I believe that the most common cause of the slice is the body outracing the arms on the downswing, thus forcing the arms out and over the target line. In order for the ball not to go straight left, the club face is kept open, which causes the ball to curve drastically from left to right for a right handed golfer.
I remember seeing Chip Beck practice this drill back in the day because he was curving the ball too much from left to right. He would draw his right foot back behind his left foot and hit balls. I tried this and it works. The right foot back slows down the hips and body and allows the arms to catch up and pass the body at impact. Try it some time if you are slicing the ball, you might be amazed!