Littermate Syndrome

The scientific literature on littermate syndrome is nonexistent. However, the anecdotal evidence observed by veterinarians, shelters, and behavior specialists is overwhelming. Littermate syndrome refers to undesirable behaviors, such as fear, aggression, and anxiety, in puppies placed together in new homes as they mature. Littermate syndrome is not a "real" syndrome per se, but a phenomenon that can happen when puppies from the same litter or different litters but similar in age grow up in the same household (Fratt, 2019). Littermate syndrome can include numerous issues, such as: 

• Codependence: separation distress when puppies are separated
• Fear of new people, dogs, objects, and experiences
• Reactivity
• Fear aggression towards new people and dogs
• Inter-dog aggression 
• Difficulty training individual dogs
• Challenges bonding with individual dogs

Often, there is debate over littermate syndrome from those who have raised numerous pairs of littermates or puppies of similar age without issue. It is, in fact, possible that owners may not experience the problems that arise from raising littermates. However, viewing these pairs as the exception rather than the rule is essential. So why might littermate syndrome happen in some pairs and not others? Several factors can influence whether puppies of the same age experience littermate syndrome; this can include: 

• Genetics: A dog's predisposition for certain behavioral traits. 
• Socialization: Not providing adequate socialization for each puppy can increase fear, reactivity, and anxiety as the puppies mature.
• Training: Double the puppies, double the training. Puppies should have one-on-one training to prepare them for the human world. Without enough individual training, challenges can arise through adolescence and adulthood.
• Individual needs: Dogs are individuals, and each puppy will have specific needs (training, enrichment, genetic, and social fulfillment) that owners must address. If an owner cannot meet those needs, puppies may find a way to meet them instead, which may be problematic for owners.
• Time: Raising puppies requires much time and energy to ensure a good quality of life. Getting a second puppy should not be viewed as "helpful" or "easier"; puppies do not raise themselves. Not having enough time for each puppy can increase the risk of behavior issues in adulthood. 

Most importantly, when it comes to behavior, there are no guarantees. Therefore, you can do everything right and raise two well-rounded individual dogs, and issues can still arise. The best way to avoid littermate syndrome is to have one puppy at a time. 

Why should breeders avoid placing two puppies? 

Dog breeders are essential for setting puppies up for success in their new homes and, therefore, should carefully consider if they should place more than one puppy in the same household. The potential risks for breeders placing multiple dogs include:

• Dogs placed in the shelter
• Dogs being returned to the breeder at an older age
• Difficulty rehoming an older dog with potential behavior issues
• Responsibility for behavioral euthanasia
• Negative impact on professional reputation

Pet guardians must recognize the risks of raising littermates or puppies of similar age in the same home. For dog breeders, if a potential owner would like to purchase more than one puppy, it is essential to do a thorough risk assessment of the home and provide additional education and resources.

References:

Fratt, K. 2019. There's No Scientific Reason to Believe Littermate Syndrome Exists. IAABC Foundation Journal (11).

Texas A&M University School of Veterinary Medicine and Biomedical Sciences. 2023. Understanding Littermate Syndrome In Puppy Pairs.