Scientists, who studied badly behaved pets concluded that dogs that growl and bite people may not naturally be hostile, It's more like depression, from low levels of the "feel good" hormone serotonin.
In humans, low serotonin levels are known to be linked to depression and anxiety. Researchers from Zaragoza university in Spain, hope their findings will lead to new ways of treating canine aggression, which is the most frequent behaviour problem seen by vets.
In the study, researchers took blood sample from 80 dogs, which had been referred to veterinary teaching hospitals after their owners reported that they had behaved aggressively towards humans.
Samples from normally behaved dogs were also taken.
Tests showed that the aggressive dogs had significantly lower concentrations of serotonin in their blood - 278 units compared to 387.
The lowest readings of all were from a group of dogs whose bad behaviour appeared to be an attempt at self-defence.
The aggressive pets also had significantly higher levels of a second blood chemical, the "stress hormone" cortisol - 21 units compared to 10.
However, it is not known what actually caused the dogs to have different levels of the hormones in their blood.
But the findings could open the door to new ways of diagnosing and treating aggression.
The researchers said that the results suggested that antidepressant drugs, which can raise serotonin levels in depressed humans, might be of help in treating aggressive dogs, in conjunction with other techniques to change their behaviour.
"The ability to identify dogs with lower levels of serotonin may help in identifying those dogs who could benefit from the use of pharmacology," the Telegraph quoted Mark Johnston, spokesman for the British Small Animals Veterinary Association, as saying.
The study is published in the journal Applied Animal Behaviour Science.