First of all about statistics. These can be interpreted and adjusted to say just about anything you want. One problem, in some of the multi-breed studies is that breeds not predisposed to the diseases studied were thrown out. Yet, this wasn't mentioned in the results making it seem that there is an across the board increase in certain diseases in neutered animals. Even if there is really an increase, it's only in breeds already aflicted with with said disease. For example-your chihuahau is not any more likely to get bone cancer because it's neutered, because chihuahuas don't really get bone cancer anyway. Another problem with statistics, the results are rarely presented in actual numbers. Let's say that the study indicates a 4 fold increase in disease X in dogs that are neutered. If the actual incidence of the disease is 1/10,000 dogs and goes up to 4/10,000 then that is still a very low incidence overall. Again, even if it were true, which may be debatable.
In specific reference to the Golden Retriever study everyone loves to talk about-lots of problems here. First of all dogs over 8 years old were excluded from this study. Apparently, the powers that be decided that if a dog gets certain diseases before they are 8 it's because they were neutered, but if they get it age nine then they were going to get it anyway just because they were old. I cannot even begin to understand this logic. But let's continue. The labrador study is designed similarly.
All of these studies, the GR study included are retrospective. This means they pulled files and looked at what data they could to draw conclusions. This study was performed at the UC Davis teaching hospital/referral center. By and large people who seek this level of care for their pets are responsible pet owners who have a neutered pet or are people with quality breeding or performance animals. The population is therefore already skewed towards the general population of neutered dogs or well-bred intact dogs less likely to have health issues becasue of good genetics. It does not address the population of dogs seen in private practice whose owners don't seek a diagnosis or referral care. Often these are intact dogs whose owners didn't want to pay for neutering either, much less a TPLO for a torn cruciate.
Additionally, they have chosen a breed of dog already predisposed to certain joint disorders and neoplasias thanks to overbreeding and poor genetics. Choose a different breed of dog or something less popular and you will likely get vastly different results.
About the results. People tend to read into it what they want, but if you actually read the results they are all over the board in respect to which sex is more or less likely to get which disease based on when or it they were spayed or neutered. And there is no way to control for factors such as weight, diet, exercise, etc. On top of this, some of these conditions are not exactly black and white.
Let's take hip dysplasia for example. It's not as if your dog has hip dysplasia or doesn't. There are several grades of normal and dysplastic hips. None of this is really mentioned in the results. But I can pretty much guarantee that if age of neutering plays a role in hip dysplasia it will be in those borderline dogs. You are not going to make a dog with genetically excellent or good hips dysplastic by neutering him at 6 months. The opposite is true as well-your terribly dysplastic backyard breeder dog is not going be cured by neutering at 12 or 18 months. Not to mention the severity of the signs is not always comparable to the severity of the x-rays. If your dog is mildly dysplastic but never limps then it does not matter that he is dysplastic, and if he never limped he never made it into the study, neutered or not. Also, this is a disease that your dog has his whole life, regardless of age of neuter, it just matters at age of diagnosis if they make it into the study or not, and again, not many dogs make it to the referral center for treatment of hip dysplasia. Most are managed medically at their local vet.
Mast cell tumor was another disease incluced in this study that is not black and white. Mast cell tumors come in three flavors-grade 1 which are virtually benign, grade 3 which are nasty and grade 2 which are in between. Lumping them all together hardly seems practical.
On the flip side there is a large study (I think from Banfield) looking at all pets (not those seen at referral centers) indicating that neutered pets live longer. This study includes those who died from infectious disease and trauma, so is of limited value as well
I think my take home message here is that you don't need to be afraid to spay or neuter your dog. Age for doing do can be discussed between you and your vet. I would also keep in mind that anything a neutered pet can get an intact pet can get too. The reverse is not true.
I held off posting this for a long time because I didn't want to spark a debate or angry comments, but I think people need to know all the facts when making a decision. Also, I get annoyed when someone won't adopt a rescue dog because it is already spayed or neutered and think it's pretty much ruined and prone to a lifetime of injuries. Even if the well bred, intact dog they already have has been injured a good portion of it's agility career.