Introduction
“Littermate syndrome” is a term commonly used to describe behavioral problems that may develop when two puppies of the same age—particularly littermates—are raised together. The term is frequently used by dog trainers, breeders, shelters, rescues, and pet owners to describe puppies that appear excessively dependent on one another, struggle when separated, have difficulty developing independence, or eventually develop conflict with each other.
However, there is an important distinction between the behaviors being real and the syndrome being scientifically established.
At this time, there is no strong peer-reviewed evidence demonstrating that “littermate syndrome” is a distinct behavioral disorder caused specifically by raising littermates together. Veterinary behavior specialists do not recognize it as a formal diagnosis. The scientific literature specifically addressing littermate syndrome is extremely limited; Penn State Extension notes that scientific literature on the syndrome itself is nonexistent.
That does not mean that raising two puppies together is risk-free.
There are legitimate developmental and behavioral concerns associated with raising two puppies at the same time. Puppies are going through an important period of social, emotional, and behavioral development, and the way they are raised can influence how they respond to people, other animals, novel experiences, and separation.
A more scientifically cautious way to discuss “littermate syndrome” is therefore:
A collection of behavioral problems that may occur when two young dogs are raised together with insufficient opportunities to develop as individuals.
What Is Littermate Syndrome?
The popular definition of littermate syndrome is that two puppies become excessively dependent upon each other and fail to develop appropriate independence from their sibling.
The term originally refers to puppies from the same litter, but the behaviors described under the label can also occur between unrelated puppies of similar ages that are raised together. Veterinary behaviorist Christine Calder, DVM, DACVB, describes the phenomenon as “littermate dependency” and notes that similar patterns can occur in pairs of puppies regardless of whether they are biologically related.
This is an important distinction.
If two unrelated puppies are brought home at eight weeks of age and develop the same behavioral problems attributed to “littermate syndrome,” genetics and litter relationship cannot be the sole explanation.
Likewise, many littermates are raised together without developing these problems.
This suggests that the developmental environment, individual temperament, learning history, socialization, management, and opportunities for independence are likely more important than the fact that two dogs share a litter.
Is Littermate Syndrome Actually Real?
The short answer is that the behavior is real but the syndrome is not established.
There is currently no scientific evidence establishing littermate syndrome as a distinct behavioral disorder.
A 2019 review of the topic in the IAABC Foundation Journal found no relevant scientific literature specifically demonstrating the existence of littermate syndrome. The author instead examined related areas of canine behavioral science that might explain the behaviors attributed to the syndrome.
Veterinary Partner, written by veterinary behaviorist Christine Calder, similarly states that littermate syndrome is not recognized by veterinary behaviorists or other leading experts as a formal syndrome.
This distinction matters because the term can make it sound as though there is a specific disorder that automatically develops when two littermates are raised together. There isn’t evidence for that.
Instead, there are several separate behavioral phenomena that can occur in puppies raised together:
- attachment to another dog
- separation-related distress
- fear or anxiety
- poor independence
- inadequate socialization
- difficulty focusing on a human while the other dog is present
- competition over resources
- inter-dog conflict
- aggression
- reduced confidence when alone
- difficulty coping with novel situations
These behaviors have different possible causes and should not automatically be assumed to have one underlying cause.
Why Can Raising Two Puppies Together Be Challenging?
Puppies are developing rapidly during their first months of life.
They are learning:
- what is safe
- how to interact with people
- how to interact with other dogs
- how to respond to unfamiliar environments
- how to recover from frightening experiences
- how to cope with frustration
- how to settle
- how to spend time alone
- how to communicate
- how to make behavioral choices
Research on puppy development demonstrates that social behavior begins developing very early. Observational research of four litters found social investigation and play emerging during the first few weeks of life, with individual differences in social and agonistic behavior becoming apparent before eight weeks of age.
This makes the developmental environment important.
When two puppies are together constantly, they have a readily available social partner. That can be wonderful—but it can also mean that the puppies have fewer opportunities to practice functioning independently.
Instead of: “Something unfamiliar happened → I look to my person → my person helps me determine what to do.” A puppy may learn: “Something unfamiliar happened → I look to my sibling.”
