Recently, comparative psychology has witnessed an explosion of research into previously―discouraged topic areas such as consciousness and metacognition in non-humans as phylogenetically distant from humans as cephalopods (Mather, 2008), rats (Foote & Crystal, 2007), and pigeons (Sutton & Shettleworth, 2008). Freed from the shackles of strict behaviorism, comparative psychologists have had license to explore topics that were previously restricted to humans, and sometimes, human's closest living relative – the chimpanzee. Indeed, empirical investigations of animal cognition were historically restricted to chimpanzees, rhesus macaques, pigeons, and the white rat (hearty species that thrived in laboratories). When other species were probed for cognitive abilities, often studies focused on only a single or a few members of that species; Alex, the African grey parrot (Pepperberg, 2002), Betty, the New Caledonian Crow (Weir, Chappell & Kacelnik, 2002), Kanzi, the bonobo (Savage-Rumbaugh & Lewin, 1994) and so on (although studies with these species have since been extended to other members of their species).

Although we applaud efforts in the field to expand the species of study and the questions and topics under investigation, much would be gained by returning to the lessons of methodological behaviorists and logical positivists, who fought to place psychology among the sciences. We see the trend in animal research, particularly with regards to cognitive research, as striving to highlight similarities with human traits and abilities. That is, the focus has been on cognitive continuity, and equally interesting discontinuities have sometimes been lost in the shuffle (see also Vonk & Povinelli, 2006). The approach of seeking evidence for human-like traits in non-human species has sometimes been deemed the―Holy Grail approach to comparative psychology (Povinelli & Vonk, 2004). We propose that, instead, investigators focus on species' ecological and social environments and ask what abilities and traits make sense within that context. We should certainly consider whether existing capacities are similar to or different from traits with closely or distantly related species; it is precisely such considerations that allow us to speculate as to the evolutionary forces at work behind the emergence of these traits. However, researchers should not be motivated by the desire to find evidence for particular patterns of behaviors simply because those behaviors exist in other organisms, particularly humans, and especially if there is no ecologically-relevant reason to find such behaviors in the species in question. Instead, we should look to see what behaviors and abilities the animal does exhibit—not design our tasks with a predetermined goal in question or―stack the deck to find evidence for traits that have not yet appeared in natural environments. An unbiased approach is more likely to illuminate both parallel and convergent evolutionary processes.

On that note, comparative psychologists could take heed from evolutionary psychologists who have remained focused on the larger goals of identifying the mechanisms underlying both human and non-human behavior at both the ultimate and proximate levels. Evolutionary psychologists have incorporated advances in cognitive science, behavioral economics, developmental psychology, linguistics, cultural anthropology, sociobiology, and ethology, and yet work from an overarching theoretical framework. The basic processes of natural selection and sexual selection can explain both human and non-human mating practices (Buss, 1989, 1994), sex differences in spatial abilities (Voyer, Voyer, & Bryden, 1995), attractiveness (Burke & Sulikowski, 2010) investment in offspring (Alvergne, Faurie, & Raymond, 2009), and so on. Comparative psychologists, while unequivocally but sometimes only implicitly accepting evolutionary theory, have not always placed their work within the same overarching framework, and sometimes take esoteric side trips, investigating questions that, while interesting, do little to explain the forces giving rise to various traits or behaviors in different populations. For instance, why should elephants exhibit mirror self-recognition (Plotnik, deWaal, & Reiss, 2006)?

