Toddlers' Aggression Strongly Associated With Genetic Factors

The development of physical aggression in toddlers is strongly associated with genetic factors and to a lesser degree with the environment, according to a new study. 

Credit: © nadezhda1906 / Fotolia

The development of physical aggression in toddlers is strongly associated with genetic factors and to a lesser degree with the environment, according to a new study led by Eric Lacourse of the University of Montreal and its affiliated CHU Sainte-Justine Hospital.

Lacourse's worked with the parents of identical and non-identical twins to evaluate and compare their behaviour, environment and genetics.

Eric Lacourse

"The gene-environment analyses revealed that early genetic factors were pervasive in accounting for developmental trends, explaining most of the stability and change in physical aggression, " Lacourse said.

"However, it should be emphasized that these genetic associations do not imply that the early trajectories of physical aggression are set and unchangeable."

"Genetic factors can always interact with other factors from the environment in the causal chain explaining any behaviour."

Over the past 25 years, research on early development of physical aggression has been highly influenced by social learning theories that suggest the onset and development of physical aggression is mainly determined by accumulated exposure to aggressive role models in the social environment and the media.

However, the results of studies on early childhood physical aggression indicate that physical aggression starts during infancy and peaks between the ages of 2 and 4.

Although for most children the use of physical aggression initiated by the University of Montreal team peaks during early childhood, these studies also show that there are substantial differences in both frequency at onset and rate of change of physical aggression due to the interplay of genetic and environmental factors over time.

Genetically informed studies of disruptive behaviour and different forms of aggression across the lifespan generally conclude that genetic factors account for approximately 50% of the variance in the population.

Lacourse and his colleagues posited and tested three general patterns regarding the developmental roles of genetic and environmental factors in physical aggression.

• First, the most consensual and general point of view is that both sources of influence are ubiquitous and involved in the stability of physical aggression.
• Second, a "genetic set point" model suggests a single set of genetic factors could account for the level of physical aggression across time. 
• A third pattern labeled 'genetic maturation' postulates new sources of genetic and environmental influences with age.

"According to the genetic maturation hypothesis, new environmental contributions to physical aggression could be of short duration in contrast to genetic factors," Lacourse explained.

About the twins cohort
This twin study was initiated by Michel Boivin of Laval University and Richard Tremblay, who is also affiliated with the University of Montreal and University College Dublin.

All parents of twins born between April 1995 and December 1998 in the Greater Montreal area (Canada) were invited to participate, which resulted in the participation of 667 monozygotic and dizygotic twin pairs.

Monozygotic means the twins originated from the same embryo -- they are genetically identical. Dizogytic means they developed in separate embryos, meaning they are not identical.

Mothers were ask to rate their twins physical aggression, by reporting behavior such as hitting, biting, kicking and fighting, at the ages of 20, 32 and 50 months.

"The results of the gene-environment analyses provided some support for the genetic set-point hypotheses, but mostly for the genetic maturation hypotheses," Lacourse said.

"Genetic factors always explained a substantial part of individual differences in physical aggression."

"More generally, the limited role of shared environmental factors in physical aggression clashes with the results of studies of singletons in which many family or parent level factors were found to predict developmental trajectories of physical aggression during preschool."

Our results suggest that the effect of those factors may not be as direct as was previously thought.

Long-term studies of physical aggression clearly show that most children, adolescent and adults eventually learn to use alternatives to physical aggression.

"Because early childhood propensities may evoke negative responses from parents and peers, and consequently create contexts where the use of physical aggression is maintained and reinforced, early physical aggression needs to be dealt with care," Lacourse said.

"These cycles of aggression between children and siblings or parents, as well as between children and their peers, could support the development of chronic physical aggression."

"We are presently exploring the impact of these gene and social environment interactions."

