OVERVIEW: Same-sex dogs feature prominently in household fighting studies, but sex, age, and sterilization do not reliably predict or cure fights between housemates.
Spaying or neutering a dog does not reliably prevent or cure fighting between dogs living in the same home. Research also complicates familiar claims that males are the fighters, females are peaceful, puppies are safe with adults, or two spayed females should get along. Certain patterns appear repeatedly in clinical studies, but a pattern among dogs referred for aggression is not a prediction for every household.
The difference matters. Someone choosing a second dog wants to know the chance of trouble before it happens. Someone whose dogs already fight wants to know why, how seriously to take the danger, and whether surgery will help. The research offers useful clues to both people, but not the certainty that many common rules imply.
Are females or males more likely to fight in the same household?
Same-sex pairs, particularly female pairs in some early clinical reports, appear frequently among dogs referred for household aggression. Sherman and colleagues (1996) examined 99 cases of aggression between dogs, 73 principally involving dogs in the same household. Females more often initiated the household cases, whereas males were more prominent in aggression toward dogs outside the home. Female-female pairs were especially common among the household cases.
Wrubel and colleagues (2011) reported that 30 of 38 referred household pairs (79%) were of the same sex. Feltes and colleagues (2020) examined a larger referral series of 305 pairs: 188 pairs (61.6%) were the same sex, and 214 (70.2%) included at least one female.
Table 1. Sex patterns among dogs referred for household aggression
Study | What was reported | What it does not establish |
Sherman (1996) | 73 household cases; females more often initiated reported household aggression. | It does not establish the risk for all female or male dogs. |
Wrubel (2011) | 30 of 38 pairs (79%) were same-sex. | It does not mean 79% of same-sex housemates will fight. |
Feltes (2020) | 97 female-female; 91 male-male; 117 mixed-sex. Together, 188 of 305 (61.6%) were same-sex; 214 (70.2%) included a female. | A female in the pair was not necessarily the initiator. |
Reading rule: These percentages describe veterinary referral cases, not the likelihood that a particular combination of dogs will fight.
These numbers support paying attention to sex composition. They do not mean that 61.6% of all same-sex households will experience fights. To establish that risk we would need to know how many same-sex and mixed-sex pairs live together without fighting, how long they have lived together, and what opportunities they have had for contact. The clinical studies mainly describe dogs whose owners already sought help. Furthermore, the presence of a female in a pair does not tell us which dog initiated an encounter.
Are fights between two females more dangerous?
There is a concerning signal, but not a settled verdict. In Sherman's household cases, female-female pairs had the highest median severity score, and some incidents required veterinary treatment. The comparison with other sex combinations narrowly missed the conventional threshold for statistical significance (p = 0.057). Female pairs did not, however, fight more frequently than the other combinations in that sample.
Table 2. Injury severity by sex pairing in Sherman et al. (1996)
Pairing (initiator first) | Cases | Median severity score |
Female-female | 34 | 4 |
Male-male | 21 | 3 |
Female initiator, male recipient | 10 | 3 |
Male initiator, female recipient | 5 | 2 |
Scale: 1 = no contact; 2 = contact without injury; 3 = minor injury; 4 = injury requiring veterinary care. Based on the worst owner-reported encounter. The overall comparison (p = 0.057) was not statistically significant at the 0.05 level; these scores are not an individual risk estimate.
That distinction is easily lost. Frequency, the decision to attack, the number of injuries, and the seriousness of the wounds are different outcomes. A pair that fights infrequently may still be dangerous if one encounter produces deep wounds. A pair with frequent warnings and no physical contact is a different problem. Neither can be understood by the phrase “female dogs fight harder.”
Feltes and colleagues also found that both female-female and male-male pairs were associated with worse clinical outcomes than mixed-sex pairs. The study's outcome was permanent separation, rehoming, or behavioral euthanasia, not a direct measurement of which sex inflicted the deepest wounds. In unadjusted comparisons with mixed-sex pairs, the odds of a poor outcome were about 2.9 times as high for female-female pairs and 2.2 times as high for male-male pairs. These are not probabilities that a particular pair will fail, and the difference between the two same-sex combinations was not statistically established. It would be premature to declare females inherently more violent or less capable of living together.