If both puppies respond fearfully, excitedly, or aggressively, they can potentially influence one another’s behavior.
Texas A&M describes this phenomenon as puppies looking to each other for information about unfamiliar stimuli rather than looking toward their human caregivers. However, this should be viewed as a possible learning and social-development process, not proof of a specific syndrome.
Signs Commonly Attributed to Littermate Syndrome
There is no validated diagnostic checklist for littermate syndrome.
Nevertheless, several behavioral patterns are repeatedly described by veterinary professionals, trainers, shelters, and owners.
1. Extreme distress when separated
One of the most frequently reported signs is intense distress when the puppies are separated.
A puppy may:
- vocalize
- pace
- scratch at doors or crates
- attempt to escape
- stop eating
- become highly aroused
- repeatedly search for the other dog
- have difficulty settling
- show substantially different behavior when the sibling is absent
Some separation distress is normal in young puppies. The concern is the intensity of the response and whether the puppy is capable of gradually learning to cope with separation.
Importantly, this should not automatically be called “separation anxiety.” Separation-related behavior has multiple potential causes and should be assessed independently.
2. Difficulty functioning independently
A puppy may appear confident when the other puppy is present but become fearful, hesitant, or unable to function normally when alone.
For example:
Together:
- confident
- exploratory
- playful
- willing to approach people
- comfortable in new environments
Apart:
- hiding
- freezing
- vocalizing
- refusing to explore
- avoiding people
- showing significant distress
This difference is particularly important because it suggests that the presence of the other dog is functioning as a source of security.
3. Excessive attention toward the other puppy
The puppies may appear to have difficulty disengaging from each other.
Examples include:
- constantly monitoring the sibling
- following each other everywhere
- becoming distracted during training
- repeatedly initiating play
- difficulty settling independently
- ignoring the handler when together
- choosing the sibling over available human interaction
Simply preferring another dog is not a problem. Dogs are social animals, and playing, sleeping together, and seeking each other’s company are normal behaviors.
The concern is when the relationship interferes with the dog’s ability to interact with the rest of the world.
4. Difficulty training independently
Handlers may notice that each puppy performs reasonably well when alone but struggles when the pair is together.
Potential signs include:
- inability to focus on the handler
- responding to the other dog rather than the handler
- increased arousal
- competing for reinforcement
- difficulty responding to known cues
- increased play or chasing
- inability to settle
This is one reason individual training is particularly important when raising two puppies.
5. Fear or anxiety in unfamiliar situations
Some dogs may appear to use their sibling as a social reference.
When separated, they may become more cautious around:
- unfamiliar people
- unfamiliar dogs
- new environments
- novel objects
- noises
- vehicles
- veterinary clinics
- unfamiliar surfaces
The presence of another confident dog can sometimes provide social support. Conversely, if one puppy becomes fearful, the other may potentially respond to that behavior and become fearful as well.
This is one reason that individual socialization experiences are important.
6. Leash reactivity
Leash reactivity is another behavior sometimes attributed to littermate syndrome.
A puppy may:
- bark at dogs
- lunge
- pull
- vocalize
- become highly aroused
- respond differently when separated from the sibling
However, leash reactivity has many possible causes, including frustration, fear, excitement, previous learning, genetics, inadequate socialization, and reinforcement history.
7. Conflict between the puppies
Perhaps the most concerning reported outcome is aggression between the dogs. Two puppies may appear inseparable when young and later begin experiencing conflict as they mature.
Potential triggers include:
- food
- toys
- resting locations
- human attention
- access to doors
- high-arousal play
- greetings
- confinement
- proximity to other dogs
- competition over preferred resources
Conflict can become more apparent as puppies mature socially and sexually. This is sometimes described as “sibling rivalry,” although that term can also oversimplify a complicated behavioral problem.
Importantly, two dogs fighting does not prove littermate syndrome.
Household dog aggression can occur between unrelated dogs, adult dogs introduced years apart, parent and offspring, and dogs that were previously highly compatible.
What About the Idea That the Dogs “Bond More with Each Other than their Humans”?
This is one of the most common explanations of littermate syndrome.