Journal Reference: 
Eric Lacourse, PhD, Michel Boivin, PhD, Mara Brendgen, PhD, Amélie Petitclerc, PhD, Alain Girard, MSc, Frank Vitaro, PhD, Stéphane Paquin, PhD candidate, Isabelle Ouellet-Morin, PhD, Ginette Dionne, PhD and Richard E. Tremblay, PhD. A longitudinal twin study of physical aggression during early childhood: Evidence for a developmentally dynamic genome. Psychological Medicine, January 2014

Sibling Bullying: Are Parents in denial about sibling aggression?

Siblings routinely pick on one another, but when does bickering become bullying - and what can parents do about it?

Sibling relationships can be difficult, and never more so than in childhood. But society often regards the scrapping and squabbling, the play fighting and not-so-playful fighting as a normal part of growing up.

"The public brushes off aggression between siblings as just rivalry," says Corinna Tucker of the Carsye Institute, University of New Hampshire.

Tucker is the lead author of a new study on the issue for the journal Pediatrics. Almost a third of the 3,600 children questioned said they had been the victim of some sort of sibling aggression in the past 12 months.

This included a range of acts from theft and psychological abuse to physical assault, either mild or severe. In comparison, research suggests that up to a quarter of children are victims of schoolyard aggression every year.

Corinna Tucker uses the term "sibling aggression" in her study, but psychologists are increasingly reaching for a familiar label for the bad stuff that goes on between brothers and sisters - bullying.

This is defined by experts as intentional acts of aggression, repeated over a period of time, where an individual or group is in a position of power over someone.

So sibling relationships would seem the perfect breeding ground for bullying, since children live together for a long period of time and there is usually an intellectual and physical power imbalance.

Although there might not be an outright malevolence, there is often reason for jealousy.

"A sibling relationship is emotionally intense - it's one of those relationships where you can love them and you can hate them at the same time," says Tucker.

"And siblings are natural competitors for family resources and parents' attention."

Laura - who grew up in a house she shared with four siblings and a foster brother - did not consider it bullying at the time, but now thinks the term captures what went on in their house in Ohio.

"My older brother - I would say he beat up on all of us," she says, her voice breaking. Although he never hurt them badly, he liked to wrestle his sisters, pinning them to the floor.

"He was bigger and stronger than us - he could put us in very powerless situations. It was really scary."

For her brother, she believes, the activity gave him a momentary sense of control at a chaotic time for the family. Her mother, who had depression, had left home.

The children were all particularly mean to the youngest sister Tracy who had been the focus of their mother's attention. In the afternoons, after school and before their father came home from work, they would tease Tracy until she phoned her mother to try to get them to stop.

"I remember us saying to her: 'Mum can't do anything about it - she isn't coming here,'" recalls Laura. "And my dad - I think he was so overwhelmed that he only ended up dealing with the most serious things."

Her father later told Laura that he had sent her foster brother back to the orphanage because he was being bullied so much by Laura's middle sisters, a pair of twins.

Research led by Dieter Wolke at the University of Warwick on large samples of British and Israeli schoolchildren has found that half the children who suffered from sibling bullying also suffered bullying at school - and that this group was particularly at risk of unhappiness or developing behavioural problems.

"If you only have sibling bullying or school bullying you are about 2.7 times more likely to have behaviour problems, but if you have both then it's 14 times more likely," says Wolke. "And the point about this is that you don't have any escape. It's 24/7."

Little is known about the long-term effects of sibling bullying but Jan Parker, a family psychotherapist and a co-author of Raising Happy Brothers and Sisters, is certain that they can be "serious and lifelong".

"We're very familiar with discussions on the impact of [school] bullying," she says. "I don't think it takes a great imaginative leap to understand the impact of it happening within your own home. You feel that you are not only let down by the person bullying you, but also by the adults in your life that are letting it go unchecked."

While sibling fights are normal, parents need to be aware of a pattern developing over time, she says. Parents are advised to talk to the children about what's happening. If they are worried, they could speak to a health professional.

They should also consider what might be causing the problems. "There is one known predictor - it's when siblings perceive there to be a wide disparity between how they are treated and how their brother and sister are treated," she says.

Robin Kowalski spoke to Health Check on the BBC World Service - listen again to the programme on iPlayer or get the Health Check podcast.