Comparative psychology is the field of study examining and juxtaposing modes of behavior in different species of organisms, originally between humans and other species, but developing to compare the behaviors of nonhuman species with each other. Researchers in various disciplines often disagree on the definition and the scope of the field. However, most of the disagreements are merely semantic as the types of research done under the label of comparative psychology are credible, relevant and accepted. Some researchers prefer to see it through the lens of evolutionary psychology, defining the discipline as the study of the evolution of behavior. In this regard, Niko Tinbergen views the field as being relevant to four focuses of research: the commonality of the subject behavior within the subject species, the relevance and influence of that behavior on the ability to reproduce, what biological or environmental factors are predictive of the behavior and what traits must an organism exhibit or processes must it undergo in order to perform it. This view isolates comparative psychology to a science primarily interested in issues relevant to biological evolution.
The disputes about its definition arise not only out of what is studied, but also regarding the shifting focus of the field given the advancement of the sciences. Ethel Tobach proposed the updated definition: "Comparative psychology is the science of the elucidation of similarities and differences in the evolution and development of the activity of all species to illumine the processes by which their activity contributes to the beneficence of their relationship to the abiotic and biotic aspects of the environment."
Additionally, the term itself has been found to be confusing, prompting reason to redefine the discipline. Tobach cites the fact evolutionary psychology is a more appropriate term than comparative psychology as the popularity of comparative psychology programs at major academic, degree-granting institutions is far more scarce than other disciplines of biology. She also cites that the term has been somewhat phased out by evolutionary psychology in popular beginners' materials for psychology.
The origins of the discipline are found in the studies of Darwin. In conjunction with George Romanes, behavior was observed alongside anatomy, though the development of general psychology as a discipline helped to split comparative psychology from comparative anatomy. Additionally, the biological theories of Darwin on evolution became a completely different discipline in the academic world, where comparative behavior study was absorbed by psychology as the sciences became arbitrarily defined in the academies. The term ethology has also been used, often times in conjunction with comparative psychology and as an alternative to evolutionary psychology. The variation in use as either a synonym to another concept or a complement to another, then seeing those roles shift underline the fluctuating definitions of the subfield altogether.
The field itself, no matter how it is termed or defined, is extremely broad. Tendencies particular to animals like nest-building and hoarding (organizing) are major subjects as trends are obviously different among species. Hibernation, tonic mobility and playing are also subject to study. Unique to animal study, reproductive behavior receives an unusually strong emphasis, perhaps to the detriment of studying other behaviors. Mating rituals, calls and migration could be included in this category. Instincts, distinct sensory abilities or traits and learning are of great interest to the field. Regarding the study of cognition, considered fundamental in psychology, researchers tend to reference human behaviors in reference to animals. This highlights what could have been an attempt to examine animal tendencies and reference them compared to humans.
Based on the definitions applied to comparative psychology, evolutionary psychology has faced criticism like other subfields of psychology that it is a "pseudoscience" or a fundamentally flawed field of study. Professor of evolutionary biology Massimo Pigliucci suggests a number of factors that could undermine the authority of the field but particularly points out that the field might make certain assumptions for the principles of evolution, forcing researchers to compensate for failed hypotheses by suggesting untestable theories in their stead. Additionally, the "comparative method" he identifies as merely a technique of evolutionary psychology is of limited viability since there is only a tiny scope of species to compare regarding human behavior, limited to primates. The lack of sufficient references would undermine a major premise of comparative psychology in his view.