Caveat had a post on this topic a while back.
MSNBC published an article entitled "Pet sterilization laws raise health concerns".
Here is an excerpt from the article.
As legislators push for more mandatory spay and neuter laws for pets as young as 4 and 6 months in hopes of reducing the number of unwanted animals, critics are crying foul over research showing that such surgeries may raise certain health risks in dogs and therefore shouldn't be required.
Studies have shown that dogs that undergo spaying (removal of the ovaries and uterus) or neutering (removal of the testicles) are at increased risks for certain cancers, thyroid disorder, incontinence and some of the same behavior issues, such as aggression, that the surgeries are said to prevent.