Most of us don’t have a problem attributing emotions to primates, dogs, horses and other vertebrates. But what about invertebrates? That seems less obvious. They have smaller, less complex brains, but is that enough to boldly claim they have no emotions? Of course, studying animal emotions is a precarious business. Studying human emotions has already proven difficult enough, and in animals it is bound to be a lot harder.
One way to go about it, is to take a look at so-called cognitive biases, biases in the processing of information that are typical of negative affective states. An example of this is the pessimistic bias, an increased expectation of punishment, greater attention to potential threats and a tendency to interpret ambiguous stimuli as if they were threats.