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Mirror, Mirror

Evidence that psychology, like biology, is conserved between human and nonhuman species augurs a shake-up for science and society

G. A. Bradshaw, Robert Sapolsky

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The case for inferential symmetry rests on evolutionary conservation—the levels of shared biology among animal species. At the genomic level, for example, chimpanzees and humans share 96 percent of their DNA—a figure that rises to 99 percent for genes that actually encode proteins. The genes that determine basic brain segments have changed little since vertebrates diverged from arthropods more than 500 million years ago. Not surprisingly, similar genes give rise to similar structures.

Merging human and animal models of brain and behavior raises an interesting point: Do homologous brain regions among vertebrates, or at least mammals, guarantee similar neural processes? This is a difficult question. If the brain were a computer, then it would be easier to predict that two computers with the same architecture and processor would show similar “physiology.” However, matching neural blueprints are no guarantee of consistent responses. Two species may both possess a structurally similar region of the brain, yet the size of that region relative to the rest of the brain may differ. For example, two parts of the brain, the basal forebrain and extended amygdala, mediate the same functions in many species. These include the recognition of potential mates and competitors, rituals for courtship and mating, parental care (when it occurs), aggression, and territoriality.

True singularities—brain features specific to only one radiation or even species—are rare, but do exist. Certain cell types—Mauthner cells in the spinal cords of some fish and amphibians and a type of frontocortical neuron in humans—can also be unique. Novel sensory systems, such as the ability to sense bioelectric fields in some sharks, or somatosensory specializations, such as echolocation in bats, are apparently specific to the niches occupied by those species.

However, these changes in structure, function and behavior are neither random nor disconnectedly modular. Rather, they are linked by common ancestry. From an evolutionary perspective, there is no reason to favor one direction of inference over the other. Furthermore, the patterns of behavior across many species are consistent enough to warrant a shared conceptual grammar that would allow scientists to make a valid inference across species. But how does this change in theory translate to practice? In other words, if old conventions of inference have lost their monopoly, then how should scientists balance similarities with singularities? The answer comes from first principles.

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