Family members show similarities in the degree and severity of aggression. Camiel van der Laan (NSCR, VU) investigated the role of genes and the environment in explaining these similarities in the development of aggression over the life course and in the transmission of aggression from parents to children. On Friday 11 March, Van der Laan obtained his PhD with IT RUNS IN THE FAMILY: A genetically informative study of individual differences in aggression.
For his research, Van der Laan used data from twins and their relatives who are registered with the Dutch Twin Register. Aggression has been measured on several occasions since 1991: of more than 80,000 people, more than 360,000 aggression has been measured in total.
Aggression over the life course
The research shows that aggression decreases on average over the life course. It is already known from previous research that physical aggression generally peaks around the age of two to four years and non-physical aggression during primary school and adolescence. Nevertheless, there are also stable characteristics over the life course: the most aggressive children are more likely to grow into the most aggressive adults. The research shows that genetic influences play a role in this relative stability of aggression over the life course. Genetic risk of aggression in childhood and puberty leads to a greater chance of aggression later in life.
Family members share genetic risk of aggression
Similarities in aggression between family members can mainly be explained by genetic similarities, i.e. by sharing genetic risk. There are no evidence that environmental influences explain the similarities in aggression within families. This was shown in a twin study of norm-breaking behavior as well as a study that looked at whether parental genetic risk creates an environment that influences their children’s aggression. Environmental influences, on the other hand, play well an important role in explaining differ in aggression within families. In other words, experiences and environmental characteristics that are unique to particular individuals within a family seem important in explaining differences in aggression.
Going with the downward trend
In the period between 1991 and 2015, aggression within the studied population decreased slightly. This fits with statistics showing a decrease in violent crime. Family members resemble each other not only in the degree and seriousness of aggression they display, but also in the extent to which they go along with this downward trend or deviate from this trend. Positive changes in society therefore do not have to work out equally well for every person and every family.
Continuity of aggression from generation to generation not attributable to environment
The research shows that a child may be unlucky enough to inherit a high genetic risk for aggression, making him or her more likely to exhibit aggressive behavior in the same situation than someone with a low genetic risk. But the continuity of aggression from generation to generation is not due to the environment they share. This puts the importance of the upbringing and behavior of parents into perspective. It is important to take this into account if we want to better understand differences in aggressive behavior and develop more effective interventions. An important next step is to investigate which environmental characteristics lead to differences within families. Genetically identical identical twins can play an important role in this, as any underlying genetic influences in them can be excluded.
Publication details and further reading
Van der Laan, C.M. (2022), IT RUNS IN THE FAMILY; A genetically informative study of individual differences in aggression. NSCR/VU.
Supervisor: Dorret Boomsma
Co-promoters: Steve van de Weijer and Michel Nivard
This promotion could be attended on Friday 11 March at 13.45 in the auditorium of the VU main building at De Boelelaan 1105.