Among ornithologists, “avian siblicide,” the murder of a nestling bird by an older brother or sister bird, is a well-known phenomenon.

Human children rarely murder their brothers and sisters, but every human parent can attest to the fact that there is a level of what scientists call “relational aggression” among their offspring.

Researchers from the University of California at Davis just completed a major study of “sibling relational aggression,” published this month in Child Development. They studied the interactions between 461 pairs of siblings and their parents, looking for factors that influenced sibling aggression. Among their findings:

  1. Just having a brother or sister influences a child’s level of aggression.
  2. An older sister or brother who is very aggressive increases a younger sibling’s chances of being aggressive too.
  3. A younger brother or sister who is very aggressive increases an older sibling’s level of aggression.
  4. If parents show hostility in their family interactions, their children’s level of aggression increases. Parental hostility related to economic pressures, so indirectly, socio-economic factors, have an impact on children’s aggression.

Aggression Runs in Families

Certain families seem to produce aggressive children. For example, one British study found that 10% of London families were responsible for 64% of adult convictions, 94% of felony theft offenses, and 100% of all robberies. A study in Australia found that having a sibling who is a juvenile delinquent doubles the chances of younger sons becoming one too.

Although parental hostility is a risk factor for childhood aggression, marital conflict between parents is not.

Other family risk factors that increase the likelihood of childhood aggression are economic pressures, single parenting, violence in the home, and maternal depression.

Gender differences in sibling aggression

In general, boys are more physically aggressive in sibling relationships than girls, but girls can be just as aggressive in non-verbal ways. In general, sister/sister relationships have less fighting than brother/brother or brother/sister combinations. There is some evidence that having a nurturing older sister protects younger children from becoming aggressive and even protects them from developing substance abuse issues, but having an overly aggressive older brother has the opposite effect.

Children tend to show more aggression toward siblings at younger ages, and then outgrow it. For example, scientists from the University of Buffalo observed children at ages four and eight with their siblings and peers. At four, the children were more aggressive toward their brothers and sisters than toward friends. However, by age eight, they were about equally aggressive toward both siblings and friends.

The Davis study and others imply that children may actually learn how to be aggressive by watching their older brothers and sisters.

Parents are responsible for controlling sibling aggression so that it does not escalate into sibling abuse, such as incest and/or severe physical or emotional exploitation. Sibling abuse is more common than people think and more common than parental abuse. Some authors believe that as many as 3% of all children are victims of dangerous abuse by their siblings. It becomes more common when children are left alone without supervision on a daily basis. Parents who promote rivalry among their children and who accept sibling aggression as a fact of life also put their children at risk for sibling abuse.

What’s a Parent to Do?

The most important role you play with your child is that of a model for behavior. Your children are more likely to do as you do, not as you say. If they see that you handle stressful situations by becoming aggressive or belligerent, they will learn this behavior. It is important to be aware of the behaviors you are teaching your child. Do you drive aggressively while screaming angry insults at other drivers? Are you rude or aggressively demanding toward others, such as restaurant or other service workers? Your children learn through these interactions.

How you handle aggression between siblings is critical. A common complaint among children is, “He started it!” If you continually punish one child, and do not properly address issues with another child who could be instigating aggressive situations, you will likely breed resentment between siblings that could result in even more aggression. Assuming the older child is the aggressor could mean that you are missing a younger child’s aggressive impulses and letting them go unchecked.

As a parent you play a critical role in teaching children how to mediate disputes without aggression. By setting rules and expectations for how your children interact with each other, they are more likely to find ways to resolve their differences without aggression throughout their lives.