Neutering and Behavior in Dogs: What the Evidence Shows

1. What the Question Actually Is

1.1 Two Different Decisions

Neutering for population control and neutering as a behavioral intervention are separate questions with separate evidence bases. The first has a clear public-health rationale. The second is a clinical claim about individual dogs, and it is the one examined here.

1.2 Why the Assumption Is Intuitive

Testosterone is associated with several behaviors that owners find difficult, and removing the gonads removes the primary source. The inference — less hormone, less behavior — is straightforward and, for a subset of behaviors, correct.

The problem is that the subset is narrower than the recommendation implies, and that the hormone modulates rather than drives most of what people want changed (a distinction the testosterone evidence sets out in detail).

1.3 What Would Count as Evidence

A behavior that is hormone-dependent should decline after gonadectomy across populations, consistently, and independently of why the dog was neutered. That last condition matters more than it sounds, and section 9 returns to it.

2. What Neutering Reliably Changes

2.1 The Short List

Three behaviors show the most consistent response: roaming, urine marking, and mounting. These are the behaviors most directly linked to reproductive function, and the classic work on gonadal hormones and objectionable behavior in dogs and cats established them as the reliable targets (Hart & Eckstein, 1997). Roaming in particular is also driven by olfactory information that surgery does not remove (the channel dogs rely on most).

2.2 The Size of the Effect

Even here the effect is partial rather than absolute. A proportion of dogs show marked improvement, a proportion show some, and a proportion show none — and the behaviors in question also have a learned component that surgery does not touch. A dog that has marked indoors for two years has a habit as well as a hormone, and habits persist through their own mechanisms (which is why removing the driver rarely removes the behavior).

2.3 What Is Not on the List

Fear, anxiety, resource guarding, reactivity on lead, and most forms of aggression are not among the behaviors consistently shown to respond to neutering (which the reactivity literature would predict). This is where the recommendation and the evidence part company.

3. The Aggression Data

3.1 The Design That Matters

The largest direct test used the Canine Behavioral Assessment and Research Questionnaire, a validated instrument, on an initial pool of 15,370 dogs. Critically, the analysis excluded dogs that had been neutered to correct a behavior problem — which removes the most serious confound in this literature, since dogs neutered because they were already aggressive would otherwise make neutering look causal (Farhoody et al., 2018).

After exclusions, 13,795 dogs were analysed for aggression toward familiar people, 13,498 toward strangers, and 13,237 toward other dogs.

3.2 The Result

Neither gonadectomy nor age at gonadectomy showed an association with aggression toward familiar people or toward other dogs. For aggression toward strangers there was a low but statistically significant increase in the odds of moderate or severe aggression in neutered dogs compared with intact dogs (Farhoody et al., 2018).

3.3 An Earlier Finding in the Same Direction

The pattern is not new. A study of 1,842 dogs neutered and adopted between six and twelve months of age, surveyed two to three years later, found that aggression toward family members and toward strangers increased in males gonadectomized before five and a half months (Spain, Scarlett & Houpt, 2004).

That study is older, smaller and used a different design, which is precisely why the convergence is worth noting: two independent datasets, fifteen years apart, produced the same directional result on early neutering in males (with early development shaping much of what follows).

3.4 How to Read That

The honest reading is not "neutering causes aggression." The effect on strangers was small, the design is cross-sectional, and questionnaire data carry their own limitations (as measurement problems go generally).

The reading that does hold is narrower and still decisive for practice: in the largest dataset available, with the obvious confound removed, neutering was not associated with less aggression toward familiar people, strangers or other dogs. Those three categories are not the whole of canine aggression, but they cover what the recommendation is usually meant to address — and in none of them did the procedure perform as expected (with aggression being situation-specific rather than trait-like in any case).

4. The Hormone Exposure Studies

4.1 A Different Question

Rather than comparing neutered against intact, a second approach asked how much of a dog's life had been spent with gonadal hormones present. The measure — percentage lifetime exposure to gonadal hormones — treats the question as continuous rather than binary (McGreevy, Wilson, Starling & Serpell, 2018). It is a more informative framing than neutered-versus-intact, and it carries a statistical problem that section 9 sets out (of the kind that recurs wherever composite measures are used).

