Male Dog Behaviour After Neutering: The Science

Updated November 28, 2020

Recently, a well-respected Sydney academic published his behavioural data on over 6000 castrated male dogs. It’s created headlines like”Neutering Causes Behavior Problems In Male Dogs”.

Can this be true? You’ve probably been told that desexing reduces behaviour problems, not causes them!

Does Desexing Lead To Aggression?

The reality is, there’s been a debate about this for some time and no-one can agree. But we need to get it right.

  • We’re talking about human-directed aggression
  • Many of those people are children
  • When children get injured, it’s often badly
  • These dogs often get euthanased

Personally, I think the study in question is flawed. It uses a voluntary online survey that was distributed to breeder groups. That’s unlikely to give a reliable snapshot of the dog population. It also doesn’t have a ‘control’ population of entire dogs.

But to be fair, no study is perfect. The answer is to not look at just one, but consider them all. To try to answer the question, that’s what I’ve done here.

Studies On Aggression & Neutering

Below is a summary of every study I can find that compares aggression towards people between entire and desexed dogs. If it looks a mess now, it’ll hopefully make sense once I explain it afterwards (yes, I read every paper).

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= desexed dogs are better
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= desexed dogs are worse
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= no difference detected
StudyPopulationSignificant Findings 
Borchelt 1983258 USA dogs diagnosed with aggressionNo difference detected
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Wright et al 198762 dogs referred for aggression problemsSexually intact dogs were more likely to be aggressive
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Gershman et al 1994178 biting vs 178 randomly selected USA dogsSexually intact dogs were more likely to bite
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Podberscek et al 1997a217 low vs 218 aggressive USA English Cocker SpanielsMore neutered dogs were in the ‘high’ aggression group
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Podberscek et al 1997b1019 randomly selected USA English Cocker SpanielsNo difference detected
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Guy et al 20013226 dogs attending Canadian veterinary clinicsAggressive behaviour more common in castrated dogs
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Reisner et al 20051053 randomly selected USA English Springer SpanielsNeutered males were more likely to bite
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van den Berg et al 2005110 aggressive USA Golden retrievers vs 118 relativesCastrated dogs were more common in the aggressive group*
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Messam et al 20071112 dogs attending vets in Kingston, JamaicaSexually intact dogs were more likely to bite
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Hsu & Sun 2010852 randomly selected dogs in TaiwanNeutered dogs scored lower on owner-directed aggression
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Casey et al 2014946 neutered vs 855 entire male UK dogsNo difference detected
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Shuler et al 2014636 dog bites reports from 47,526 registered USA dogsSexually intact males were more likely to bite
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McGreevy et al 20186235 neutered dogs recruited by online USA surveyStranger-directed aggression increased with lower hormonal exposure
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Confused? You’re not alone. Here are my thoughts and impressions:

Studies on single breeds are of limited use. Golden Retrievers are such placid dogs that their aggression is likely to be of a different nature. Aggression in Springers is suspected to be genetic in origin, especially in the show lines.  I place little relevance on these papers, so bear with me while I remove them…

As aggression gets more serious, results are clearer. Of the remaining studies, let’s look only at the dogs being treated for aggression or reported for dog bite to people. Now it looks like this:

StudyPopulationSignificant* Findings 
Borchelt 1983258 USA dogs diagnosed with aggressionNo difference detected
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Wright et al 198762 dogs referred for aggression problemsSexually intact dogs were more likely to be aggressive
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Gershman et al 1994178 biting vs 178 randomly selected USA dogsSexually intact dogs were more likely to bite
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Shuler et al 2014636 dog bites reports from 47,526 registered USA dogsSexually intact males were more likely to bite
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Whether you agree or not I hope you can see the reasoning. Random surveys of dogs are going to give different results to studies looking at acts of aggression. But I haven’t quite finished with the confusion.

A reason that entire males might be unfairly blamed is that poorer quality dog owners may also be less likely to desex their dogs. That means that some of the aggression might just be because the owner is an idiot and has raised the dog badly.

