Using a comparative approach to behavior allows one to evaluate the target behavior from four different, complementary perspectives, developed by Niko Tinbergen . First, one may ask how pervasive the behavior is across species? Meaning, how common is the behavior in animals? Second, one may ask how the behavior contributes to the lifetime reproductive success of the individuals demonstrating it? Meaning, does it result in those animals producing more offspring than animals not showing the behavior? These two questions provide a theory for the ultimate cause of behavior. Third, what mechanisms are involved in the behavior? Meaning, what physiological, behavioral, and environmental components are necessary and sufficient for the generation of the behavior? Fourth, a researcher may ask about the development of the behavior within an individual. Meaning, what maturational, learning, social experiences must an individual undergo in order to demonstrate a behavior? These latter two questions provide a theory for the proximate causes of behavior. For more details see Tinbergen's four questions.
Charles Darwin was central in the development of comparative psychology; it is thought that psychology should be spoken in terms of "pre-" and "post-" Darwin because his contributions were so influential. Darwin’s theory lead to several hypotheses, one being that the factors that set humans apart, such as higher mental, moral and spiritual faculties, could be accounted for by evolutionary principles. In response to the vehement opposition to Darwinism was the “anecdotal movement” led by George Romanes who set out to prove that animals possessed a “rudimentary human mind”.
Near the end of the 19th century several scientists existed whose work was also very influential. Douglas Alexander Spalding, who was called the “first experimental biologist” worked mostly with birds—studying instinct, imprinting, and visual and auditory development. Jacques Loeb emphasized the importance of objectively studying behavior, Sir John Lubbock is credited with first using mazes and puzzle devices to study learning1 and Lewis Henry Morgan is thought to be “the first ethologist in the sense in which we presently use the word”.
Throughout the long history of comparative psychology, repeated attempts have been made to enforce a more disciplined approach, in which similar studies are carried out on animals of different species, and the results interpreted in terms of their different phylogenetic or ecological backgrounds. Behavioral ecology in the 1970’s gave a more solid base of knowledge against which a true comparative psychology could develop. However, the broader use of the term "comparative psychology" is enshrined in the names of learned societies and academic journals, not to mention in the minds of psychologists of other specialisms, so it is never likely to disappear completely.
A persistent question with which comparative psychologists have been faced is the relative intelligence of different species of animal. Much effort has gone into explaining that this may not be a good question, but it will not go away. Indeed, some early attempts at a genuinely comparative psychology involved evaluating how well animals of different species could learn different tasks. These attempts floundered; in retrospect it can be seen that they were not sufficiently sophisticated, either in their analysis of the demands of different tasks, or in their choice of species to compare. More recent comparative work has been more successful, partly because it has drawn upon studies in ethology and behavioral ecology to make informed choices of species and tasks to compare.
Amphibians and reptiles
Today an animal's psychological constitution is recognised by veterinary surgeons as an important part of its living conditions in domestication or captivity.
Common causes of disordered behaviour in captive or pet animals are lack of stimulation, inappropriate stimulation, or overstimulation. These conditions can lead to disorders, unpredictable and unwanted behaviour, and sometimes even physical symptoms and diseases. For example, rats that are exposed to loud music for a long period will ultimately develop unwanted behaviours that have been compared with human psychosis, like biting their owners.
The way dogs behave when understimulated is widely believed to depend on the breed as well as on the individual animal's character. For example, huskies have been known to completely ruin gardens and houses, if they are not allowed enough activity. Dogs are also prone to psychological damage if they are subjected to violence. If they are treated very badly, they may become dangerous.
The systematic study of disordered animal behaviour draws on research in comparative psychology, including the early work on conditioning and instrumental learning, but also on ethological studies of natural behaviour. However, at least in the case of familiar domestic animals, it also draws on the accumulated experience of those who have worked closely with the animals.
Many of these were active in fields other than animal psychology; this is characteristic of comparative psychologists.