At its most benign, family jealousy between siblings reflects a competition for resources—coupled with the bonds of kinship, which are equally strong. St. Augustine, in his Confessions, described having “personally watched and studied a jealous baby. He could not yet speak and, pale with jealousy and bitterness, glared at his brother sharing his mother’s milk. Who is unaware of this fact of experience?”

This heady mix can lead to all sorts of jealous rivalry and internecine warfare. It’s evident in the animal kingdom, where actual family cannibalism also takes place. The animal behaviorist Scott Forbes makes some fascinating links between sibling rivalry in animals and humans. This is not sexual jealousy, but involves real birds and bees. Forbes describes how herpetologists, ornithologists, and mammalogists found that “infanticide—including siblicide—was a routine feature of family life in many species,” most commonly seen in birds. Some birds lay two eggs “to insure against failure of the first egg to hatch. If both hatch, the second chick is redundant to the parents, and a potentially lethal competitor to the first-hatched progeny.” The healthy older chick often kills the younger to eliminate the competition, and some parents actually encourage siblicide when the death of the nest-mate doesn’t naturally occur.

After all, if resources are scarce, it’s better that the strongest offspring survive and that their potential efforts go to ensuring that happens. (It’s the old story of genetic replication again: Surviving offspring are more likely to have the strongest genes, and they are the ones that have the best chance of reproducing later and passing those genes on.) Forbes thinks that such extreme jealous reactions are not common in the human species, but “the more modest forms of sibling rivalry that are ubiquitous in species with extensive parental care—the scrambles for food and begging competitions—resemble more closely the dynamics that occur in human families.”

Perhaps this fundamental link is why sibling jealousy is such a common theme in life and letters. Forbes paints it out as a normal, natural, and largely unperturbing force in human life, but some of our oldest stories point to its violence. The most famous of all is the murder of Abel by Cain. If you believe the book of Genesis, this was the first jealous murder in human history—the first to be born caused the first to die. According to the biblical story (Genesis 4:1-8), Cain murdered his brother out of jealousy. God was pleased with Abel’s offering of a lamb, but not with Cain’s offering of some leftover harvest and his selfish expectation of something in return—God’s love, or some special standing in his eyes. Cain is disappointed and enraged, and kills his brother in the field. There is the jealous triangle of Cain, Abel, and God, and Cain’s heightened emotional reaction when he feels the threat of losing God’s love to Abel. This particular type of jealous family conflict echoes in many other stories: Joseph and his brothers, Jacob and Esau, Thyestes and Atreus, Romulus and Remus.

The conflict between brothers sometimes stands as an emblem for civil war, including John Steinbeck’s rewriting of Cain and Abel in East of Eden. This great novel focuses on two families, each with biblical resonances. Charles Trask, a farmer, beats his younger brother Adam nearly to death after being rejected by their father, and Adam’s sons, Caleb and Aron, do not treat one another any better. In a climactic scene, Adam rejects Caleb’s gift of money, telling his eldest son that “I would have been so happy if you could have given me—well, what your brother has—pride in the thing he’s doing, gladness in his progress.” In a fit of jealousy, Caleb takes Aron to a brothel and reveals to him their mother, Cathy, who is a prostitute there, leading the disillusioned and disgusting Aron to enlist in the army. He’s killed in the last year of the First World War.

It is not only in literature that such vehement sibling jealousy is to be found. The legendary actor-sisters Joan Fontaine and Olivia de Havilland had a famously jealousy-ridden relationship. They admitted to loathing one another from childhood. At the apex of their jealousy triangle was their mother Lillian, who raised them alone after her husband’s many infidelities and eventual elopement with their Japanese housekeeper, and who flip-flopped in her affection for the girls for most of their lives. Joan, the younger of the sisters, said, “I regret that I remember not one act of kindness from her all through my childhood,” and she spoke in her autobiography of her sister’s unhappiness at having to share parental attention with the new baby. Lillian seems to have encouraged the rivalry between her girls. The fact that they both became Oscar-winning actresses, sometimes vying for the same roles, and one year even competing against each other for the Oscar for best actress (Joan won), was not surprising at all. But it hardly helped the family dynamic. Even Lillian revived her stage career once the “de Havilland” name was getting well-known.

