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Antisocial behaviour refers to a wide range of behaviours that disregard societal norms, including clinical disorders. There are numerous reasons why some people exhibit anti-social behaviour. In her doctoral thesis, Katre Sakala focused on two aspects of antisocial behaviour that deviated from societal norms: behaviour that required police intervention, and drug use. The study is based on longitudinal data from the Estonian Children Personality Behaviour and Health Study (ECPBHS), which allows for long-term observation of people's behaviour.

Previous research has found that attention-deficit/hyperactivity disorder (ADHD) diagnosed in childhood is associated with antisocial behaviour. However, Sakala did not base her research on diagnosed disorders but rather on the symptoms of attention-deficit and hyperactivity. According to her findings, the strongest predictors of later antisocial behaviour were substance use disorder, reactive aggression, concentration difficulties at age 15 and fighting in elementary and middle school. Taking into account the concentration difficulties that many students face in school (without engaging in aggressive behaviour), this knowledge may eventually help prevent antisocial behaviour.

Sakala focused on the association between monoamine oxidase (MAO) and antisocial behaviour as biological risk factors. Monoamine oxidase is an enzyme that helps break down neurotransmitters like serotonin and dopamine, influencing mood and behaviour. When analysing the connections between MAO and problematic behaviour, it was found that males with lower platelet MAO activity may exhibit antisocial behaviour. A similar connection was observed in females, but only when they displayed more antisocial behaviour. It is important to note that the link between drug use and low platelet MAO activity was evident only in males, who also had a higher risk of using drugs earlier. These findings support previous research that lower platelet MAO activity is linked to problematic and risky behaviour, including substance abuse. However, this link was observed for the first time in regular school students, owing to a representative sample and longitudinal data.

Previous scientific studies have shown that males with the MAOA-L genotype who were maltreated as children are more impulsive and antisocial. In her thesis, Sakala investigated the potential link between these two behaviours and the MAOA gene and its methylation (DNA methylation is a process by which a gene is silenced).

Sakala's analysis of the results revealed that, while methylation of the MAOA gene in adolescence is associated with impulsive and unrestrained behaviour as well as quick decision-making and thrill-seeking, this association was only evident in males from complex home environments who carried the corresponding genotype. Several studies have suggested that the MAOA-L genotype in males may be associated with antisocial behaviour, but previous findings from the ECPBHS study have shown that males with the same genotype may cope better. As a result, the current findings may imply that MAOA methylation can be a marker of developmental plasticity of the MAOA-L genotype, indicating that different behaviour patterns emerge under favourable conditions compared to challenging ones.

In her research, Sakala found that gender was a consistent predictor of antisocial behaviour, but the influence of gender was mediated by a number of other factors, including substance use and aggression. The significance of monoamine oxidase as a biomarker for behaviour may be dependent on gender because the gene encoding the enzyme is located on the X chromosome. The thesis findings show a link between low platelet MAO activity and problematic behaviour in males, but a similar link also appears in females only when they engage in more antisocial behaviour.

Katre Sakala defended her doctoral thesis titled “” on 16 May. This thesis was supervised by Professor Jaanus Harro from the University of Tartu. The opponents were Dagmar Kutsar, associate professor at the University of Tartu, and Professor Gregers Wegener from Aarhus University.