There is some intuitive logic behind it: if two puppies spend nearly all of their waking hours together, they have enormous opportunities to interact with each other.
But the scientific evidence does not establish that puppies raised together necessarily form an unhealthy attachment to one another or that this automatically prevents attachment to humans.
Dogs are capable of maintaining multiple social relationships.
The more defensible concern is that if the puppies receive insufficient individual interaction with humans, insufficient individual training, and insufficient individual exposure to the world, they may have fewer opportunities to develop those skills.
That is a substantially different claim from: “Littermates bond to each other instead of bonding with people.”
The former is a reasonable developmental concern. The latter has not been demonstrated as a universal phenomenon.
How Common Is Littermate Syndrome?
We don’t know.
This is one of the most important facts to communicate to adopters and rescue volunteers.
There is no reliable prevalence estimate for littermate syndrome.
There is no large epidemiological study establishing that:
- X% of littermates develop it
- X% develop aggression
- X% develop separation problems
- Littermates are X times more likely to develop behavioral problems than single puppies
The reason is straightforward: researchers cannot calculate the prevalence of a condition that has not been clearly defined or consistently diagnosed.
Penn State Extension specifically notes that the scientific literature on littermate syndrome itself is nonexistent.
There is currently no good evidence supporting a specific level of risk.
Does That Mean There Is No Increased Risk?
Not necessarily. This is where the discussion becomes more nuanced.
The absence of evidence for a distinct syndrome does not mean that raising two puppies simultaneously carries no additional behavioral challenges.
Two puppies require substantially more time and management than one.
If the household cannot provide each puppy with sufficient:
- individual socialization
- individual training
- individual enrichment
- individual rest
- individual handling
- individual experiences
- appropriate separation
- human interaction
then developmental problems become more plausible.
The problem may therefore be less: “These puppies are littermates.” and more: “These puppies are experiencing the world primarily as a pair rather than as two individuals.”
That distinction is important for rescue organizations because it changes both how we assess risk and how we intervene.
What Does the Science Tell Us About Early Development?
Although research specifically on littermate syndrome is lacking, there is considerable evidence that early developmental experiences influence later behavior.
For example, a study comparing adult dogs separated from their litters at approximately 30–40 days with dogs separated at approximately two months found greater odds of several undesirable behaviors in the dogs separated earlier, including destructiveness, excessive barking, fearfulness on walks, noise reactivity, possessiveness, and attention-seeking.
This research does not demonstrate littermate syndrome. Instead, it demonstrates something broader and better established: Early social experiences matter.
The quality and quantity of those experiences can influence later behavior.
Littermate Syndrome Is Not the Only Possible Explanation
This is particularly important when evaluating shelter or rescue dogs.
A dog showing these behaviors may instead be experiencing one or multiple of these:
- separation-related distress
- generalized anxiety
- fear
- inadequate socialization
- frustration
- environmental stress
- resource guarding
- arousal-related behavior
- inter-dog aggression
- pain or medical problems
- genetic predisposition
- previous traumatic experiences
- insufficient reinforcement history
- poor management
- lack of appropriate enrichment
The behavior should be assessed first. The label comes second.
Why Do Problems Sometimes Appear Later?
One reason the littermate syndrome narrative is so compelling is that some dogs appear completely fine as puppies and develop significant conflict later.
This can happen for several reasons that do not require a unique syndrome.
As dogs mature:
- their social behavior changes
- their environment changes
- their tolerance for particular interactions may change
- competition may become more important
- reproductive hormones may influence behavior
- learned behaviors become more established
- individual personalities become more apparent
Research on puppy social development shows that individual differences in social and agonistic behavior can emerge at varying stages of development.
Therefore, a pair that plays beautifully as eight-week-old puppies does not necessarily tell us what their relationship will look like as adult dogs. This is true for littermates and for unrelated dogs.
How would you Assess for Littermate Syndrome?
Rather than asking: “Do these dogs have littermate syndrome?” a better assessment is: Can each dog function independently?
Look at the dogs individually.
Can each dog:
- eat without the other dog present?
- rest without the other dog?
- walk with a handler independently?
- accept reinforcement from a person?
- respond to known cues independently?
