among their children so neere as they may, and not shew more affection to one then to another, least thereby they provoke them to anger and desperation.” In his 1622 book Of Domesticall Duties, the English minister William Gouge addressed the subject at greater length, counseling parents to treat their children equally in a passage that sounds a lot more like a legal contract than a philosophy of love:

The parties to whom parents are to performe all the forenamed duties are expressed under this word (Children) which hath not any speciall respect to prioritie of birth, to constitution of body, to affection of parent, or any such thing, as if first borne, proper, beautifull, darling, or the like children were only meant, but all that are begotten and borne of parents, all their children are meant. Whence I observe, that Parents ought to have an impartiall respect to all their children, and performe dutie indifferently and equally to all.

Let’s remember, however, that the 17th-century definition of “equal” is a lot different from ours. Gouge, for instance, says it’s fine to favor one child over another if the favored child is good and the unfavored bad: “It is no partialitie to like grace and goodnesse in a childe, and for grace and goodnesse sake to love his childe so much the more, as also for impietie and obstinacy in rebellion to have his heart the more alienated from his childe: this is rather a vertue in a parent. Partialitie is when on by and undue respects one childe is preferred before another.” And while he urges parents to distribute their affection and attention equally, the property should not be. The loot, he says, should go largely to the eldest, for several reasons: Scripture says so; it’s the law of the land; it keeps the estate intact; and firstborns are generally just more awesome than the rest. (Known as primogeniture, this has been the general rule since ancient times and still persists in parts of the world, as well as on Game of Thrones.)

Here I should point out that Of Domesticall Duties is a Puritan work whose primary reference is the Bible, which is not exactly a handbook for treating your children fairly. While we frantically strive to keep sibling conflict to a low simmer, biblical parents tend to stir the pot with many-colored coats and schemes to divert birthrights. Though they don’t agree on much, this is something both the Bible and evolutionary theory endorse. Evolutionary biologists have argued that parents are incentivized to encourage sibling conflict as a way of sussing out the strongest child, the one who’s most likely to survive and therefore the best to invest resources in. Unsurprisingly, the losing children tend not to agree. In the Bible, as in life, this typically leads to conflict, at which point you root for the one you like best and maybe arm him with a slingshot, like any loving parent would.

Sibling fighting passed nearly unremarked until about a hundred years ago. For the first time in history, parents wanted to do something about it rather than, you know, make it worse with blatant favoritism. For the first time ever, this was actually possible. Western parents had more time and attention to give now that families were smaller. The number of births per white American woman, for instance, declined from an average of seven in 1800 to 3.56 in 1900. Child mortality had decreased, permitting parents to focus on the less pressing issues. Also, now that parents were encouraged to do the child-rearing themselves as opposed to outsourcing it to nursemaids, tutors, and the like, they discovered how contentious children actually are, and set about finding ways to ameliorate it.

Starting in the 1930s, the topic really caught on, when the psychiatrist David M. Levy coined the term sibling rivalry and published a book on the subject. Rivalry, however, is a pretty mild description for what Levy observed when he presented his patients with celluloid dolls that were supposed to represent their younger siblings. The patients promptly destroyed and dismembered the dolls in a massacre so wide-scale, Levy had to switch from celluloid to a more durable clay. And it wasn’t just spoiled American brats who did this; Levy’s experiments with the Kekchi Indians in Guatemala produced the same results.

By the mid-1940s the term, and the concept, began to enter the public consciousness, with more and more books and magazine articles addressing the issue. No surprise: Parenting had become child-centered. What happened when “child” turned into “children,” and there wasn’t room in the center for both? It was as if the Messiah got a brother, and parents didn’t know what to do.

As usual, Dr. Spock set the tone. In The Common Sense Book of Baby and Child Care, he discusses the matter at length, focusing on the primal drama of the second child’s arrival. Spock has been credited with introducing the enduring analogy of the new baby as a mistress or second wife: a cute, unemployed drinker the current wife is not so thrilled to greet. And indeed, his advice is to treat the new baby as such, waiting until you’re alone to hold her and playing it cool: “Treat her casually. Don’t act excited about her. Don’t gloat over her. Don’t talk a lot about her. As far as possible, take care of her while the older one is not around.” Did Spock’s approach work? Probably not; as he laments in his 1985 edition—emphasis his—“Jealous quarreling among brothers and sisters has been tolerated much more recently than it was in previous generations.”

Still, there weren’t a lot of better ideas. Some were strange: According to a 1979 New York Times article, in Mexico, sibling rivalry was treated with garlic, though the piece didn’t specify how. Other experts mostly just elaborated on Spock’s approach, in terms that can’t help but rattle the modern ear. Writing in 1970, the psychologist Fitzhugh Dodson notes that the arrival of a younger sibling might make children feel “gypped” (!) and prompt them to regress, as his own daughter did when she asked that her Coke (!) be served in a baby bottle. Dodson complied.

Then there’s Rudolf Dreikurs, whose philosophies my own mother turned to when her daughters’ constant bickering drove her to enroll in a night-school class called “Discipline Without Tears.” The thinking, I suppose, was that the class would give her the tools to deal with us, or at the very least get her away from our shouting every Tuesday and Thursday from seven to eight. A disciple of the Austrian psychotherapist Alfred Adler, Dreikurs advocated individuality and family democracy, and his technique for dealing with sibling conflict was enormously simple. He wrote an entire chapter on the subject, but everything you need to know is in the title: “Stay out of fights!” By refereeing, he argued, you just prolong the conflict and encourage it to happen again.

More recently, research has backed Dreikurs up. As multiple studies have shown, siblings fight more when a parent is present. When parents routinely intervene, the fights are more savage and last longer—which suggests that the historical record’s neglect of the issue was perhaps the right way to deal with it. When Spock observed that children were fighting “much more,” maybe it wasn’t because his methods didn’t work, but because he had methods in the first place, because we were talking about it at all, because we were devoting chapters and books and therapy sessions to it. Sibling conflict interventions have become an industry, but the industry hasn’t minimized the conflict so much as multiplied it. As the psychologist Laurie Kramer has pointed out, the countless cartoons and books that are supposed to teach squabbling siblings to reconcile actually increase the squabbling, by modeling fights and giving siblings new ideas for torturing one another.

I’ve read many thousands of pages on managing sibling conflict, and Dreikurs’s method is the one I’ve come to favor, though I’m not sure if it’s because it works, because it’s what my parents did, or because it literally requires me to do nothing. I often fail, however, because doing nothing is often the hardest thing to do. It is very, very difficult to listen to two screaming children and think, Carry on; and it is very, very easy to scream yourself.

The best advice, perhaps, is what my parents routinely told my sister and me when we were irritating each other: “If you ignore her, she’ll stop.” We ignored the advice then; I still do. It can be awfully hard to stay out of sibling fights, and most parents don’t. We’ve made it our problem, and now it is.


This article has been adapted from Jennifer Traig’s book Act Natural: A Cultural History of Misadventures in Parenting.

* This article originally stated that humans were the sole species to have lifelong sibling relationships.

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