Platelet monoamine oxidase activity in association with childhood aggressive and hyperactive behaviour: the effect of smoking?

Monoamine oxidase (MAO) activity in platelets is known to be a marker of certain personality traits and behavioural preferences in adults, but less is known of the association of platelet MAO and behaviour in children. Platelet MAO activity was measured in 1129 randomly sampled 9- and 15-year old children by a radioenzymatic method with β-phenylethylamine as the substrate. Ratings of children’s Aggressiveness, Motor Restlessness and Concentration Difficulties were obtained from teachers. Smokers had significantly lower MAO activity and they were also given significantly higher scores in Aggressiveness, Motor Restlessness and Concentration Difficulties than non-smokers. In 15-year but not 9-year-old children, significant associations were found between platelet MAO activity and behavioural ratings. However, these associations disappeared when smokers were excluded from the analysis. It is concluded that the causal relationship between behavioural measures and platelet MAO can only be revealed with longitudinal studies, considering smoking as a confounding factor.