Table 3. Sex pairing and poor outcomes in Feltes et al. (2020)
Pair type | Unadjusted odds ratio versus mixed-sex pairs | What was measured |
Male-female | 1.0 (reference) | Baseline comparison group |
Male-male | 2.2 | Poor outcome: permanent separation, rehoming, or behavioral euthanasia |
Female-female | 2.9 | Same composite outcome, not post-fight reconciliation |
Interpretation: An odds ratio is not an individual probability. These unadjusted comparisons do not prove causation; female-female and male-male pairs were not significantly different from each other. This is not a ranking of wound severity.
Is a male and female the safest pairing?
Mixed-sex pairs were less prominent among the problem cases and had comparatively better outcomes in Feltes's referral series. That is a useful consideration when choosing a housemate. It is not a guarantee of compatibility. Male-female pairs can engage in threats, one-sided attacks, and serious reciprocal fighting, and individual differences can outweigh a simple sex-based rule.
Breed, size, health, reproductive condition, past experience, and the arrangement of daily life may all matter. A dog cannot be judged by sex alone, and two dogs cannot be evaluated as a relationship merely by adding together their individual traits.
Could breed and body size be skewing these results?
Yes. Breed composition and body size could partly influence findings about which dogs fight and which pairs have worse outcomes. But the research has not established that breed explains away the apparent sex differences. To answer that, investigators would need to compare similar dogs and households while accounting for breed or ancestry, weight, age, reproductive status, and opportunities for contact.
Sherman and colleagues (1996) described 38 breeds among initiating dogs in the broader referral sample and found that some breeds appeared more often than predicted using American Kennel Club registration figures. They also reported that the larger dog started fights in approximately 85% of household cases involving different sizes. This is an interesting description of the referred cases, not proof that size caused the attack. AKC registration counts cannot tell us how frequently each breed actually lived in a multidog household without fighting.
Wrubel and colleagues (2011) recorded a variety of breeds but studied only 38 pairs, far too few to make strong comparisons among breeds while also separating sex, age, and sterilization status. Feltes and colleagues (2020) included 610 dogs, of which approximately 46% were mixed-breed, and recorded breed information. However, the study reported one-variable-at-a-time associations with poor outcomes. The increased odds reported for female-female and male-male pairs were not adjusted simultaneously for breed, body size, and other potentially related characteristics.
Table 4. How breed and size complicate the household-aggression studies
Study | What was recorded | What remains unresolved |
Sherman (1996) | Breed composition and size patterns in referred cases; larger dog initiated many different-size pair cases. | The sex and injury patterns were not isolated from breed, size, or referral effects. |
Wrubel (2011) | A range of breeds among 38 pairs. | Too few pairs to estimate reliable breed-by-sex or breed-by-sterilization effects. |
Feltes (2020) | Breeds and mixed-breed dogs among 610 dogs; sex-pair outcome associations. | Reported sex odds ratios were unadjusted for breed and other potentially correlated factors. |
Morrill (2022) | Breed explained about 9% of variation across the behavioral traits studied. | Not a test of the prevalence or injury severity of household dogfights. |
Reading rule: Breed is a plausible confounding factor, not a demonstrated explanation of the sex associations. Reported referrals cannot provide a population risk ranking by breed.
Imagine two groups of dogs in which female-female pairs are, on average, larger than mixed-sex pairs. More serious wounds in the female pairs could partly reflect size and physical capability rather than a sex-specific tendency to inflict severe injuries. That is a possible confounding explanation, not a finding demonstrated by these studies. The opposite is possible too: a sex association might remain after breed and size are properly accounted for. The available comparisons cannot settle the issue.
Nor should we replace a sex stereotype with a breed stereotype. Morrill and colleagues (2022) found that modern breed labels explained about 9% of variation across the behavioral traits they measured, leaving substantial differences among individual dogs of the same breed. Their research did not directly measure serious fights between cohabiting dogs. It therefore cannot tell us the relative likelihood of a particular breed initiating household violence, producing severe wounds, or reconciling afterward.
Do puppies start fights, or do adults attack puppies?
Both can occur, but the literature more consistently identifies younger dogs as initiators in referred household cases. In Wrubel's study, the younger dog was identified as the instigator in 20 of the 27 pairs for which that distinction could be made (74%). Sherman reported younger initiators in 42 of 73 household cases (58%). In Feltes's series, the dog identified as the aggressor averaged 16 months younger than the recipient and was more often acquired later.