Although both comparative and evolutionary psychologists are interested in the mechanisms underlying psychological capacities and behavioral traits exhibited by various species and specific human and non-human populations, and the evolutionary forces that gave rise to such traits and behaviors, these two fields have taken somewhat disparate trajectories, rather than culminating in a single field of study. We propose that much would be gained if the fields were merged into a new science of Comparative Evolutionary Psychology. The goals would remain—the new science would explore the ultimate and proximate causes of behavior in humans and their close and distant relatives, and the relatively newfound appreciation for an eclectic approach to empirical questions and methodologies would be retained. However, a return to strict adherence to the objectivity of the past is needed if we are to move forward with any kind of credibility as an impartial science. This means that we must embrace the very nature of evolution—an appreciation for both continuity and discontinuity, even in closely-related species such as humans and other great apes. In doing so, we may have to abandon holy grail type pursuits and be open to finding evidence for abilities that our closest relatives have that we do not, and vice versa. We may also find evidence for human-like abilities in distantly-related species. We must adopt more open-minded approaches to our studies, such that we do not grasp firmly on to―preferred hypotheses that fit dogma, but rather are willing to formulate new theories and hypotheses that emerge from a consideration of an animal's evolutionary history.

In our forthcoming volume, the Oxford Handbook of Comparative Evolutionary Psychology, we have brought together an exemplary group of contributors who embrace this approach to science, although they have presented diverse viewpoints on these and other timely issues in the field. Contributors discuss cognitive specializations in cephalopods (Mather), cetaceans (Jaakola), reptiles (Wilkinson & Huber), corvids (van Horik, Clayton & Emery), canines (Miklosi & Topal) and primates (Cartmill & Maestripieri), as well as humans (Dunbar & Sutcliffe; Sell). We selected for inclusion several―hot topic areas in the field today in which researchers are tackling challenging issues by working with multiple species and various methodologies. For instance, forays into the field of memory, particularly episodic memory and metacognition, return us to the century-old query: is man alone in pondering his own existence and reflecting on his past? Contributions from Raby and Clayton; Smith, Coutinho, Boomer, and Beran; Crystal; and Feeney and Roberts update the reader on the many exciting developments and controversies in this area. Building on the seminal work of Vygotsky (1978) and Boyd and Richerson (1985), Tomasello (2000) has suggested that human culture is the foundation for uniquely human cognition, such as the full suite of theory of mind and co-operation. The volume includes a section of the most up to date research on prosociality (Silk & House), cooperation (Warnken & Melis), culture (Hopper & Whiten; Mesoudi & Jensen), imitation (Subiaul), and morality (Sheskin & Santos) in both humans and non-humans (Boesch). Communication has also been a central area of study for centuries. Without the ability to communicate, many other abilities would be meaningless, or useless. Much public interest has centered on the question of whether any other species has a language that comes close to that of humans. Such questions are addressed by Lyn and Pepperberg regarding apes and parrots, respectively. Cartmill and Maestripieri address the gestural communication in apes, and Zuberbuhler reviews the work on primate communication, focusing largely on studies conducted in the wild. Cocroft provides insight into the fascinating world of social insects and how communication serves vital functions for these species—in subserving mating behaviors. Barrett, and Radenovich and Andrews lead off the volume with thoughtful essays on where the field should and could go while Cheney and Seyfarth and Shettelworth close out the volume with provocative contributions summarizing the past, present, and future of comparative evolutionary psychology.

These extraordinary scientists epitomize the approach to comparative evolutionary psychology that we advocate. Most work with a variety of species and many adopt both field and laboratory studies, adopting multifaceted approaches to their questions of interest. To advance both fields together, we suggest that more progress will be made if researchers are open to methodologies and approaches of those in closely related disciplines and who work from different perspectives. To practice this open-minded perspective, we suggest dissembling the barriers that arose with the construction of false dichotomies between nature/nurture, behaviorism/nativism, field/laboratory, ideographic/nomographic, and human/non-human research, and most importantly, perhaps, a willingness to abandon the idea that other species are most valuable when they most closely approximate humans (cf. Savage-Rumbaugh & Lewin, 1994). By doing so, we can better appreciate the diversity of life on this planet, and we will be better positioned to under-stand the processes of evolution—as they apply to both the behavior and psychology of organisms. Thus, comparative evolutionary psychology can be a single united field for the study of evolved traits in all species.

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