They are regarded as a man's best friend, but scientists say dogs are selfish creatures who will play with owners, but not help them.
Researchers from the University of Portsmouth studied 24 family dogs to see how useful they were to their owners, finding that they were more interested in themselves than coming to the aid of a human.
Scientists concluded that, although dogs sometimes helped their owners, it was "mainly self-interest that was driving the dogs' behaviour".
To test each animal, a notebook a human had been using was hidden in an opaque tub in one corner while the pet watched and the person left the room. When they returned and searched for their notebook, if the dog indicated where it was, they were being helpful.
A toy was placed in another container in a different corner and, if the dog chose this, they were considered selfish. Also hidden in a third corner was a stapler, which was considered neither selfish nor useful.
When the notebook user returned and searched for the item they had "lost", the dogs indicated the toy more often than the notebook or stapler.
Of the latter two items, the canines were just as likely to go for one as the other. Patrizia Piotti, lead researcher, of the University of Portsmouth said: "We assessed whether dogs would abandon an object that they find interesting in favour of an object useful for their human partner, a random novel distracter, or an empty container. Results showed that it was mainly self-interest that was driving the dogs' behaviour."
She added that, if there were no toys around, a different group of dogs were found to have better results. They looked at the spot where the notepad was hidden for half a second longer than the stapler.
However, this was only if the researcher was speaking to them in a high-pitched voice, suggesting the dogs were merely excited.
The research was published in the journal PLoS ONE.
Clive Wynne, at Arizona State University in Tempe, said: "Does the dog take an interest in an object that a human is interested in, or only in objects that dogs are interested in? "That got a clear-cut result: dogs only like objects that dogs like."