Table 5. Younger dogs and reported initiation
Study | Younger dog finding | Important distinction |
Sherman (1996) | 42 of 73 household cases (58%) identified the younger dog as initiator. | Younger does not mean puppy. |
Wrubel (2011) | 20 of 27 identifiable initiators (74%) were the younger dog. | Only 27 of the 38 pairs had this classification. |
Feltes (2020) | Identified aggressors averaged 16 months younger than recipients. | An age difference is not the age at first fight. |
Reading rule: None of these findings gives the probability of a puppy attacking an adult dog, or vice versa.
“Younger” is not synonymous with “puppy.” A four-year-old attacking a six-year-old belongs in that category. The studies do not establish a rate of attacks by puppies on adults. Sherman also described five cases in which an adult dog attacked a recently introduced puppy approximately two to three months old. The assumption that an adult will automatically tolerate a puppy is therefore unsafe, especially when the adult's history or current behavior raises concerns.
At what age do household fights begin?
No universal age of onset has been established. Sherman's initiating dogs had a median age of approximately three and a half years at presentation. That is not the same thing as the age when they first became aggressive. Referral often follows a history of problems rather than the first incident.
Adolescence and social maturation can coincide with changes in strength, persistence, activity, and relationships. Later in life, pain, illness, sensory changes, and reduced mobility may alter how an older dog responds to an approach or physical contact. The important question is what changed before the first conflict, not whether a dog passed a supposed age at which fighting was inevitable. We also do not have reliable age-specific population probabilities for serious household attacks.
Doesn't neutering males reduce fighting?
Sometimes it reduces particular behaviors. That is different from establishing that it prevents household dogfights. Testosterone influences reproductive behavior, and castration commonly reduces roaming associated with mating, sexual behavior, and some urine marking. The World Small Animal Veterinary Association's 2024 guidelines summarize a questionnaire-based study in which intermale aggression reportedly decreased after castration in 57% of dogs, while 4% were reported to become more aggressive.
Those figures do not mean that 57% of fighting male housemates were cured. “Intermale aggression” can cover encounters with unfamiliar males as well as familiar ones, and owner questionnaires do not necessarily distinguish threats from physical violence. Nor does a behavior that changes after surgery establish that every other component of an established household conflict has changed.
Notably, Sherman and colleagues reported that neutering was recommended more often for aggression toward dogs outside the household than for the household cases. A large analysis by Farhoody and colleagues (2018) found no statistically significant association between gonadectomy, or age at gonadectomy, and owner-reported aggression toward other dogs after accounting for measured factors. That study had its own limitation: it considered dog-directed behavior broadly, not exclusively injurious fights between housemates. Taken together, the studies do not justify promising that neutering will stop two male dogs from fighting.
A further limitation affects the sterilization evidence: Farhoody and colleagues (2018) explicitly did not include breed differences as a variable in their analysis. Their overall null result does not establish that gonadectomy has identical effects in every breed or lineage. Nor does the omission demonstrate that breed would have changed the result. The breed-specific question remains open.
Will spaying two females prevent or cure their fights?
There is no convincing household-specific evidence that it will. That does not make reproductive physiology irrelevant. Sherman and colleagues recorded ten household cases associated with estrus or pregnancy-related circumstances, eight involving female-female pairs. Some involved a dog in heat or pregnancy; others involved a mother with puppies. The investigators described associations in case histories, not a controlled test of spaying as treatment.
A reproductive cycle can change the circumstances in which dogs encounter one another, and maternal behavior or overt pseudopregnancy can sometimes include aggression. These are reasons to record reproductive status carefully and obtain appropriate veterinary assessment. They are not grounds for assuming that surgery will undo a longstanding social conflict or erase a history of injuries. In Sherman's series, aggression in some maternal cases continued after the puppies were weaned.
There is also a neglected distinction in the word “sterilization.” Preventing reproduction is not the same as removing the ovaries or testes and changing gonadal hormone production. The medical and behavioral implications depend partly on the procedure. A decision about surgery should consider reproduction, health, age, and the individual dog's circumstances rather than treating a behavioral promise as an established medical fact.
Table 6. What sterilization research can and cannot answer
Evidence | Reported finding | Limit for household fighting |
Castration of males | A questionnaire study reported reduced intermale aggression in 57% and increased aggression in 4%. | Not a cure rate for male housemates with established fights. |
Farhoody et al. (2018) | Adjusted dog-directed aggression result: OR 0.89; 95% CI 0.58 to 1.39; p = 0.617. | No statistically significant association; not limited to severe housemate attacks. |
Spaying of females | No convincing evidence that spaying prevents or cures female-female household fighting. | An absence of demonstrated efficacy is not proof of no hormonal effects. |
Sherman (1996) | Ten household cases involved estrus or pregnancy-related circumstances; eight were female-female. | A reproductive association is not a controlled test of surgery. |
Clinical implication: Record exact surgery type, timing, reproductive history, and the sequence of each dog’s behavior. Do not infer that sterilization alone restores compatibility.
