[url=http://www.nij.gov/topics/crime/gun-violence/Pages/welcome.aspx]Specifically it's either intergang violence or the commission of felonies. The weapon of choice is usually a handgun.[/url]
So, simple solution: Make being in a gang illegal. If you are found to belong to a gang then you are jailed for life. Cure the disease instead of treating the symptom.
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I'd say poverty, poor education, as well as poor enforcement of gun laws due to neutered law enforcement agencies are bigger causes than gangs since gangs would lose a lot of members with these measures.