Dogs that are extra friendly share certain genetic similarities with people who are born with a developmental disorder sometimes called the "opposite of autism", which makes them hyper-social, researchers said.
The report in the journal Science Advances pinpointed changes in two genes that are related to extreme social behaviour in dogs, and also in people who are born with Williams-Beuren Syndrome.
People with this condition tend to be highly outgoing, gregarious, empathetic, interested in prolonged eye contact, prone to anxiety and may have mild to moderate learning disabilities and intellectual impairment.
The findings offer new insights into how dogs became domesticated and split paths from their wolf ancestors thousands of years ago.
"It was once thought that during domestication, dogs had evolved an advanced form of social cognition that wolves lacked," said co-author Monique Udell, an animal scientist at Oregon State University.
"This new evidence would suggest that dogs instead have a genetic condition that can lead to an exaggerated motivation to seek social contact compared with wolves."
SURVIVAL OF THE FRIENDLIEST
Researchers studied 18 domesticated dogs and 10 captive grey wolves to see how sociable they were towards people and how they performed in problem-solving tasks.
Given the task of lifting a puzzle box lid to get a sausage treat, the dogs were more likely to stare longingly at the people nearby. The wolves were more likely to figure out how to get the treat themselves.
Researchers then took blood samples to see how the wolves' and dogs' genetic traits lined up with their personalities.
They found variations in two genes - GTF2I and GTF2IRD1 - that "appeared to be connected to dog hyper-sociability, a core element of domestication that distinguishes them from wolves", said the report.
In dogs, unique genetic insertions called transposons in these genetic regions were linked to a strong tendency to seek out human contact. Some of these transposons "were only found in domestic dogs, and not in wolves at all".
In people, the deletion of genes from this region in the human genome is linked to Williams-Beuren syndrome.
"We haven't found a 'social gene', but rather an important (genetic) component that shapes animal personality and assisted the process of domesticating a wild wolf into a tame dog," said a statement by co-author Bridgett vonHoldt, an assistant professor in ecology and evolutionary biology at Princeton University.
HOW WOLVES BECAME DOGS
Dr Adam Boyko, an assistant professor at Cornell University and expert in dog genetics, said in an e-mail: "It may be one of the first studies to ever identify the specific genetic variants that were important for turning wolves into dogs.
"That said, the overall sample size in the study is small, so validating the association of these variants in a much larger cohort of diverse dogs would be needed to prove that these are, in fact, the variants in the region driving both the association and the signature of positive selection."
A separate study in journal Nature Communications suggested that dogs first split from wolves about 40,000 years ago. Some animals approached hunter-gatherer camps in search of food, researchers said. "Those wolves that were tamer and less aggressive would have been more successful at this" and more likely to befriend humans, explained the report.