A reason that desexed males might be unfairly blamed is that when behaviour problems in males develop, most vets will recommend desexing. That will push more aggressive dogs into the ‘neutered’ groups as desexing after the event is unlikely to work. This is suspected to have happened in the Golden Retriever study.

Additionally, it is feasible that aggression at the vet could have started with a stressful or painful desexing experience, especially in past times when pain control was poor.

Which Males Become Aggressive?

In conclusion, the ‘truth’ will probably be that desexing is good for some types of aggression but worse for others. I suspect it slightly increases the incidence of fear aggression (which often goes unreported) and reduces the more dangerous forms of human-directed aggression.

A clue to this is the dog bite statistics again. When we go one step further and look at fatal dog bites, the majority are children and they mostly involve undesexed males (Sacks).

One of my current concerns regards the delaying of desexing until over 12 months in large breeds. Let me be clear: I support it. However I have seen two lovely Rottweilers become dangerous during the waiting period, and they are likely to stay that way.

There’s no point worrying about a 5% risk of joint problems when you end up with a far worse problem. I advised the delay but I also believe these two dogs should have been desexed as soon as their behaviour started changing. Their futures are now bleak.

I don’t expect to sway anyone’s opinion, and I don’t even want to. All I want is to get the idea across that this debate is a lot more complicated than it looks. It’s full of people with vested interests on both sides only quoting the studies that agree with them.

You are welcome to argue!

Further Reading

You might also like:

*Significant meant the measured difference between the groups had a p-value of <0.05. It’s how we (somewhat arbitrarily) decide if a result is real or not. To learn more, read our guide to making sense of scientific research.

I can supply copies of the references (below) on request. If you know of any other studies that compare aggression in neutered and entire male dogs, please let me know!

Have something to add? Comments (if open) will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. Meet his team here.

Borchelt, P. L. (1983). Aggressive behavior of dogs kept as companion animals: classification and influence of sex, reproductive status and breed. Applied Animal Ethology, 10(1), 45-61.

Casey, R. A., Loftus, B., Bolster, C., Richards, G. J., & Blackwell, E. J. (2014). Human directed aggression in domestic dogs (Canis familiaris): Occurrence in different contexts and risk factors. Applied Animal Behaviour Science, 152, 52-63.

Gershman, K. A., Sacks, J. J., & Wright, J. C. (1994). Which dogs bite? A case-control study of risk factors. Pediatrics, 93(6), 913-917.

Guy, N. C., Luescher, U. A., Dohoo, S. E., Spangler, E., Miller, J. B., Dohoo, I. R., & Bate, L. A. (2001). Demographic and aggressive characteristics of dogs in a general veterinary caseload. Applied Animal Behaviour Science, 74(1), 15-28.

Hsu, Y., & Sun, L. (2010). Factors associated with aggressive responses in pet dogs. Applied Animal Behaviour Science, 123(3-4), 108-123.

McGreevy, P. D., Wilson, B., Starling, M. J., & Serpell, J. A. (2018). Behavioural risks in male dogs with minimal lifetime exposure to gonadal hormones may complicate population-control benefits of desexing. PloS one, 13(5), e0196284.

Messam, L. L. M., Kass, P. H., Chomel, B. B., & Hart, L. A. (2008). The human–canine environment: a risk factor for non-play bites?. The Veterinary Journal, 177(2), 205-215.

Podberscek, A. L., & Serpell, J. A. (1996). The English Cocker Spaniel: preliminary findings on aggressive behaviour. Applied Animal Behaviour Science, 47(1), 75-89.

Podberscek, A. L., & Serpell, J. A. (1997). Environmental influences on the expression of aggressive behaviour in English Cocker Spaniels. Applied Animal Behaviour Science, 52(3-4), 215-227.

Sacks, J. J., Lockwood, R., Hornreicht, J., & Sattin, R. W. (1996). Fatal dog attacks, 1989-1994. Pediatrics, 97(6), 891-895.