4.2 The Male Result

Across 6,235 male dogs castrated before ten years of age for reasons other than behavioral management, forty behaviors differed between intact and castrated dogs. Twenty-five were associated with hormone exposure and fourteen with age at castration.

Only two behaviors were more likely in dogs with longer hormone exposure: indoor urine marking and howling when left alone. Longer exposure was associated with significantly reduced reporting of twenty-six mostly unwelcome behaviors — of which eight related to fearfulness and seven to aggression (McGreevy et al., 2018).

4.3 What That Inverts

The expected pattern is that hormones drive unwanted behavior and removing them reduces it. The observed pattern was the reverse for nearly everything measured except the two behaviors that are directly reproductive.

The authors framed the practical consequence as a paradox: castration may reduce the number of unwanted dogs while increasing the likelihood of problem behaviors that make individual dogs more likely to be surrendered (McGreevy et al., 2018).

5. Where Males and Females Differ

5.1 The Parallel Study

The same method was applied to 8,981 female dogs spayed before ten years of age, again excluding behavioral indications. Twenty-three behaviors differed between intact and spayed dogs, twelve associated with hormone exposure and five with age at spay (Starling, Fawcett, Wilson, Serpell & McGreevy, 2019).

5.2 The Weaker Signal

Two behaviors were more likely with longer hormone exposure — chewing and howling. Ten mostly unwelcome behaviors were reported less often, of which one related to fearfulness and three to aggression (Starling et al., 2019).

Compare the male figures: eight fearfulness-related and seven aggression-related. The direction is the same; the magnitude is not.

5.3 Why This Matters Practically

Advice about neutering is routinely given without reference to sex, and these two studies used identical methods on the same instrument. The behavioral case against early neutering is substantially stronger for males than for females — which is not how the discussion is usually conducted (and individual variation complicates it further).

6. Timing and Physical Health

6.1 The Shift in the Question

The health literature has moved from whether to when. Neutering in the first year became routine in the U.S. and much of Europe, and subsequent work indicates that for some breeds this is associated with increased risks of debilitating joint disorders and certain cancers (Hart, Hart, Thigpen & Willits, 2020a).

The joint disorders examined are hip dysplasia, cranial cruciate ligament tear or rupture, and elbow dysplasia. The cancers are lymphoma, mast cell tumor, hemangiosarcoma, and osteosarcoma.

6.2 The Breed-Specific Signal

The pattern first became visible in single-breed work. An evaluation of gonadectomized Vizslas examined both cancer risk and behavioral disorders and found elevated risk with earlier gonadectomy, with age of onset also affected (Zink et al., 2014).

Single-breed studies of this kind cannot be generalised, and that limitation turned out to be the finding: the effect sizes differ enough between breeds that a single recommendation does not survive contact with the data.

6.3 Breed and Body Size

The findings across 35 breeds do not generalise into a single recommendation, which is the point of the paper: the risks differ substantially by breed. A parallel analysis of mixed-breed dogs across five weight categories found small breeds generally showing little vulnerability compared with larger-bodied dogs (Hart et al., 2020b).

Body size, not breed identity as such, carries much of the pattern (which is consistent with what breed explains generally).

6.4 Where Behavior and Health Converge

Both literatures point the same way on timing even though they measure different outcomes, and neither was designed with the other in mind (convergence across independent methods being the strongest evidence available here). Early neutering is associated, in several large studies, with more unwanted behavior in males, and in larger breeds with elevated joint and cancer risk. Neither finding argues against neutering as such. Both argue against doing it early by default.

7. What This Means for a Decision

7.1 The Question to Ask First

If the reason for neutering is a behavior problem, the first question is which behavior. Roaming, marking and mounting have a reasonable hormonal case. Fear-driven or defensive behavior does not, and the data suggest it may worsen (with fear learning proceeding on its own logic).

7.2 The Reversibility Problem

Surgical neutering is permanent, and the behavioral effects — in whichever direction — cannot be undone. Where the behavioral case is uncertain, temporary hormonal suppression allows the effect to be observed before the decision is made. This is a standard veterinary option and is underused.