Why are confident rules about dog aggression so unreliable?
One problem is that researchers and owners do not always mean the same thing by “aggression” or “fight.” Some study scales include growling, snarling, lunging, or snapping without contact. Others score actual bites, and still others rely on an owner's account of the worst incident. A noncontact dispute, a brief reciprocal fight, and an attack that continues after one dog tries to escape should not automatically be counted as equivalent events.
Another problem is that the dog labeled the “instigator” may not be the dog that first approached, obstructed retreat, produced a threat, made contact, or caused the injury. Without a chronology of both dogs' actions, those roles can be confused. If an incident happened beside a food bowl, for instance, the presence of food describes the setting; it does not by itself prove which dog wanted what or why violence occurred.
A third problem is the missing comparison group. Referral clinics see owners with a problem, not every household in which dogs live peacefully. And when an owner reports improvement after neutering, medication, training, separation, or management changes, we must ask what else changed. Dogs may fight less often simply because they were no longer allowed to meet in the circumstances that produced conflict. That may be successful injury prevention, but it is not proof that the underlying relationship was repaired.
What seems more important when a household is already fighting?
The dogs' actual behavior and the consequences of previous encounters deserve more weight than a demographic stereotype. In Feltes's outcome analysis, a history of bites that broke the skin was associated with worse outcomes. So was attacking immediately upon seeing the other dog, although that finding involved a small subgroup. These are associations in referred cases, not a calculator for predicting an individual dog's future.
For me, the first question would not be which dog is the alpha, whether the pair is female-female, or whether both dogs have been altered. I would want a chronology. What happened before the first injury? Did one dog withdraw while the other continued? Was an injury preceded by a warning, a collision, or a painful interaction? Did the behavior change after a new dog arrived, one matured, someone became ill, or the household routine changed? And how do these dogs behave during the many ordinary encounters that never become fights?
The answer may remain incomplete even after a careful assessment. That is preferable to attaching a confident but unsupported cause to a dog simply because of its sex, age, or surgery history.
What remains unknown about which dogs fight and recover?
We still lack good estimates of the absolute chance that a randomly selected pair of females, males, or mixed-sex dogs will develop injurious household conflict. We do not have reliable age-specific rates for puppies attacking adults or adults attacking puppies, nor a validated way to predict which pair will progress from threats to injury. The evidence is also too thin to rank the sexes by the wounds they cause while adequately accounting for size, opportunity, and the particular actions of each dog.
The surgical question remains especially uncertain. We do not have strong controlled evidence that neutering or spaying prevents the first serious household fight, reduces the severity of later bites, or restores a relationship already damaged by repeated attacks. Nor can studies of post-conflict affiliation settle the matter: Cools and colleagues (2008) observed reconciliation-like interactions in some captive dog groups, while Cafazzo and colleagues (2018) observed avoidance after conflict among the captive dogs they studied. Neither measured the long-term safety of male-male, female-female, or mixed-sex household pairs after serious injury.
These are not minor gaps to be filled with confident guesses. A useful future study would follow households with and without fights, distinguish threats from bites and reciprocal fights from unilateral attacks, track each dog's reproductive and medical history, and record both safe contact and separation. Until then, a careful unknown is more useful than a reassuring but untested rule.
We also lack well-controlled studies that jointly examine breed or ancestry, body size, sex pairing, and reproductive surgery while tracking actual injuries and later compatibility. Those missing comparisons limit every simple ranking of which dogs will fight or benefit from sterilization.
When should owners seek professional assessment?
Skin-breaking bites, repeated attacks, pursuit after a dog retreats, aggression immediately upon seeing a housemate, or a sudden change in an older dog's behavior warrant prompt attention. Prevent additional contact that could cause injury, arrange a veterinary examination when medical or pain-related causes may be involved, and seek qualified behavioral assessment of the relationship. Do not stage another confrontation to find out who starts it, and do not assume that putting the dogs back together after surgery is a safe test.
A useful history includes the date and sequence of incidents, ages and acquisition dates, any surgery and its timing, reproductive cycles when relevant, veterinary findings, injuries to either dog, how encounters ended, and how much safe contact has actually occurred. Such details help distinguish a change in the dogs' behavior from a change in their opportunities to fight.