Shuler, C. M., DeBess, E. E., Lapidus, J. A., & Hedberg, K. (2008). Canine and human factors related to dog bite injuries. Journal of the American Veterinary Medical Association, 232(4), 542-546.

Van den Berg, L., Schilder, M. B. H., De Vries, H., Leegwater, P. A. J., & Van Oost, B. A. (2006). Phenotyping of aggressive behavior in golden retriever dogs with a questionnaire. Behavior genetics, 36(6), 882-902.

Wright, J. C., & Nesselrote, M. S. (1987). Classification of behavior problems in dogs: distributions of age, breed, sex and reproductive status. Applied Animal Behaviour Science, 19(1-2), 169-178.

20 Replies to “Male Dog Behaviour After Neutering: The Science”

  1. My almost 6mo spoodle has shown territorial behaviours and has broken my 4yo skin on his cheek – he had a kangaroo ear and my kid bent to pat him and hi snapped, jumped and went straight for his face. Days later he had some plastic he picked off the floor and bit me on my hand when trying to remove it. He had spent a night – his second night away from us at a kennel.

    He also nipped at my (same) son when we first got him, he must have been 14 weeks old. Also after spending a night back at the breeder’s as we had a trip planned.

    We are using positive training: thanks, off, take. And I have been able to safely remove objects off him twice now with no reaction.

    He is truly adorable but I think it’s clear he’s not fan of my 4yo and he’s got a temper. Tairo is 3/4 toy poodle 1/4 spoodle.

    I wanted to go ahead with vasectomy. My vet called back wanting to discuss pros and cons but I can tell from her tone in her voicemail, she will push for desexing him. He was scheduled soon when he turns 6mo.

    After reading your article I am inclined towards desexing. But still torn. So much information is overwhelming. Thanks immensely for educating and sharing evidence-based information in a way that is easy to understand.

  2. My 2.5 YO toy cavoodle suddenly seems to have undergone a personality change. First up, this is after he went missing for 5 days and spent 2 nights at a stranger’s house (lovely woman) 1 night in the pound and 3 nights at my ex-partner’s place. It could be the root of the problem? But I feel certain that he was properly cared for during this time. Now he has become overtly aggressive! Like any dog, if you try taking his bone away he gets wild with anger – barking mad! Gnashing teeth. On a few occasions I have found him eating stuff he shouldn’t like avocado skins and he becomes like a demon dog from hell. Otherwise he is sweet, affectionate, and adorable, BUT now he has become territorial. He sits too close to the radiator under my desk and goes psychotic if I try to gently move him away with my foot. Yesterday I had to physically go under my desk and I was scared he was gonna bite my face. I have resorted to using a toilet plunger so he bites that harmless thing and not me. Now he stands right in my way as I try to walk across the room and even that turns into a growling standoff. My friends and vet say get him desexed ASAP but after reading so many contradictory stories on here – I don’t want him to become even worse.

    1. Hi Jeremy. From my experience, if he had been desexed at 6 months, it’s very unlikely he would have become the way he is now. I would get him desexed ASAP, but there’s no telling whether it will work or not this late.

  3. I have a six year old Staffy X -most likely with Lab. She was speyed at about 12 months and for her first three years was very friendly with all other dogs. She loves adults, cats also and especially groups of kids.
    At about three she became extremely aggressive with other dogs, started humping legs and would regularly cock her leg to pee as well as squat. Also at about this time to complicate understanding she experienced aggression from several other dogs which may have played into the development of her aggression. I have spoken to about 10 other Staffie X owners who have encountered similar development of aggression in their bitches …speying procedure varying from 6-12 months. Not exactly a representative group. When I was a lad (1950’s) I spent a lot of time on country properties and I recall a number of working dog breeders saying that bitches should be allowed to have a litter before speying because they worked “better” and didn’t start fights . The conclusions from professional articles I have read there is quite a strong opinion based on large US trial samples standardised for environment , quality of training, health etc. that earlier speying deprives the bitches body of experiencing the development of a normal adult hormonal environment. This conclusion, when you think about it, makes good sense. Another parallel finding from these studies was that in general desexed dogs and bitches tend to be responsible for more dog and human aggression than intact dogs. I am aware there is no general consensus on this topic because of the difficulty of statistical standardisation. Anyone got any thoughts or experiences on this subject.

