Behavioral Effects of Neuter



Many guardians of neutered dogs report significant behavior changes after their dog fully recovers from the neuter surgery. These are generally guardians who chose to neuter their best friend based upon one or more of the following reasons:

  1. the recommendation of their vet that neuter would provide the best possible long term physical and psychological health for their dog
  2. the assertion of vets and behaviorists that their dog would behave better and become more manageable once neutered (see section “A New Study Sheds Some Light”  below for further discussion of outcomes)
  3. state law requiring that all male dogs adopted from shelters/rescues must be neutered
  4. a contract with a breeder that requires neuter at a prescribed time after purchase of the male dog

Alternatively, many people acquire their male dog after he has been neutered and rely on the assertions of shelter staff that once out of the stressful shelter environment and secure in his new home, he will calm down and be better behaved. In either case, guardians may find their dog exhibiting unacceptable behavior, and wonder what it is they have done wrong.

Unfortunately, as observed by Dr. Stanley Coren in Psychology Today, the latest peer-reviewed studies show the behavior changes in male dogs created by neuter are not beneficial as generally asserted by vets. In fact, the studies show that neuter actually increases aggression, fearfulness and anxiety in the recipient, as well as making them more difficult to train.

In the San Francisco Bay Area, where almost every male dog encountered in public places (e.g., dog parks, neighborhood walks) has been neutered, a relatively new problem is being observed. Specifically, almost every male dog is described by their guardian as “shy” (i.e., fearful, skittish, retreats from new people or situations), or “does not do well with people/dogs he does not know” (i.e., fearful and aggressive towards people or other dogs that scare him). From our perspective, the body language these dogs exhibit is not that of a confident, happy dog. In fact, this type of behavior in dogs seems unnatural and antithetical to the species. For example, who would ever expect to meet a skittish or unfriendly Golden Retriever?

Another common behavior issue guardians of fearful dogs share is reaction to environmental issues (e.g., thunder, fireworks, other loud noises, new places). The behavior their dogs exhibit ranges from mild anxiety to a full-blown “panic attack“. Separation anxiety is another undesirable behavior often exhibited in fearful dogs. Most guardians have no idea why their dog reacts as they do, and most are looking to do what they can to help their dogs. As a last resort, some guardians find themselves relinquishing these dogs back to a shelter because the dog’s behavior is not manageable for them. The San Francisco Society for the Prevention of Cruelty to Animals (SF SPCA) is the SF Bay Area’s largest and most progressive adoption agency, and their response to the widespread behavioral problems is to offer specialized “bravery classes for shy dogs“. However, they have one very significant caveat:

These classes are not suitable for dogs who have shown fear-based aggression or reactivity towards people or dogs, including lunging, snapping or having a history or incident of having bitten previously.” 

Can the evidence of significant canine behavior problems here in the SF Bay Area be tied to our high percentage of spayed or neutered dogs? Are the SF SPCA bravery classes being filled with behavior impaired students that the SF SPCA helped to create through their aggressive spay/neuter campaign? Our study of the stress-response system in spayed/neutered dogs leads us to believe spay/neuter adversely affects the dog’s behavior and personality. In fact, the documented changes in the stress-response system would predict an increased level of fear and anxiety, which in some dogs can result in aggression.

When we examined the peer-reviewed studies on the topic of behavior after neuter (see below), we find the studies confirm behavior is adversely impacted by spay/neuter and these changes are serious and potentially of epidemic proportions. So let’s see what the experts have to say…


The study “Non-reproductive Effects of Spaying and Neutering on Behavior in Dogs57 utilizes the “Canine Behavioral Assessment and Research Questionnaire (C-BARQ)” 15 , the only peer-reviewed, reliable, standardized method for evaluating and screening dogs for the presence and severity of behavioral problems. The overarching conclusion of the study is:

For most behaviors, neutering is associated with worse behavior, contrary to conventional wisdom.

Specifically, the study determined:

  • Neutered males are more aggressive towards people
  • Neutered males are more fearful and sensitive to touch/handling
  • Neutered males beg for and steal food more often
  • Neutered males are more aggressive towards other dogs
  • Neutered males mark territory less often
  • Neutered males roll in and eat feces more often
  • Neutered males lick people and objects more often
  • Neutered males self-groom and bark excessively

Another study, “Behavioral and Physical Effects of Spaying and Neutering Domestic Dogs16, also utilizes C-BARQ. The study concludes:

“The overall trend seen in all these behavioral data was that the earlier the dog was neutered, the more  negative the effect on the behavior.”

