Brunner et al found that male members of one dutch family were particularly violent aggressive criminals, and all were found to have a defect on the MAOA producing gene. This therefore supports a hereditory/genetic link
However - the link is only correlational, suggesting that other unknown elements may be the cause of their behaviour. Also, this was only found in male family members and doesnt take into account the behaviour of the females of the family who, according to the study, should also have aggressive behaviour due to genetic links.
Twin Studies - Coccaro et al - conducted a study into male twins. On comparing MZ and DZ twins they found that genes acocunted for approx. 50% of aggressive behaviour. Therefore this also supports the genetic link.
However - this study does not take into account external factors which may effect the behaviour of offspring. For example, MZ twins will have shared the same environment both before birth and in the upbringing, and so this may be the cause of the similarity in aggressive behaviour. Also, only males were studied, and so results may not ben applicable to females thereby reducing external validity. Furthermore, according to the theory MZ concordance rates should be 100% if the link is genetic, so this therefore reduces effectiveness
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