By Amy Tokic
You might’ve heard of “littermate syndrome” — it’s the idea that raising two puppies from the same litter together can cause problems down the line. The concerns usually centre on the two dogs becoming so attached to each other that they struggle to bond with people or other animals, along with issues like aggression, separation anxiety, and trouble focusing during training.
It’s a popular topic among dog trainers, shelter workers, breeders, and other folks in the animal world — and interestingly, it doesn’t seem to matter whether the puppies are actual littermates. Any two puppies (or dogs) raised together at the same time seem to show this same pattern.
That said, here’s the catch: veterinary behaviourists and other leading experts don’t actually recognize this as a real “syndrome.” What’s more likely going on is that these behaviours just reflect how genuinely challenging it is to raise two puppies at once, rather than pointing to some special condition tied to being littermates.
How to tell if you’re dealing with Littermate Syndrome
When two puppies grow up together from an early age, they tend to form an intense bond — they essentially learn to read and mirror each other’s emotions. But that closeness can come at a cost, often showing up as behavioural issues in one or both dogs, like:
- Fear around people and other dogs
- Severe separation anxiety
- Trouble with crate training
- Panic in new situations when left alone
- Reactivity on the leash
- More frequent fighting between the two dogs than you’d typically see in non-sibling pairs raised together
The tricky part is that these problems usually don’t show up right away: they tend to ramp up once both dogs hit mental maturity. It’s also worth noting that this pattern can bring out aggression in breeds you wouldn’t normally expect it from. Labradors, for instance, are rarely aggressive — but raise two Labrador littermates together, and you’ll often see a real spike in aggression and fear-based behaviour.
The good news is that littermate syndrome can be managed, but it takes serious effort. If you’re bringing home two puppies, know that you’re not just doubling your work, you’re tripling it: you’ll need to train each dog individually and then work with them together. And here’s the key thing to understand — this isn’t just a case of a dog “misbehaving.” Littermate syndrome actually interferes with a dog’s normal developmental process, which is why it takes so much work to undo once it’s set in. At that point, you’re not just correcting bad behaviour — you’re trying to socialize a dog after their natural socialization window has already closed. It’s a real challenge.
The bottom line
The best approach is just to avoid the problem altogether. There’s nothing wrong with having two dogs close in age — it can actually be great! Just space them out by about six months, and you’ll make life so much easier, both for you and for them.
