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According to the United Nations Office on Drugs and Crime, approximately 95 percent of those who commit homicide are men. While behavior is shaped by the environment one grows up and lives in, genetics also play a role. For example, scientists have discovered genes that appear to increase one’s tendency to exhibit aggressive behavior. One of the genes, called MAOA, is located on the X chromosome. In one recent study involving a group of male prisoners in Finland, scientists found that the prisoners who inherited a variant of this gene were between 5 percent and 10 percent more likely to have committed a violent crime. Men only have one copy of the gene, since men only have one X chromosome. Women, however, have two copies of the X chromosome and therefore two copies of the gene. Therefore, women who inherit the variant allele will most likely also have a normal allele to counteract its effects. It is important to note that many men inherit the variant copy of MAOA and only some commit violent crimes. The environment seems to play a much more critical role. You can read more about nature/nurture roles in crime in this article.