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Family Dynamics

Why Sibling Conflict Can Turn Deadly

We understand why siblings fight but struggle to explain murderous hatred.

Key points

  • Sibling conflict can be seen in species including eagle chicks and cattle egrets, among whom it often turns deadly.
  • Among humans, fratricide is rare, but when it occurs, specific circumstances may lead to an older sibling killing a younger one, and vice versa.
  • The recent deaths of three brothers in Ireland, two by murder and one by suicide, highlight some of the motivations that can propel fratricide.

While many family groups seem harmonious, sibships often harbor deep division and lasting conflict. Evolutionary psychologists explain this in terms of early competition over parental investment (1) but struggle to explain its persistence after parents are gone.

Sibling Conflict Among Animals

Much has been made of the practice of siblicide by eagle chicks. The mother may lay two eggs but typically only one is raised. If both eggs hatch, the older chick typically kills the younger, and weaker, one. While this grisly event unfolds, the parents fail to intervene.

This startling phenomenon seems wasteful but it may actually be an insurance strategy. If one egg fails to hatch, or the chick is not viable, the other is there to replace it. In the parlance of chroniclers of monarchy, there is an heir and a spare.

Eagles must carry food long distances to the nest and would find it difficult to raise two chicks simultaneously.

This sort of skulduggery is not peculiar to eagles. Siblicide occurs in other bird species as well, including cattle egrets and boobies. The cattle egrets typically lay three eggs and the number may get reduced to two by siblicide in years when food is scarce. Pigs are born with sharp teeth that they use on siblings to establish a favorable teat position where they can receive more milk. Competition over parental investment goes farther among spotted hyena pups when food scarcity leads to siblicide that produces growth enhancement for the survivors.

Even if many other species come into the world equipped to handle sibling competitors violently, this does not necessarily mean that humans react in similar ways. Yet, our myths give us fair warning about the worst to expect from sibling interactions.

Cain and Abel

When animal siblings do each other in, the practice is sometimes referred to as Cainism referring to the biblical story of Cain killing his brother Abel.

While fratricide is a real-world occurrence, it is rare, and killings by sisters are rarer still. We may be inhibited from violence against close relatives because it damages our long-term inclusive fitness calculated as the number of copies of our genes propagated into future generations.

Such inhibition is supported by evidence that siblicide is more common among half-siblings and step-siblings than it is for full siblings. Of course, families with events such as divorce, bereavement, or remarriage have more potential sources of stress and conflict.

Another intriguing pattern for siblicide is the influence of age of killers relative to victims. When both are young, the typical pattern is for the older sibling to kill the younger one. When they are adults, it is more likely that the younger is the killer. The first pattern follows the eagle chicks in the sense that the older sibling — who is generally bigger and stronger — kills the younger one.

Among adults, the fact that older children often acquire resources of money, houses, and land means that they can be an impediment to younger siblings acquiring this form of delayed parental investment. This phenomenon may have played out in a shocking multiple fratricide that occurred recently near Mitchelstown, County Cork, in Ireland.

Fratricide in the Fields

Three Hennessy brothers, who had grown up on a small, 50-acre family farm were found dead. Two of them, William (66) and John (59), still worked on the farm, while Patrick (60) worked in a tire shop.

The brothers were quiet, law-abiding, and well-liked in their local community where they had been well-known from participation in local sport. Apparently there had been some tension related to the sale of a six-acre field, and over how grazing land was to be used.

The precise sequence of events was pieced together based largely on discovery of the bodies. One brother was found in the house with severe head injuries. Another died of similar wounds in a shed nearby. Police concluded that they had been bludgeoned to death with an ax handle.

The body of the third brother, John, was found in the Funcheon river where he had died by drowning. Police speculate that John killed both of his older brothers before committing suicide.

The oldest brother, William, had inherited the farm on which John continued to work. The land was boggy and mostly not suited to cultivation. This meant that both brothers led economically marginal lives making a pittance from selling firewood and reselling hay.

Having no steady job, no land, and no prospects, it appears that the youngest sibling flipped over a seemingly minor irritant that highlighted his lack of status in the family and in the community.


1 Barber, N. (2020). Evolution in the here and now: How adaptation and social learning explain humanity. Guilford, CT.:Prometheus/Rowman and Littlefield.