If you work in rescue or looked into getting a puppy, you’ve probably heard: “Never adopt littermates—they’ll develop littermate syndrome.”
The reality is more complicated but the risk if real.
What Is Littermate Syndrome?
“Littermate syndrome” is a popular term used to describe behavioral problems that can sometimes occur when two puppies are raised together.
Commonly reported concerns include:
- Excessive dependence on each other
- Distress when separated
- Difficulty focusing on people
- Fear or anxiety when alone
- Difficulty developing independence
- Competition over resources
- Conflict or aggression between the dogs
However, littermate syndrome is not a recognized veterinary diagnosis, and there is currently no scientific evidence establishing it as a distinct behavioral disorder.
There is also no reliable percentage of littermate pairs that develop these problems.
So, Is It Real?
The behaviors are real. The “syndrome” is not scientifically established.
Two puppies that spend nearly all their time together may have fewer opportunities to learn how to function independently. They may look to each other for information about unfamiliar situations, become highly dependent on each other, or simply have difficulty learning to focus on people when together.
These issues do not only apply to littermates. It can also occur between unrelated puppies raised together, so being littermates is not necessarily the cause.
What to Watch For?
Don’t worry about two puppies being close. Playing, sleeping together, following each other, and seeking each other out are normal.
Instead, look for signs that one dog cannot function without the other.
Ask:
- Can each dog walk independently?
- Can each dog eat and take treats independently?
- Can each dog train with a person?
- Can each dog explore a new environment without the other?
- Can each dog settle independently?
- Can each dog interact comfortably with people when alone?
- Can they tolerate short, appropriate periods apart?
Be especially observant if a dog:
- Becomes extremely distressed when separated
- Constantly monitors or follows the other dog
- Cannot focus on people when together
- Becomes significantly fearful when alone
- Shows escalating conflict with the other dog
Why Is It Recommended Not to Adopt Two Puppies at the Same Time?
The recommendation to avoid adopting two puppies at the same time is not because littermates are destined to develop “littermate syndrome.” The bigger concern is that raising two puppies simultaneously is significantly more demanding and can make it harder to give each puppy the individual experiences it needs during an important period of development.
Puppies Need Individual Experiences
Puppies are learning constantly. They need opportunities to experience the world independently and build relationships with their humans.
When two puppies do everything together, it can be easy for them to:
- Rely heavily on each other for confidence
- Look to each other when encountering something unfamiliar
- Become distracted by each other during training
- Miss opportunities to develop independence
- Receive less individual socialization and enrichment
This doesn’t mean that puppies should never interact. It means they also need opportunities to learn “I can do this without my sibling.”
Two Puppies Are More Than Twice the Work
Two puppies don’t simply require twice as much food and veterinary care. Their training and developmental needs can multiply the workload.
Ideally, each puppy should receive:
- Individual training
- Individual walks
- Individual socialization
- Individual enrichment
- Individual handling and cooperative-care practice
- Individual time with people
- Individual opportunities to rest
- Gradual practice being separated
Trying to provide all of this for two puppies simultaneously can be challenging, even for experienced owners (or trainers).
Training One Puppy Is Easier Than Training Two
Two puppies can easily distract one another. One puppy gets up → the other follows. One puppy barks → the other joins. One puppy finds something interesting → the other wants to investigate.
Training can become much more difficult when the puppies are constantly reinforcing each other’s excitement. Teaching each puppy individually first allows the owner to build a strong foundation before expecting them to work successfully together.
Socialization Can Become “Pair Socialization”
This is an important distinction. Two puppies may appear confident because they have each other. But confidence when accompanied by another dog does not necessarily mean confidence when alone.
Each puppy should experience new people, environments, surfaces, sounds, handling, and other appropriate experiences as an individual.
Independence Takes Practice
Independence is a skill. Dogs don’t automatically know how to be comfortable alone simply because they live with another dog.
Teaching puppies that they can safely spend time apart can help prevent the other dog from becoming their only source of security.
The Goal Isn’t to Prevent a Bond
There is nothing wrong with two dogs having a close relationship. In fact, a strong relationship between household dogs can be a wonderful thing.
The goal is to create: Two confident individual dogs who also enjoy each other’s company.
Rather than: Two dogs who need each other to function.
Why Waiting Can Be Easier
For many families, adding one puppy and allowing that dog to mature before adding another makes the process much easier.
The first dog can develop:
- Basic training skills
- Household routines
- Appropriate social skills
- Comfort with being alone
- A relationship with the family
- The ability to settle
- Good communication with its humans
Then, when another dog is introduced, the owner is working with an established individual rather than trying to raise two developing puppies at the same time.
The Bottom Line
Littermate syndrome is not a scientifically established diagnosis. Not every pair of littermates develops behavioral problems, we don’t know exactly why it occurs, and we don’t currently know how common these problems are.
It is possible to raised two puppies together successfully.
The recommendation to adopt one puppy at a time is primarily about developmental needs, time, management, and the ability to provide sufficient individual experiences—not because every pair of puppies will develop littermate syndrome.
For most families, raising one puppy well is easier than raising two puppies well. And when it comes to puppies, quality of individual experiences matters more than simply having another puppy around.