So which dogs are most likely to fight?
The responsible answer is narrower than the popular advice. Same-sex pairs appear prominently in the clinical literature, female-female fights showed a concerning severity pattern in one study, and younger or later-acquired dogs often appeared as initiators. Reproductive circumstances were documented in some cases, and castration may reduce some forms of intermale aggression. None of this establishes a universal risk percentage, an inevitable age of onset, or a reliable surgical cure.
For someone choosing another dog, sex is one consideration, not a substitute for examining the actual animals and the home they will share. For someone whose dogs already fight, the sensible next step is to reconstruct what the dogs did and protect them from further injury. The question is not merely which kinds of dogs fight. It is what happened between these particular dogs, what changed, and what the evidence does not yet allow us to conclude.
Glossary
Aggression: A term defined differently across studies. Some researchers include threats without contact; this article distinguishes threatening displays from attempted or completed physical attacks.
Reciprocal fight: An episode in which two or more dogs physically engage in violent combat, rather than one dog attacking a recipient that does not fight back.
Unilateral attack: Physical aggression directed by one dog against another without corresponding physical aggression by the recipient during that phase of the incident.
Gonadectomy: Surgical removal of the testes or ovaries, often referred to as neutering or spaying. It alters gonadal hormone production as well as fertility.
Sterilization: Prevention of reproduction. Some procedures prevent reproduction without removing the gonads; the terms are not biologically interchangeable.
Referral sample: Cases presented to a specialty service. The proportion of particular dogs in that sample is not their probability of fighting in the general population.
Poor outcome: A study-defined endpoint. In Feltes et al. (2020), it included permanent separation, rehoming, or behavioral euthanasia; it was not a direct measure of post-conflict reconciliation.
Reconciliation: A measurable post-conflict return to affiliative interaction under a particular study definition. It does not necessarily mean that a pair with serious injuries can safely share a household.
Confounding: When a factor related to both the dogs being compared and the measured outcome could distort an apparent association. Breed, size, management, or referral patterns could complicate a sex comparison.
Breed or ancestry: A dog’s recorded breed category or genetic background. A breed label is not a behavioral diagnosis or a validated predictor of a particular household fight.
Scholarly bibliography
Cafazzo, S., Marshall-Pescini, S., Lazzaroni, M., Virányi, Z., & Range, F. (2018). The effect of domestication on post-conflict management: Wolves reconcile while dogs avoid each other. Royal Society Open Science, 5(7), 171553. https://doi.org/10.1098/rsos.171553
Cools, A. K. A., Van Hout, A. J.-M., & Nelissen, M. H. J. (2008). Canine reconciliation and third-party-initiated postconflict affiliation: Do peacemaking social mechanisms in dogs rival those of higher primates? Ethology, 114(1), 53–63. https://doi.org/10.1111/j.1439-0310.2007.01443.x
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
Feltes, E. S. M., Stull, J. W., Herron, M. E., & Haug, L. I. (2020). Characteristics of intrahousehold interdog aggression and dog and pair factors associated with a poor outcome. Journal of the American Veterinary Medical Association, 256(3), 349–361. https://doi.org/10.2460/javma.256.3.349
Maarschalkerweerd, R. J., Endenburg, N., Kirpensteijn, J., & Knol, B. W. (1997). Influence of orchiectomy on canine behaviour. Veterinary Record, 140(24), 617–619. https://doi.org/10.1136/vr.140.24.617
Morrill, K., et al. (2022). Ancestry-inclusive dog genomics challenges popular breed stereotypes. Science, 376(6592), eabk0639. https://doi.org/10.1126/science.abk0639
Romagnoli, S., et al. (2024). WSAVA guidelines for the control of reproduction in dogs and cats. Journal of Small Animal Practice, 65(7), 424–559. https://doi.org/10.1111/jsap.13724
Sherman, C. K., Reisner, I. R., Taliaferro, L. A., & Houpt, K. A. (1996). Characteristics, treatment, and outcome of 99 cases of aggression between dogs. Applied Animal Behaviour Science, 47(1–2), 91–108. https://doi.org/10.1016/0168-1591(95)01013-0
Wrubel, K. M., Moon-Fanelli, A. A., Maranda, L. S., & Dodman, N. H. (2011). Interdog household aggression: 38 cases (2006–2007). Journal of the American Veterinary Medical Association, 238(6), 731–740. https://doi.org/10.2460/javma.238.6.731
