More than 40 years ago, Japanese roboticist Masahiro Mori (1970/2005) proposed the “uncanny valley” hypothesis, which predicted a nonlinear relation between robots’ perceived human likeness and their likability. Although some studies have corroborated this hypothesis and proposed explanations for its existence, the evidence on both fronts has been mixed and open to debate. We first review the literature to ascertain whether the uncanny valley exists. We then try to explain the uncanny phenomenon by reviewing hypotheses derived from diverse theoretical and methodological perspectives within psychology and allied fields, including evolutionary, social, cognitive, and psychodynamic approaches. Next, we provide an evaluation and critique of these studies by focusing on their methodological limitations, leading us to question the accepted definition of the uncanny valley. We examine the definitions of human likeness and likability, and propose a statistical test to preliminarily quantify their nonlinear relation. We argue that the uncanny valley hypothesis is ultimately an engineering problem that bears on the possibility of building androids that may some day become indistinguishable from humans. In closing, we propose a dehumanization hypothesis to explain the uncanny phenomenon.

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The uncanny valley hypothesis by Japanese roboticist Masahiro Mori posits a nonlinear relation between human replicas' human likeness and the emotional responses they elicit. In three studies, we corroborated the uncanny valley hypothesis, using the uncanny phenomenon as a vehicle to shed a new light on human animacy perception. In Study 1, 62 participants rated emotional responses and human likeness of 89 artificial and human faces. In Study 2, another 62 participants conducted a visual looming task with the same 89 faces allowing for the measurement of perceived threat. Results support the uncanny valley hypothesis, suggesting that the uncanny feeling may serve a function to wary humans of the potential danger of entities crossing the animate-inanimate boundary. In Study 3, 36 participants sorted faces as either real or unreal as quickly as possible in a reaction time sorting task allowing for the measurement of categorical uncertainty associated with animacy perception. Faces associated with longer sorting reaction times were also those associated with the highest ratings of negative emotions, suggesting that categorical uncertainty in animacy detection is related to the uncanny feeling. Results are discussed in light of human animacy perception and new directions for future research are suggested.

Human replicas highly resembling people tend to elicit eerie sensations—a phenomenon known as the uncanny valley. To test whether this effect is attributable to people’s ascription of mind to (i.e., mind perception hypothesis) or subtraction of mind from androids (i.e., dehumanization hypothesis), in Study 1, we examined the effect of face exposure time on the perceived animacy of human, android, and mechanical-looking robot faces. In Study 2, in addition to exposure time, we also manipulated the spatial frequency of faces, by preserving either their fine (high spatial frequency) or coarse (low spatial frequency) information, to examine its effect on faces’ perceived animacy and uncanniness. We found that perceived animacy decreased as a function of exposure time only in android but not in human or mechanical-looking robot faces (Study 1). In addition, the manipulation of spatial frequency eliminated the decrease in android faces’ perceived animacy and reduced their perceived uncanniness (Study 2). These findings link perceived uncanniness in androids to the temporal dynamics of face animacy perception. We discuss these findings in relation to the dehumanization hypothesis and alternative hypotheses of the uncanny valley phenomenon.