Specifically, the study found:

  • Neutered males are significantly more aggressive regardless of age of neuter
  • Neutered males are significantly more fearful regardless of age of neuter
  • Neutered males are significantly more anxious regardless of age of neuter
  • Neutered males are more difficult to train
  • Neutered males are less responsive to cues

Additionally, the study noted:

“The other three behavioral categories examined (miscellaneous behavior problems, attachment and attention seeking behavior, and separation-related behavior) showed some association with neutering, but these differed more substantially depending on the age at which the dog was neutered.”

Likewise, a study conducted in 2014, “Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas”,109 concluded the younger the age at gonadectomy (spay or neuter), the earlier the mean age at diagnosis of a behavioral disorder, or fear of storms. It is clear to us that sterilization should not entail removing hormone producing glands, and this study’s conclusions seem to concur:

“Additional studies are needed on the biological effects of removing gonadal hormones and on methods to render dogs infertile that do not involve gonadectomy.”

The study, “Behavioral Assessment of child-directed canine aggression56, evaluated dogs who had already bitten a child. The study concludes:

“Historical evidence of fearful or anxious behavior in response to loud noises and thunderstorms or separation from the owner may signal a predisposition to biting in threatening situations related to anxiety or fear…

“…Fear-related aggression was the most common primary behavioral diagnosis in the dogs….Most dogs (93%)…both male and female were neutered [or spayed].  Although our data did not include age at neutering [or spaying] or whether the surgery occurred before or after the appearance of aggressive behavior, it is apparent that neutering [or spaying] does not guarantee a reduction of aggression in dogs.”

The aforementioned studies document the behavioral changes observed in neutered male dogs. If you have followed our discussion of the “Canine-Human Connection“, it will not surprise you to find depression, anxiety and decreased quality of life are the most common psychopathological conditions in young hypogonadal (i.e. having deficient levels of testosterone) men. Further, cognitive functions were significantly worse in these young men–showing worse executive function, attention, visual scanning abilities and psychomotor speed. 147,148 In one of these human studies testosterone replacement therapy was initiated and it was observed to improve depression, anxiety and quality of life after 6 months of treatment.147  Another study in young and middle-aged men utilizing different methods of assessing mood status over 2 years of testosterone replacement therapy reported improvement in cognitive function only.149  These findings lead us to conclude that testosterone replacement therapy (as we advocate) may help improve the negative impact upon behavior and cognitive function resulting from neuter.

Veterinarians are reticent to provide testosterone supplementation to male dogs. This is true even when their testosterone levels are at zero. The specialist treating Billy refused our request for testosterone replacement, citing concerns Billy may become dangerously aggressive. In 1991, it was thought that higher levels of circulating testosterone in humans and animals were associated with increased physical aggression.162 Alternatively, data in a 2002 study showed that even supplementing with levels of testosterone moderately above normal physiologic levels did not lead to an increase in aggression or mood disturbances (as reported by participants and/or their friends and family). Instead, for the first time, this 2002 study identified highly negative repercussions experienced by testosterone deficient patients instead.163 However, much of this research has been observational, retrospective and/or cross-sectional in nature, making it difficult to render conclusions about the causal relations between testosterone and male behavior. We are, however, fortunate to have a reader who supplemented her pup Mercury with testosterone when his neuter drastically changed his behavior, and provided us video evidence of the before and after.  The following is a composite of the emails/comments we received from her:

Dear Healthy and Happy Dog,

My dog Mercury developed fear and aggressive behavior soon after being neutered. He shutdown completely in agility training among other issues at home. My vet agreed to testosterone shots after I read your website about it with respect to neutering.

Two weeks after first testosterone injection, Mercury has started marking again, getting excited, if you know what I mean, and is back to barking in his crate at the arrival of a show and seeing dogs. This is faster than I thought as I was reading with respect to people, it could take up to 3 weeks before any improvements are noticed. But he is small. I suppose his young age is playing a big role with it taking effect quickly too. I also am taking him off of Zoloft which was prescribed by the vet early on.

