Child tantrums are not the result of bad parenting as new study reveals toddlers’ aggression is strongly associated with genes.
The development of physical aggression in toddlers is strongly associated with genetic factors and to a lesser degree the environment, according to a new study led by Eric Lacourse of the University of Montreal. Lacourse worked with the parents of identical and non-identical twins to evaluate and compare their behaviour, environment and genetics.
Mothers were asked to rate the physical aggression of their twins by reporting behaviour such as hitting, biting, kicking and fighting, at the ages of 20, 32 and 50 months.
“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 not changeable. Genetic factors can always interact with other factors from the environment.”
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 exposure to aggressive role models in the social environment and media. However, the results of studies on early childhood physical aggression indicate that physical aggression starts during infancy and peaks between the ages of two and four. 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 and rate of change of physical aggression due to 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 tested three general patterns regarding the developmental roles of genetic and environmental factors in physical aggression. First, 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 labelled ‘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.
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, early physical aggression needs to be dealt with care,” Lacourse said. “These cycles of aggression between children and parents could support the development of chronic physical aggression.”