It was René Descartes in the 1600s who described animals as having no emotional capacity. In his eyes they were little more than complicated automata. Voltaire disagreed completely, asserting that if the animal forms had a similar biological structure to our own, then they must have a similar emotional structure as well. His was, at the time, the minority view. But how much of our belief in animal emotions is a projection of our own emotions? Looking for human characteristics in animals is known as anthropomorphism, and can be a very easy trap to fall into if you're trying to make an objective decision.

Part of the difficulty has been the way in which animals express emotions. Many have fewer facial muscles, can't cry, make different noises, or use less familiar techniques to provide an indication of what's going on inside their heads. When a monkey smiles, it is usually out of fear or submission, not out of friendliness. When a dog "smiles" it is usually because you are about to be bitten. Modern research strongly suggests that animals do indeed feel emotions, and indeed suffer from a wide range of emotion-related ailments, such as chronic stress and depression.