    1. Hi James. The situation with female dogs is not as clear as with the males mentioned in this article. I’ve written before about aggression after desexing in females, and the jury is still out on whether there is a link. It’s not out of the question though, especially in certain breeds.

  4. I think it vairies between dogs. I recently got my dog neutered. He was 180# & 2 years and 8 months at the time. He was a sweetheart and very affectionate prior to his neutering. He also grew up as a pup with 2 other older dogs. About 2 weeks after, he behavior and aggression has gotten very bad to the point I may have to have him euthanized. He has begun to destory the house. It started with small things like shoes and clothes, but escalated to a computer printer, dining room table & chairs, the couch and a filing cabinet. He has also severly attacked the other 2 dogs in the house several times, one of them needed treatment and stitches at the animal hospital. He is much larger than the other 2 dogs and his behavior gets worse everyday. I began making him stay outside during the day, but he tore off the steps to the deck and pulled the door off to the shed and now the other 2 dogs need taken out front since the backyard is inaccessible. I’m not sure what else I can do with him. This behavior is like nothing I’ve ever seen out of him before.

    1. Hi Joey. That’s terrible to hear. It’s extremely unusual aggression (if that’s even what all of this is) and probably has a complex origin. As much as it may be hard to organise, getting a behaviour expert in is all I can suggest.

    2. I think you’re blaming the neutering for simple lack of proper training. A dog that destroys household items is a BORED dog with EXCESS energy. Your dog is older now right? He’s big he’s strong and he’s clearly not getting enough exercise. He’s not getting the disciplined life he requires.

      He attacks your other dogs? Because you have not established that you’re the one he needs to listen to.

      I tell you from experience. I had a dog I got from the shelter who had been returned twice for destroying property. He was a great dog for me because I had 5 acres and he got to run a lot and got to play with the other dogs, he was very active. He never destroyed anything of mine.

      I have a dog now that attacked my other dog. Because I did not give him structure. I made sure to exercise him plenty daily and I had him always sit down and wait to be given his meal. He was not to take any toy unless I gave it to him. And if he had a toy and I asked for it, he had to give it to me. If he even looked at the other dog wrong, I immediately redirected his attention to me and praised him when he looked at me instead attentively. Any time he STARTED to act in a way that I did not like, I immediately got his attention and told him to sit down and look at me. For leadership.

      In other words, I became his leader.
      He has never ever attacked the other dog since. Not even once.

      So, it’s not that your dog is neutered. I don’t buy that. It’s that you’re not fulfilling his needs as a dog. He needs structure.

      1. We have a 6 month old German shepherd he gets walked and exercised every day thrown the ball and so on. I too get him to sit and not eat till told to do so, gets made to sit at all intersections when street walking and kept on an extremely short leash. He has continually jumped and bitten others since a pup, has been continually been smacked at the right time and it is only me he doesn’t bite or or jump on. It appears to be more of a dominance thing with him so mainly jumps on the Girls in our family. Unfortunately i cannot be there all the time to constantly chastise his behaviour so he will getting de-sexed in the not to distant future. He is my 8th German Shepherd.

      2. Hi Mark. The bad news from me is that if he has been doing this since a very young age, male hormones are probably playing very little role. I think you should really find a good dog behaviour specialist in your area. I strongly advise finding on using positive methods as they are known, and in fact, shown to deliver better results, however, this sounds like a hard one to solve. Good luck.

