Spaying and neutering are among the most common veterinary procedures. Most of the clients I was seeing with our students at the vet faculty in Paris were coming because they wanted to fix some behavioural issues in males (randomly: dominance, aggressive behaviour, territoriality,…) and stop the heats in females (I used to say this was the best way to avoid dealing with blood on the carpet!). This was always an interesting discussion for both the students and the clients. Because out there, there are still a lot of misconceptions !
Especially concerning the effect on behaviour. Yes, for sure, behaviour in males in influenced by testosterone, but there are also plenty of environmental factors to consider! For instance, spaying/neutering an aggressive male or female can be a double-edged sword: it has been proven that in some cases, the aggressive behaviour will be reinforced! And for plenty of other of those behavioural issues, there will just be no visible effect (I am sure you all see at least once a “castrated” male dog trying to mount a female… or even another male!).
However, there are plenty of other advantages that can be highlighted (see the figure below), and that’s why I think all dogs that are not intended to be bred should be spayed and neuter. That’s what we wanted all our vet students to know this by-heart. That’s what I think you should tell this as well to all your new owners!