7.3 What Neutering Cannot Do

It cannot resolve a learning history, and most behavior that owners want changed is maintained by consequences rather than by hormones (which is what makes the behavior persist). Where the behavior is anxiety-driven, the intervention that matters is the one addressing the anxiety (as the anxiety literature sets out), and where pain is involved, neither surgery nor training will substitute for a diagnosis (a route that is missed with striking regularity).

8. Summary at a Glance

Three behaviors respond reliably — Roaming, urine marking and mounting; the classic hormone-dependent set (Hart & Eckstein, 1997).

Aggression toward people or dogs is not among them — With behavioral indications excluded, neutering showed no association with aggression toward familiar people or other dogs, and a small significant increase toward strangers, across roughly 13,000–14,000 dogs per analysis (Farhoody et al., 2018).

Longer hormone exposure meant fewer problems, not more — In 6,235 males, longer exposure was associated with reduced reporting of 26 mostly unwelcome behaviors, including 8 fearfulness-related and 7 aggression-related (McGreevy et al., 2018).

Only two male behaviors went the expected way — Indoor urine marking and howling when left alone (McGreevy et al., 2018).

The female signal is weaker — In 8,981 females, 10 unwelcome behaviors were reduced with longer exposure, of which 1 related to fearfulness and 3 to aggression (Starling et al., 2019).

The health question is about timing — Early neutering is associated with elevated joint disorder and cancer risk in some breeds, with major breed differences (Hart et al., 2020a).

Body size carries much of it — Small breeds showed little vulnerability compared with larger-bodied dogs (Hart et al., 2020b).

Both literatures converge on timing — Neither argues against neutering. Both argue against neutering early by default.

9. Research Gaps and Critical Appraisal

All of this is owner-reported and cross-sectional. Every study cited here uses questionnaire data at a single time point. They establish associations across large populations and cannot establish that neutering caused any behavioral difference.

The hormone exposure measure is confounded with age. Percentage lifetime exposure is calculated as a proportion of the dog's age at reporting. A dog neutered at six months and reported at ten years has a low value; the same dog reported at eighteen months has a high one. The measure therefore correlates inversely with current age at any fixed neutering age, and age has independent effects on behavior.

Confounding by indication is handled but not eliminated. Excluding dogs neutered for behavioral reasons is the right move (Farhoody et al., 2018; McGreevy et al., 2018), and it relies on owners correctly reporting why the procedure was done years earlier.

No randomised trial exists and none is likely. Randomising dogs to neutering or not, at assigned ages, would answer the causal question and is not going to happen for practical and ethical reasons. This limitation is permanent, not temporary.

C-BARQ measures owner perception. It is a validated instrument, which means it measures consistently — not that it measures the dog's behavior independently of the owner's interpretation (a general constraint on this kind of data).

Breed-specific health data do not transfer. The joint and cancer findings are breed-specific by design, and the majority of breeds have not been examined. Applying a Golden Retriever result to an unstudied breed is not supported.

Effect sizes are frequently omitted in secondary coverage. The stranger-directed aggression increase was described by its own authors as low. Reporting that a study "found neutering increases aggression" without that qualifier misrepresents it.

10. Conclusion

The recommendation that neutering will calm a dog down has an unusual property: it is widespread, it is offered by professionals, and the largest datasets addressing it point the other way. With dogs neutered for behavioral reasons excluded, neutering showed no association with reduced aggression in any category tested, and a small increase toward strangers. In more than six thousand males, longer exposure to gonadal hormones was associated with fewer unwanted behaviors across twenty-six measures, including eight relating to fear and seven to aggression — the reverse of the expected pattern, with the only two exceptions being the directly reproductive ones. The female data run in the same direction and considerably weaker, which is itself worth knowing given how rarely the advice distinguishes by sex. None of this makes neutering wrong, and none of it is causal evidence: these are questionnaire studies, the hormone exposure measure is entangled with age, and the randomised trial that would settle it will never be run. What the evidence does support is a narrower and more useful position than either camp usually takes. Neutering reliably addresses roaming, marking and mounting. It does not reliably address fear, anxiety or aggression, and may make the first two worse in males neutered early. The question worth asking is therefore not whether but which behavior, and when — with the timing question now carrying weight from two independent literatures that were not designed to agree (and with early experience shaping much of what gets attributed to hormones).

