Dogs bred on intensive puppy farms grow up to be more aggressive, fearful and anxious than pets from reputable breeders, a scientific study has found.
Animal behaviour experts have conducted the first UK study of how the first few weeks of a dog's life in a puppy farm can have a serious effect on its temperament as an adult.
The Newcastle University team assessed pugs, Chihuahuas and Jack Russells from reputable breeders and from puppy farms.
The puppies' backgrounds were assessed by asking the owners questions such as if they saw the mother, if health documents were available and the age at which they got the puppy.
The owners were then asked to assess their adult dog using a test called the Canine Behaviour and Research Questionnaire. This included whether their dog was aggressive; if it was afraid of new things or loud noises and whether it suffered separation-related problems. The test also assessed obedience.
Dr Catherine Douglas, a lecturer in animal science and research supervisor said puppy farms were usually businesses that produced a lot of dogs for sale, but the study also considered smaller breeders with poor welfare standards.
She said: "There has been some research around the health problems associated with dogs from puppy farms but very little research into long-term effects on adult dog behaviour.
"We found that across all behaviour categories, including trainability, dogs from less responsible breeders had significantly less favourable behaviour and temperament scores than puppies from responsible breeders."
That was likely to be down to the poor environment both before and after birth.
Dr Douglas said: "Previous studies have shown that mothers who are stressed or poorly fed during pregnancy had more anxious and less trainable offspring.
"Early separation from the mother has been found to be detrimental and the direct effect on the puppies of their early experiences in these less-than-ideal environments is also likely to be a factor."
The research supported the RSPCA advice "Don't buy a puppy because you feel sorry for it - another will be bred to replace it".
The research will be presented at Universities Federation for Animal Welfare's Recent Advances in Animal Welfare Science in York next month.