SHOULD I NEUTER OR SPAY MY DOG ?
The first thing I would say is that if there is a genuine serious behavioural or clinical reason that requires your dog being neutered or spayed, and that it will make a difference, then get it done.
I am not a vet but a behaviourist, lifelong dog owner and rescue centre volunteer, all the following comes from my own personal experience, observations, formal learnings and research.
Testosterone, Oestrogen and Progesterone (the hormones you will remove if you neuter your dog) are present in all dogs in varying degree given the dogs sex, male or female. These hormones give the dog it's sex but also greatly influence other things too. A dogs boldness, confidence, calmness, shyness, reactivity, nervousness, fearfulness and eventual social maturity (2-3 years old dependent on breed) is profoundly affected by these hormones.
So, if you do decide to neuter or spay your dog you do not want to do it until they have reached social maturity, male and female. This would be two years old for a smaller breed, up to three years old for a giant breed dog (unless a genuine serious behavioural/clinical reason makes it imperative). If you are spaying a female dog make sure you carry it out three months after their last season (midway between seasons), as this allows the dogs hormonal and bodily parameters to return to normal before you carry it out, if you don't, later behavioural problems due to hormonal in-balances may be the result.
Far to many dogs are being neutered and spayed too early in their lives, at the risk of future serious behavioural and indeed physical problems.
Vets will often say, “ Oh you must be a responsible owner, there are too many pups in the world, blah blah”. What rubbish this is. I have always had male dogs, they have all remained intact throughout their long, sound and healthy lives, and not one puppy was fathered by them, as I am indeed a responsible owner, and not allowed such to happen. Dare I say that neutering of pets is a major part of a vets income. Also many dog breeders will insist that a puppy of theirs is neutered or spayed if you take one, only I think, to stop owners breeding their own dogs and so losing some of the breeders own market; there is no thought for the poor young puppies future development. If a dog breeder insists on this, look elsewhere.
The nervous/fearful dog:-
If you have a dog, male or female, that is already a nervous dog, then getting them neutered or spayed will almost certainly make them even more nervous or fearful. The hormones that you will remove from the dog are ones that affect a dog's boldness, confidence and calming, they bolster up it's reactions to perceived scary things, if you take them away then it's rather obvious what might happen. If there is no genuine reason other than just because a vet or someone else is saying “get them done”, then think hard, why risk worse future problems for you and your nervous/fearful dog or pup.
The aggressive dog:-
Many people will say “Oh it's aggressive, get it done”. What you need to do first is find out why the dog is being aggressive, then decide on a plan of action. There can be many reasons for aggression and often getting your dog done is not the answer. Many times a dog learns to be aggressive, the dog thinks that aggression is good for it and gets it what it wants, a dogs own early learning and perception of what it thinks is happening to them at that specific time is causing their problems, not the testosterone that is flowing through their bodies, often this might mean a mal-adapted dog that will need work with a behaviourist and a vet.
What I would say though is that once an aggressive period starts the length and intensity of it can be fuelled by testosterone, so if your dog truly has outwardly dangerous aggression, like dominance aggression, then getting them done might lower the intensity and length of it, which is not a bad thing in this situation, just be sure the cause of the aggression is not fear based though, for you may well make it worse if you get them done..
There have been some (not many) studies on the effect of neutering or spaying aggressive dogs and the only real results at this time, that are well founded, concerns the spaying of young aggressive female dogs, especially those already showing aggression towards the owner. If the female dog is under twelve months (especially under six months old) of age when it is spayed, there is a marked increase in it's aggression afterwards (O'farrel & Peachey 1990).
The sexually over exuberant male:-
I will admit that neutering a male dog that is carrying out highly driven, over the top sexual behaviours, driven by hormonal influences (not owner influence, attention seeking, boredom and frustration etc), can reduce or sometimes stop things. Again I would ensure that the driving force is indeed hormonal and that the behaviours are highly driven and impacting on the life of dog and owner. I would recommend trying a chemical castration first, to see if it really makes a difference, before getting your dog actually done.
IF you, or another decides that neutering or spaying is required, and it is not for medical reasons, but for behavioural ones, then think about a chemical neutering first. A vet can chemically castrate/spay your dog with a drug and you can see if this makes any difference to the said behaviour before going down the non-reversible, dog altering route.
In conclusion I am against the general neutering of dogs unless there is a genuine medical/clinical or behavioural problem that warrants it, if you can, especially for behavioural problems, try a chemical neutering first.
If you or others do decide to neuter your dog then do it when the dog has sexually and fully socially matured, 2-3 years old for a dog (small breeds 2 years, large breeds three years), and at least after 2-3 seasons for a bitch (2 seasons for small breeds, three seasons for large breeds), with bitches make sure spaying is carried out 3 months after the last season (midway between seasons) .