- explore a novel environment independently?
- interact with unfamiliar people independently?
- tolerate brief, appropriate separation?
- recover from mild stress without the other dog?
- settle without the other dog?
Also observe what happens when they are together.
Do they:
- constantly monitor one another?
- become significantly more aroused together?
- become significantly more fearful together?
- reinforce each other’s barking or lunging?
- compete over resources?
- interrupt one another’s interactions with people?
- struggle to disengage?
- show escalating conflict?
These observations provide much more useful information than simply labeling the dogs.
What Should We Do If We Already Have Two Puppies?
Having two puppies does not mean the situation is doomed. It means the humans need to be intentional. The goal should be to raise two individual dogs who also happen to live together.
That means deliberately providing:
- Individual training
- Each puppy should learn skills independently.
- Individual walks
- Each puppy should regularly experience the world without the other dog.
- Individual socialization
- Each puppy should have opportunities to encounter people, environments, surfaces, sounds, dogs, and other experiences independently.
- Individual enrichment
- Each puppy should have opportunities to solve problems and engage with enrichment without relying on the sibling.
- Individual rest
- Each puppy should learn that relaxation does not require physical contact with the other dog.
- Individual human relationships
- Each puppy should have meaningful one-on-one interaction with members of the household.
- Appropriate separation
- Separation should be taught gradually and positively rather than suddenly forcing prolonged isolation.
The objective is not to prevent the puppies from enjoying each other. The objective is to make sure that the other puppy is not required for the dog to feel safe, confident, or functional.
Should Littermates Be Separated?
This is one of the most controversial questions surrounding the topic. There is no scientific basis for saying that every pair of littermates must be separated. There are many dogs that are raised successfully with siblings or same-age companions.
At the same time, most professionals (most I know) recommend avoiding acquiring two puppies simultaneously because of the additional developmental, physical, emotional, time, money, and management demands.
If serious aggression or extreme separation distress has already developed, however, the question becomes much more complicated. Those cases should be evaluated individually, ideally with a qualified behavior professional or veterinary behaviorist. Rehoming one dog may sometimes be appropriate, but it should not be presented as an automatic treatment for “littermate syndrome.”
What We Can Confidently Say
Based on the current evidence, several conclusions are reasonable.
- Littermate syndrome is not a recognized veterinary diagnosis.
- There is insufficient scientific evidence to establish it as a distinct behavioral disorder.
- The behaviors attributed to littermate syndrome are real.
- Dogs can develop separation-related distress, fear, poor independence, training difficulties, and aggression.
- Those behaviors have many possible causes.
- Being raised with a littermate is only one potential factor.
- The problem is not necessarily limited to littermates.
- Similar issues can occur when unrelated puppies of similar ages are raised together.
- Not every pair develops problems.
- Many dogs successfully grow up with siblings or same-age companions.
- There is no reliable prevalence number.
- We cannot currently say what percentage of littermate pairs develop “littermate syndrome.”
- Individual development matters.
- Providing each puppy with independent experiences, socialization, training, enrichment, and relationships can help reduce the risk of excessive dependency.
- Behavior should be evaluated rather than labeled.
- A dog displaying fear, aggression, or separation distress needs an assessment of the actual behavior and its contributing factors.
The Bottom Line
“Littermate syndrome” is a useful term for describing a collection of problems that some people observe when two puppies are raised together, but it should not be treated as a scientifically established diagnosis.
The strongest evidence does not support the idea that being littermates automatically causes behavioral problems.
Instead, the more defensible concern is that raising two puppies simultaneously can create an environment in which the puppies have fewer opportunities to develop as individuals. If they become heavily dependent on each other, they may struggle with independence, separation, socialization, training, or—sometimes—conflict with one another.
For rescue organizations and adopters, this distinction is important.
Rather than telling people:
- “Never adopt littermates because they will develop littermate syndrome.”
A more accurate message is:
- “Two puppies can be successfully raised together, but they need to be raised as individuals. The goal is not to prevent them from bonding with each other. The goal is to make sure each dog can confidently function without the other.”
That approach acknowledges the behavioral concerns without presenting an unproven syndrome as scientific fact.