Key Insights (Takeaways)

• Neutering was not associated with less aggression in the three categories the largest dataset examined. With dogs neutered to correct behavior problems excluded, gonadectomy showed no association with aggression toward familiar people or other dogs, and a low but significant increase toward strangers, across roughly 13,000 to 14,000 dogs per analysis (Farhoody et al., 2018). The exclusion matters: without it, dogs neutered because they were aggressive would make the procedure look causal.

• Longer hormone exposure was associated with fewer unwanted behaviors, not more. Across 6,235 male dogs, longer lifetime exposure to gonadal hormones was associated with reduced reporting of 26 mostly unwelcome behaviors — 8 relating to fearfulness and 7 to aggression — while only indoor urine marking and howling when alone went the other way (McGreevy et al., 2018). This runs counter to the expectation behind the standard recommendation — though the study establishes association, not cause.

• The effect is substantially weaker in females. The same method applied to 8,981 spayed females found 10 unwelcome behaviors reduced with longer hormone exposure, of which one related to fearfulness and three to aggression (Starling et al., 2019), against eight and seven in males. Advice that does not distinguish by sex is not tracking the evidence.

• Three behaviors do respond reliably, and they are the reproductive ones. Roaming, urine marking and mounting remain the defensible targets (Hart & Eckstein, 1997) — partially, and with the learned component untouched by surgery.

• The timing question now carries more weight than the yes-or-no question. Early neutering is associated with increased joint disorder and cancer risk in some breeds, with small breeds showing little vulnerability compared with larger-bodied dogs (Hart et al., 2020a; 2020b). Behavior and physical health point in the same direction on timing, from two literatures that were not designed to agree.

References

Farhoody, P., Mallawaarachchi, I., Tarwater, P. M., Serpell, J. A., Duffy, D. L., & Zink, C. (2018). Aggression toward familiar people, strangers, and conspecifics in gonadectomized and intact dogs. Frontiers in Veterinary Science, 5, 18. https://doi.org/10.3389/fvets.2018.00018

Hart, B. L., & Eckstein, R. A. (1997). The role of gonadal hormones in the occurrence of objectionable behaviours in dogs and cats. Applied Animal Behaviour Science, 52(3–4), 331–344. https://doi.org/10.1016/S0168-1591(96)01126-4

Hart, B. L., Hart, L. A., Thigpen, A. P., & Willits, N. H. (2020a). Assisting decision-making on age of neutering for 35 breeds of dogs: Associated joint disorders, cancers, and urinary incontinence. Frontiers in Veterinary Science, 7, 388. https://doi.org/10.3389/fvets.2020.00388

Hart, B. L., Hart, L. A., Thigpen, A. P., & Willits, N. H. (2020b). Assisting decision-making on age of neutering for mixed breed dogs of five weight categories: Associated joint disorders and cancers. Frontiers in Veterinary Science, 7, 472. https://doi.org/10.3389/fvets.2020.00472

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. https://doi.org/10.1371/journal.pone.0196284

Spain, C. V., Scarlett, J. M., & Houpt, K. A. (2004). Long-term risks and benefits of early-age gonadectomy in dogs. Journal of the American Veterinary Medical Association, 224(3), 380–387. https://doi.org/10.2460/javma.2004.224.380

Starling, M., Fawcett, A., Wilson, B., Serpell, J., & McGreevy, P. (2019). Behavioural risks in female dogs with minimal lifetime exposure to gonadal hormones. PLoS ONE, 14(12), e0223709. https://doi.org/10.1371/journal.pone.0223709

Zink, M. C., Farhoody, P., Elser, S. E., Ruffini, L. D., Gibbons, T. A., & Rieger, R. H. (2014). Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas. Journal of the American Veterinary Medical Association, 244(3), 309–319. https://doi.org/10.2460/javma.244.3.309