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Most veterinarians and dog breeders agree that puppies shouldn’t be separated from the rest of their litter for adoption before 2 months of age. A new study corroborates that advice, showing that puppies removed from their broods earlier were more likely to develop behavioral problems as adult dogs.

It’s an important issue, the authors note, because pets’ behavioral problems affect their relationships with owners and the risk of later abandonment.

Veterinary researchers in Naples, Italy, interviewed the owners of 140 dogs, ages 18 months to 7 years. Half of the dogs had been removed from their litters and adopted between 30 and 40 days, while the other half were not adopted until they were 60 days old.

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The owners, who were all clients of veterinary practices around Naples, participated in telephone surveys that asked about their pets’ provenance, breed and observed problematic behaviors.

About 50% of all dogs included in the study were purchased at pet shops, while 33% came from friends or relatives and 16% were acquired from breeders. None were adopted from shelters, nor had any been traumatized as puppies.

Overall, the researchers found, attention seeking and reactivity to noises were the most commonly reported problem behaviors. Younger dogs (under 3 years old) were also significantly more likely to be destructive and to tail-chase than older animals.

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But the dogs that had been separated from their litters early — regardless of breed, size or whether they had been neutered — were significantly more likely to exhibit most of the problem behaviors that the researchers asked about. These included:

Destructiveness

Excessive barking

Fearfulness of walks

Reactivity to noises

Possessiveness of food and toys

Attention seeking

Aversion or aggression toward strangers

Play biting

Tail chasing

Soiling the house

The researchers did not determine exactly why early removal from a litter may cause dogs to act out as adults, but they surmised that some dogs may have a genetic predisposition to certain conditions, including fear, anxiety and phobia of noises, and that early environmental experiences may increase the likelihood that they will develop these conditions or go on to have disordered behavior.

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“It is generally accepted that dogs go through a sensitive period, the socialization period, during which social experiences and stimuli have a greater effect on the development of their temperament and behavior than if they occur in later life,” wrote the authors.

The study was published in the journal Veterinary Record.

Meredith Melnick is a reporter at TIME. Find her on Twitter @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter @TIME.