Things between the sisters apparently came to a climax with their mother’s death from cancer in 1975, when they ceased once and for all to speak to one another. Joan was blunt in her feelings: “I married first, won the Oscar before Olivia did, and if I die first, she’ll undoubtedly be livid because I beat her to it,” she told the Hollywood Reporter in 1978. The jealous sins of the parents really do seem to be visited on the children, for Joan fell out with both of her children, Deborah and Marita. This was not just a sisterly jealous squabble. Joan was not reconciled with her sister at the time of her death in 2013 and seems not to have made amends with her children, either.

Lillian de Havilland was just one of millions of parents who, in the first half of the 20th century, would come under more scrutiny than ever before. The prevention of sibling jealousy was in the scientific air. Historian Peter Stearns puts the rise in sibling rivalry in 1920s America down to a number of significant factors. Parents were far more likely to recognize it at this time. A whole raft of “scientific” expertise had arisen warning against the early intrusion of jealousy into childhood and advising parents how to curb it. Sibling rivalry became a formal concept, and the responsibility for dealing with it lay in parental hands—no wonder it sold bucket-loads of books and magazines to anxious mothers and fathers. Parents were not helped with young children in the household either. The new trend for grandparents to live separately and the decline in servants pushed the responsibility back onto a new nuclear family. And the falling birth rate meant fewer older siblings were available to assist, and rivalry for adult affection may have grown fiercer with smaller sibling groups.

Interestingly, Stearns also argues that this was precisely the point at which adult jealousy was being reevaluated: New tensions about the inappropriateness and danger of adult jealousy were displaced onto the childhood arena. He argues that it was now vital that children’s early impulses were monitored and “cured” to guard against a future abnormal personality. Adult jealousy was “swallowed” by this new socially efficacious category. Sibling rivalry was something that people should worry over and could “do something” about.

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Sigmund Freud was behind the new focus on the rivalry and jealousy at the heart of the infant experience. He was fascinated by sibling rivalry: “The elder child ill-treats the younger, maligns him and robs him of his toys; while the younger is consumed with impotent rage against the elder, envies and fears him, or meets his oppressor with the first stirrings of a love of liberty and a sense of justice.” His Oedipus complex was one of the great cultural formations of the 1890s, and became one of the most influential ideas of psychology in the 20th century. It was named after the Greek mythological figure Oedipus, who unwittingly kills his father and marries his mother. Examining neurotic and normal children, and reflecting on the potency of the myth, Freud postulated that every child is a mini Oedipus: “It is the fate of all of us, perhaps, to direct our first sexual impulse towards our mother and our first hatred and our first murderous wish against our father.”

This idea of Freud’s is often ridiculed. Most people believe that the Oedipus complex should be hastily forgotten about; that, because there is no verifiable scientific proof of such speculation about child development, they are without value. Freud himself certainly thought the Oedipus complex could be witnessed and verified—and in his own life. In a letter to his friend Wilhelm Fliess, he wrote, “I have found, in my own case too, [the phenomenon of] being in love with my mother and jealous of my father, and I now consider it a universal event in early childhood.” Ernest Jones, a pupil of Freud’s and his biographer, went even further, dramatically reconstructing the origins of Freud’s psychological speculation:

Before he [Freud] was two years old, for the second time another baby was on the way, and soon visibly so. Jealousy of the intruder and anger for whoever had seduced his mother into such an unfaithful proceeding were inevitable. Discarding his knowledge of the sleeping conditions of the house, he rejected the unbearable thought that the nefarious person could be his beloved and perfect father.

Is this really proof? Or does this tell us more about Freud’s own psychological makeup than anything else? There is no concrete scientific evidence to support Freud’s theory, yet the Oedipus complex does have the appeal of myth and poetry. It says something important about how individual human subjects become part of the wider social structure—how they have to tear themselves away, often violently, from their first primary caregivers to live in the world at large. The “family romance,” as Freud called the incessant intergenerational rebellion, is an essential component of natural selection. Freud himself believed that mythology and literature have a unique ability to reflect an elemental, psychological truth—that jealous rivalry between children and parents is enacted and dramatized again and again over the centuries.

This article has been adapted from Peter Toohey's Jealousy.