weakness - biologically reductionist, ignores the environmental factors, realistically it's interactionist. precise mechanisms are unclear, MAOA-serotonin-aggression link is unclear, research linked low serotonin w high aggression, same w low MAOA and aggression, however low MAOA should in turn lead to higher serotonin meaning the relationship between the 3 is a mystery. twin studies lack validity, twins don't necessarily share the same environment, equal environments assumption, ignores how the twins are treated by others, family, friends, who they surround themselves w, they have control over that, concordance rates are inflated, could also be researcher bias where they make concordance rates for mz twins higher cos that's what they think they should do