Good points by Trentonjoe. Fewer walks may be the fruit of a better pitcher, not the precipitating factor.
However, I noticed booger's initial point in a Jekyl/Hyde rookie pitcher the Texas Rangers had this season (Derek Holland, I think was his name). I went to a game where he pitched 7 strong innings, walking no one and giving up only one run against the Red Sox. Quite promising, no? Well, the next game he went out and walked 6 Athletics and got pulled from the game in the 3rd inning, behind by several runs.
The question is, did he walk fewer (and therefore pitch more effectively) due to a change of mindset (I'll attack the hitters today) or due to a better ability on that particular day to control the ball?