88% reduction in complaints by requiring cameras.

In Rialto California police officers were required to wear a camera in order to document their interactions with civilians. In these areas, public complaints against officers dropped by 88%. 

Rialto’s police officers also used force nearly 60 percent less often

http://www.nytimes.com/2013/04/07/business/wearable-video-cameras-for-police-officers.html?pagewanted=all&_r=0

http://carloz.newsvine.com/_news/2013/07/19/19560742-study-police-force-where-12-of-officers-wear-mini-cameras-see-88-drop-in-citizen-complaints-those-so-equipped-use-force-60-less-often