Dogs are seen as less threatening if they have dark eyes.
Dogs with darker eyes are considered to be less threatening
Experts believe that humans could have driven their popularity as man's best friend descended from wolves over thousands of years of domestication.
Yellow is the most common eye colour for wolves and a team from Teikyo University of Science in Japan collected 22 images of grey wolves and 81 pictures of domestic pooches from different breeds.
They then gathered 12 pictures of dogs including Labrador Retrievers and Welsh Corgis and "recoloured" their eyes to be dark or yellow.
76 participants were asked to rate the images and it was revealed that canines with darker eyes were considered to be friendlier and less threatening than the same dogs with lighter-coloured eyes.
Researcher Dr. Akitsugu Konno said: "We speculate that a darker iris makes it more difficult to distinguish the size of the pupil and thus gives the illusion of a large pupil, which is associated with our perception of being more infant-like."