and the chronicing aresive over time are the ones who during adolescence are the chronicallying aresive. >> that four percent. >> it's only four percent of males. so approximately -- if we-- if we think about what are the factors going on here what are the environmental and genetic factors that are going on what we have been able to observe by using twins so we use mono-- twins that have the same genetic endowment. and we use from ternlt -- fraternal twins that are similar, like brothers and sisters 50% of them they share 50% of their genes. what we see is that the identical twins are much more similar in terms of physical aggression than the fraternal twins. it is a good indication that there are important genetic differences. and it appears that it's about 50/ -- 50% in terms of explanation of frequency of physical aggression comes from genetic endowment 50% from environmental endowment. from the brain per -- perspective we've been looking at brain imaging serotonin synthesis in the prefrontal lobes. and what we observe is that the chronicallying aresive have problems in terms of sero