'We think dogs bark due to this internal conflict and mobbing behaviour, but domestic dogs bark more because they are put, and put themselves into, conflicting situations more often,' she says.

The reason traces back to the first dogs that started hanging around human food dumps about 8,000 to 10,000 years ago.

They would have experienced a serious disadvantage if they had run a mile away every time a human or other animal approached.

'In evolutionary terms, dogs self-selected the behaviour of sticking around, overcoming their fear and being rewarded by getting to eat that meal before some other dog got it,' Lord said.

'Thus these animals allow people to get unusually close. The scared ones die while those less scared stay, eat, survive and reproduce. So they inherit the tendency.'

These findings were published in a special issue of Behavioural Processes.