Dog lovers have known since time began that pets get jealous, but now a new study has shown that it is true. A professor from the University ofCalifornia conducted research into what makes dogs jealous. Dr Christine Harris first decided to investigate canine jealousy after she noticed strange behaviour among her parents' three border collies. She noticed that when she stroked two of the pooches at the same time, they became aggressive towards each other. Both dogs tried to knock her hand away from the other so they could receive maximum attention. She said: "To me, that really fit with the core motivation of jealousy….I was petting them with both my hands, not just one. They wanted exclusive attention. That got me to thinking about jealousy in dogs.