Since we started the Testosterone shots last March, Mercury has returned back to his pre-neuter self. He is back to running agility and he is happy and healthy!!


Certainly, this is merely observational evidence. However, sometimes a picture is truly worth a thousand words:

Currently a review of the evidence suggests that the relatively modest doses of testosterone required for clinical purposes are not associated with changes in aggressive or angry behavior. Moreover, reports of AAS (androgenic-anabolic steroids) abusers exhibiting high levels of aggressiveness and experiencing episodes of mania or hypomania after taking huge doses of AAS should not be compared with or extrapolated to the effects of therapeutic doses of testosterone.

In our own experience, supplementation with testosterone to normal physiologic levels did improve Billy’s fears and anxiety, and certainly improved his quality of life. Billy never exhibited fear aggression, but rather fear caused him to retreat from perceived danger. Testosterone supplementation allowed Billy to be more comfortable with new people, dogs and experiences, and in fact he began to enjoy new things! Testosterone also allowed Billy to experience loud noises, fireworks, thunder, etc. without panicking and in fact with little or no anxiety. We also noticed a significant improvement in Billy’s cognitive functioning. We found his periods where he appeared to “zone out” disappeared, and he became much more responsive to us in general.

Although it is difficult to perform studies on dogs to assess quality of life and levels of depression and anxiety (as we did in the human studies discussed above), understanding how neuter affects the developing stress response system can be another tool to help explain why hormone balancing for Billy changed his behavior so dramatically.

Highly potent stressors (i.e. neuter) early in life while the dog’s stress response system is still developing, can have detrimental effects on behavior and personality that are permanent.85 Please see our “Stress Response System” page for a more complete explanation. On our “Countering the Effects of Neuter” page we will explore steps you can take to minimize neuter’s effect on your dog’s endocrine system.


Previously, we mentioned that quite often the vet advises the guardian/owner of an intact male dog that neuter will improve his behavior. The current peer-reviewed studies explored on this page claim otherwise. However, social media is replete with testimonials where the guardian/owner extols the value of neuter surgery for helping their male dog to behave better. We are often left pondering the disconnect between the actual science and some people’s personal experience.

Several of our readers have reported to us that after following our advice to keep their male dog intact, somewhere between the ages of 5-12 months, the dog’s behavior spirals out of control. These formerly well-behaved puppies suddenly seem to have little or no ability to control their impulses. They become blatantly disobedient when their guardian/owner attempts to rein in the errant behavior. Was this even more evidence of  the canine-human connection? Are male dogs exhibiting bad behavior because they are going through the canine equivalent of human puberty? Human parents are well aware of the difficulty in dealing with young boys at the time they are reaching sexual maturity. However, over time, these young boys become young men and their behavior tends to normalize. Likewise, those guardians/owners who hung tough and resisted the pressure applied by the veterinary industry to neuter their puppy, reported that eventually the puppy just seemed to grow out of this bad behavior stage. Perhaps those reporting better behavior after neuter had the procedure done at the recommendation of their vet when their male dog was in the throes of the dramatic hormonal changes of canine puberty. The dog’s behavior may seem improved for the immediate circumstance.  However, over time, as hormonal balance was lost, we felt confident their dogs would fit into the study results reported here.

In May of 2020, a new study174 was published in Biology Letters, and discussed by the researchers online at “The Conversation” (a network of not-for-profit media outlets that publish news stories written by academics and researchers):

“…dogs have a passing phase of reduced obedience towards their owners during puberty…Perhaps the most important thing to note for dog owners is that these behaviour changes were a passing phase. By the time dogs were 12 months old, their behaviour had returned to how they were before puberty, or in most cases, had improved…This is crucial for any new dog owner to be aware of, because sadly, adolescence is the peak age when dogs are abandoned and end up in animal shelters. It’s also extremely important that owners don’t punish their dogs for disobedience or start to pull away and disengage from them at this time, as this would be likely to make problem behaviour worse in the long run, as it does in people.”

We believe the results of this new study provide further guidance for dog guardians/owners who decide on hormone-sparing sterilization for their male dog rather than neuter.