Two puppies at once: littermate syndrome and what the evidence shows

Many people are told never to take two puppies from the same litter, because the pair will develop “littermate syndrome”: bonded to each other instead of to the family, anxious when apart, hard to train and likely to fight as adults. The worry behind the warning is reasonable. The label is not: we found no published research showing that such a syndrome exists, or that littermates are worse off than two unrelated puppies of the same age.

What “littermate syndrome” claims, and what the evidence says

The term is used for a cluster of problems: two puppies raised together that cling to each other, panic when separated, ignore their owners, react fearfully to new things, or start fighting as they mature. It appears in training books, websites and breeder advice, but not in the scientific literature.

Kayla Fratt, writing in the journal of the International Association of Animal Behavior Consultants (IAABC) Foundation, searched for research and found none that compared sibling pairs with non-sibling pairs. She concludes that it is unproven either way, and warns that calling it a “syndrome” makes the problems sound inevitable and can lead to dogs being rehomed before simpler explanations — too little individual socialization and training, poor management, unmet needs — have been addressed. An article by the Board of Canine Behaviorists likewise concludes that current evidence does not support littermate syndrome as a formally recognized behavioral disorder, but describes the behaviors as real risks when two young dogs mature together without enough individual training and independence. It suggests that the same pattern can appear in two unrelated puppies of a similar age.

In short: being littermates is not shown to be the problem. Raising two puppies of the same age at the same time is harder, and the extra work is often underestimated.

The real risks of raising two puppies together

Less individual socialization and training. The early weeks and months are when puppies most readily adapt to new people, animals and places. The American Veterinary Society of Animal Behavior (AVSAB) describes the first three months of life as the primary socialization period, and says that incomplete socialization in that window can increase the risk of later fear, avoidance or aggression. With two puppies, it is easy to take them everywhere together, so that neither learns to cope alone. Our article on puppy socialization covers what breeders do in the first weeks.

Dependence on each other. Puppies that are always together may find it hard to be apart, at the vet, on walks or when one is ill. Practicing short separations from the start helps.

Fighting as they mature. Wrubel and colleagues analyzed 38 pairs of dogs seen at a veterinary teaching hospital for aggression between dogs in the same household. Most pairs, 30 of the 38 (79%), were of the same sex, and 26 (68%) involved at least one female. Where the instigator could be identified, it was most often the younger dog (20 of 27) or the newer arrival (19 of 27). Fights were triggered by things such as owner attention, food, excitement and found items. The study did not look at littermates specifically, and some of the pairs were of different ages, but it shows that same-sex pairs are common among dogs that fight at home. In a larger review of 1,644 dogs seen for behavior problems, Bamberger and Houpt found that males were overrepresented for most diagnoses but not for aggression between dogs.

Competition in the litter. Rausch and colleagues surveyed 293 breeders in Canada and the United States about how they managed puppies’ access to food, milk and toys. Competitive behavior around toys was more often reported when littermates played together and when there was competition around food. The study cannot show cause and effect, but it suggests that how resources are managed in the litter is worth thinking about.

If you are raising two puppies

• Give each puppy its own time every day: training, play and walks with one puppy at a time, alongside time together.
• Socialize them separately as well as together, so that each learns to meet new people, dogs and places on its own. See early learning and first training and teaching a puppy to walk on a loose leash.
• Practice being apart from the start: separate beds or crates, and short periods in different rooms.
• Feed them apart and give each its own toys and chews, so they do not have to compete.
• Watch how they play. Our article on reading your dog’s body language explains the signals to look for, and when to step in.
• Ask for help early if play turns into fighting, or one puppy seems afraid of the other.

For breeders: selling two puppies to the same home

Breeders differ on whether to sell two puppies to the same home. There is no evidence for a blanket rule either way, so it is a judgment about this home and these puppies. Points worth discussing honestly with the buyer:

• Time. Does the household have time for individual training, walks and socialization for two puppies, every day, for many months?
• Sex. In the case series above, most pairs of dogs that fought at home were of the same sex. A buyer set on two females, or two males, should know this.
• Temperament. Two confident, pushy puppies, or one that is much more timid than the other, may be a harder pair. Your knowledge of the litter helps here; see evaluating puppies for placement.
• Alternatives. Would one puppy now, and a second later, suit the family better?
• A plan. If you agree, give the buyer written advice on separate time, feeding apart and what to do if problems start, and make clear that you want to hear from them early. Our article on preparing for handover covers what to send home with each puppy.

When to call your veterinarian

• Play between your dogs turns into fights, or either dog is injured.
• One dog seems afraid of the other, stops eating, or hides.
• Either dog shows a sudden change in behavior: pain or illness can make a dog irritable. Your vet can refer you to a veterinary behaviorist.
• If in doubt, call — your vet would rather hear from you early.

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Sources & further reading

1. Fratt K. There’s no scientific reason to believe littermate syndrome exists. IAABC Foundation Journal, Issue 11, 2019. journal.iaabcfoundation.org
2. Sullivan S, Board of Canine Behaviorists. Littermate syndrome: fact, fiction, and what the research actually says. caninebehaviorists.org
3. Wrubel KM, Moon-Fanelli AA, Maranda LS, Dodman NH. Interdog household aggression: 38 cases (2006–2007). Journal of the American Veterinary Medical Association 2011. doi.org
4. Bamberger M, Houpt KA. Signalment factors, comorbidity, and trends in behavior diagnoses in dogs: 1,644 cases (1991–2001). Journal of the American Veterinary Medical Association 2006. doi.org
5. Rausch Q, White S, Widowski TM, Coe J, Jacobs J, Niel L. Resource management practices of Canadian and American domestic dog breeders and associations with puppy competitive behaviour: a cross-sectional study. Animal Welfare 2026. doi.org
6. American Veterinary Society of Animal Behavior — Position statement on puppy socialization (2008). avsab.org

Where evidence is limited, recommendations rest on established practice and expert judgment, and may be updated as knowledge develops. This is general information, not veterinary advice: talk to your veterinarian about your own dog.

Next steps

• Read next: Puppy socialization.
• Related: Reading your dog’s body language · Evaluating puppies for placement.