Aggressive children might be experiencing turmoil due to one or more mental health problems. That’s why we shouldn’t label children as bad, researchers believe.

Kids who frequently show anger could be struggling with mental health problems

What is the underlying cause when children are violent and aggressive? Norwegian researchers have now investigated this. 

So far, researchers have not come up with a good answer to the cause of aggressive behaviour in children.

But now, a new study has looked at children with a diagnosis of conduct disorder, as well as children who have problems with violence and aggression, but who have not received this diagnosis.

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    “I wanted to find out how aggression and violence develop through life. What is actually behind this kind of behaviour?” says Natalia Tesli. 

    She has led the study undertaken by the University of Oslo and the Norwegian Institute of Public Health.

    How do aggression and violence arise?

    In her doctoral thesis, Natalia Tesli studied MRI images of the brains of convicted violent offenders. She compared these with images of the brains of people who had not committed violence.

    Tesli found a significant difference in the brains between the two groups. You can read more about that study here.

    Now she wanted to look at how aggression develops throughout childhood.

    She chose to study children and adolescents who had been diagnosed with conduct disorder, the diagnosis most often given when children and teens act out and are aggressive.

    They must have exhibited socially unacceptable behaviour with a lot of aggression for at least six months to receive the diagnosis. 

    “This is a serious diagnosis that the healthcare system in Norway is quite cautious about giving,” says Tesli.

    Genetic risk detected

    Children who have been bullied also have an increased genetic vulnerability to developing mental disorders later in life, Natalia Tesli has found in a new study.

    The researchers found several surprising connections in the study led by Tesli.

    They discovered that conduct disorder significantly aligns with the genetic risk present in children. 

    The children with conduct disorder had an increased genetic risk of mental health problems that usually start in childhood, such as ADHD and autism.

    They also had an increased risk for diseases that occur in adulthood, like schizophrenia, bipolar disorder, and depression.

    Linked two data sources

    Those who are diagnosed with conduct disorder have had contact with child and adolescent psychiatric services, often due to aggressive behaviour and violence. 

    The Norwegian Patient Registry (NPR) records all diagnoses assigned by the specialist healthcare services.

    But the researchers also wanted to find youth who exhibited less severe behavioural problems.

    To find them, they used data from the Norwegian Mother, Father and Child Cohort Study (MoBa). About 100,000 children are included in this study.

    Parents asked about aggression

    What is conduct disorder?

    Serious conduct disorder in a child or teen is characterised by a pattern of behaviour that includes significant lying, truancy, threats and violence against others, bullying of others, vandalism, criminal acts, and other antisocial behaviour.

    A Norwegian population survey of nine to ten-year-old children found that 0.5 per cent had such difficulties. 

    International studies of adolescents indicate a prevalence of serious conduct disorder of around three per cent. 

    (Source: Pål Zeiner, specialist in child and adolescent psychiatry)

    Parents in the MoBa study completed a questionnaire when their children were five and eight years old.

    Parents were asked whether their children had difficulties with outbursts of anger, or temper tantrums.

    One of the questions was: Has your child had problems with violence against other children, at school or in kindergarten? They were also asked about violence against animals.

    “These are behaviours that aren't necessarily picked up by the healthcare system,” Martin Steen Tesli says. 

    He is a specialist in psychiatry, a researcher at the Norwegian Institute of Public Health, and one of the co-authors of the study.

    Multiple diagnoses

    When the researchers linked the data from NPR and MoBa, they found that about 600 of the children could be said to have conduct disorder. 

    But many of these children – as many as 65 per cent – also had other mental health diagnoses.

    "We see that some can have up to four different diagnoses, given at different times," Martin Steen Tesli says. 

    We found a strong link between having a behavioural disorder and ADHD, says Martin Steen Tesli.

    “We observed a particularly strong connection between having conduct disorder and ADHD. Over 14 per cent of the children with conduct disorder also had an ADHD diagnosis.”

    Diagnosis detected in genes as well

    Blood samples were taken from the parents who answered questions and from their children.

    The blood samples were used to determine the genetic risk for mental health problems. 

    "Remarkably, we found the diagnoses and the parent's answers reflected in the children's genes," says Natalia Tesli.

    Victims of bullying also found

    The researchers posed two questions to find out more about the difference between genetic and environmental factors:

    • How much education do the parents have?
    • Has the child been subjected to bullying?

    “Both are environmental factors that we know are very significant for children's development,” says Natalia Tesli.

    The researchers then found yet another link to genetics.

    Children who have been bullied also have greater genetic vulnerability to developing mental health problems later in life. These are disorders such as schizophrenia, bipolar disorder, and depression.

    The effect was even greater if the parents of bullied children also had a lower level of education.

    The researchers point out that it is important to bear in mind that our behaviour is always a result of both our genes and the environment we grow up in.

    An expected link

    Pål Zeiner is a specialist in child and adolescent psychiatry.

    “We know quite a bit about the potential causes behind serious conduct disorders in children and adolescents," he says. 

    “Learning difficulties, language difficulties, ADHD, parental mental health issues, poor or neglectful child rearing, limited financial resources, and association with other youths with similar difficulties – all of these could substantially heighten the risk of developing conduct disorders.”

    The research also indicates a considerable hereditary aspect to ADHD and autism.

    “Genetic predisposition plays a role in the development of mental health problems. When people with a genetic predisposition live in environments that are not good, the sum of genetic and environmental factors can lead to the development of mental health problems," Zeiner says.

    Critical of participant selection

    However, Zeiner is critical of whether the study sample is representative of children with behavioural disorders. Only 41 per cent of the people asked to participate in the MoBa survey agreed to participate, and a considerable number dropped out by the time the children reached school age.

    “The remaining participants are less likely to exhibit the risk factors associated with serious conduct disorders compared to the general population, such as low education attainment, low income, and being a single parent,” Zeiner points out.

    The Norwegian Patient Registry data, derived from the specialist health service, represents a very small number of diagnosed cases of serious conduct disorder, casting doubt on whether they can represent all children with conduct disorders accurately, he suggests.

    About the study

    To identify children and adolescents with the diagnosis of conduct disorder, the researchers used data from the Norwegian Patient Registry (NPR). All the diagnoses that the specialist health service assigns to patients are recorded there.

    To find children and teens who do not have as serious behavioural difficulties, the researchers used data from the Norwegian Mother, Father and Child Cohort Study (MoBa). This is one of the world's largest health surveys. It follows children and their parents over time. One of its main purposes is to find the causes of diseases. Around 100,000 children are included in this study.

    "Unfortunately, this makes the study somewhat weaker," he believes.

    Confident in the results

    “It's important to acknowledge that the MoBa sample has its limitations, including dropout and representativeness issues,” believes Natalia Tesli.

    At the same time, she points out that this data set ranks among the world’s largest in terms of genetic data in children and adolescents. 

    “By looking at both diagnoses and traits associated with behavioural problems, we have approached this with two different methodologies. We can therefore be all the more confident in our results,” she says. 


    Tesli et al. Conduct disorder - a comprehensive exploration of comorbidity patterns, genetic and environmental risk factorsPsychiatry Research, vol. 331, 2024. DOI: 10.1016/j.psychres.2023.115628


    Translated by Ingrid P. Nuse

    Read the Norwegian version of this article at

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