  5. Hi Andrew, as a fellow Vet, I commend you on your considered articles. The debate about desexing is one that does concern me and I do think blanket recommendations properly do not work for all dogs, I am a big advocate for behavioural screening during puppyhood to determine where the best time to desex may fall relative to breed disease risk factors (including mental health issues) and the families lifestyle, do they have little kids? etc, I would be interested in hearing your thoughts on the study that found suprelorin use to result to increased aggression in street dogs (note obviously they have different levels of management) and also your thoughts on the paper that found female dogs to be 6 x more likely to do cruciate. I do think this whole issue will continue to get clouded with regards to decisions especially if we start using “healthspan” or disease-free intervals rather than lifespan as validated outcomes for research. Well done taking the time to research and write these articles. Best Wishes, Matthew Muir BVSc/BVetBiol (Hons) MRCVS

  6. I am currently exercising my sons American Staffy, male just over 12 months. He is very aggressive towards other dogs and while never off lead, he is very strong. I worry what would happen if an off leash dog approached us and I want to muzzle him. I have been pushing for him to be de-sexed but now from this research, I feel that it may be of no benefit. He growled at my son one night when my sons pregnant partner was asleep and my son entered the room. Dog was asleep beside her. I am worried about this dog and family. Any ideas?

    1. Hi Sandy. On the contrary, I believe that the evidence supports a decision to Desex/neuter your dog, especially where it concerns aggression that is not involving fear and is possibly directed towards children.

  7. My 14 month old collie lab cross became aggressive after neutering. He’d never shown any signs of aggression before but now but 2 weeks after the OP he aggressive towards other dogs and especially our family members. I see lots of evidence to support this but nothing to say what can be done to deal with the problem. Any advice?


    1. Hi Tony. That’s a devastating outcome. Although I have not had to deal with it personally, I would always advise referral to a veterinary behaviour specialist if I saw it happen. The sooner you act, the greater the chance to turn him around so well done for the quick response. There are probably quite a few possible reasons (including inadvertent distress or pain from the surgical procedure) but I’m not ruling out the loss of testosterone itself. The most likely explanation however is that his neutering time coincides with the period of social maturation when behaviour problems often appear. These are hard to advise on which is why no-one is offering solutions online. Good luck.

  8. How refreshing! A piece that actually rationally presents the multifaceted desexing issue and looks at all the potential influencing factors on data.
    This is the first non-biased, logical article I’ve read on the subject. Thank you!!

    I have a 9 month old (70kg) Great Dane and my vet has been pushing so hard for me to have him desexed. I have to argue every time that I want to wait until at least 12 months for joint issues and to reduce the chances of osteosarcoma as much as possible. I made the mistake of telling him that my puppy occasionally shows fear based responses to tall male strangers (backing away, tail between legs and ‘boofing’ noises). I’m using clicker training and rewards to try to counter condition him and it’s working well. However, my vet is convinced this is aggression and has told me I’m “irresponsible” for not having him desexed yet. I’ve read the studies and am under the impression that castration may worsen fear based aggression but he strongly disagrees.

  9. Not just responding to this article, but your entire blog; everytime I read something here I am better informed as a pet companion. Your informative and neutral style is a rarity in this internet age. Thank you for sharing your study, knowledge and experience.

  10. Good to see a rational approach and voice when it comes to attempting to make sense of the studies.
    Like so much in the medical world, things are rarely simple…..example skin allergies are not necessarily related to the food eaten – allergies are so much more complex, even if there is a genuine food allergy, it is often much more complex than an allergy to protein – which most people think of as meat but there are proteins within proteins

    Likewise, Behavioural problems are so much more complex than age of desexing or if someone mistreated an animal at some stage in its life (usually a bearded man with glasses, wearing a baseball cap and maybe a high Vis vest whilst holding an umbrella)………we now know behaviour can start with the parents of an animal and how they themselves have been treated or maybe had an illness during pregnancy.

    The fact is you have raised some valid points on the validity of the data – it would be very interesting if the questions you ask would ever get taking into account with the data collected.

    People will enviably believe what they wish to believe – but in all I think this is a very considered and reasoned piece on the debate

    1. Thanks Gillian. Dr Paul McGreevy has authored some great studies and I certainly don’t want to detract from his work. It’s more an example of the need to identify the limitations of any research. I hope one day to ask him directly about this one!

Comments are closed.