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FROM THE FIELD

The Uncanny ValleyBy Masahiro Mori (Translated by Karl F. MacDorman and Norri Kageki)

A Valley in OnesSense of AffinityThe mathematical termmonotonicallyincreasing function describes a rela-tion in which the function y f x increases continuously with the

variable x. For example, as effort xgrows, incomeyincreases, or as a carsaccelerator is pressed, the car movesfaster. This kind of relation is ubiqui-tous and easily understood. In fact,because such monotonically increas-ing functions cover most phenomenaof everyday life, people may fall underthe illusion that they represent allrelations. Also attesting to this falseimpression is the fact that manypeople struggle through life by persis-

tently pushing without understandingthe effectiveness of pulling back. Thatis why people usually are puzzledwhen faced with some phenomenonthat this function cannot represent.

An example of a function that doesnot increase continuously is climbinga mountainthe relation between thedistance (x) traveled by a hiker towardthe summit and the hikers altitude

(y)owing to the intervening hillsand valleys. I have noticed that, inclimbing toward the goal of makingrobots appear like a human, our affin-ity for them increases until we cometo a valley (Figure 1), which I call theuncanny valley.

Nowadays, industrial robots areincreasingly recognized as the drivingforce behind reductions in factorypersonnel. However, as is well known,these robots just extend, contract, androtate their arms; without faces or legs,they do not look human. Their designpolicy is clearly based on functionality.From this standpoint, the robots mustperform functions similar to those ofhuman factory workers, but whether

they look similar does not matter. Thus,given their lack of resemblance tohuman beings, in general, people hardlyfeel any affinity for them. (Note: How-ever, industrial robots are considerablycloser in appearance to humans thangeneral machinery, especially in theirarms.) If we plot the industrial robot ona graph of affinity versus human like-ness, it lies near the origin in Figure 1.

In contrast, a toy robots designermay focus more on the robots appear-

ance than its functions. Consequently,despite its being a sturdy mechanical

figure, the robot will start to have aroughly human-looking external formwith a face, two arms, two legs, and atorso. Children seem to feel deeplyattached to these toy robots. Hence,the toy robot is shown more than half-way up the first hill in Figure 1.

Since creating an artificial human isitself one of the objectives of robotics,

various efforts are underway to buildhumanlike robots. (Note: Others believethat the true appeal of robots is theirpotential to exceed and augment humans.)For example, a robots arm may be com-posed of a metal cylinder with manybolts, but by covering it with skin andadding a bit of fleshy plumpness, we canachieve a more humanlike appearance.

As a result, we naturally respond to itwith a heightened sense of affinity.Many of our readers have experi-

ence interacting with persons withphysical disabilities, and all must havefelt sympathy for those missing a handor leg and wearing a prosthetic limb.Recently, owing to great advances infabrication technology, we cannot dis-tinguish at a glance a prosthetic handfrom a real one. Some models simulatewrinkles, veins, fingernails, and even

fingerprints. Though similar to a realhand, the prosthetic hands color is

Digital Object Identifier 10.1109/MRA.2012.2192811

Date of publication: 6 June 2012

98 IEEE ROBOTICS & AUTOMATIONMAGAZINE JUNE2012

Editors note: More than 40 years ago, Masahiro Mori, a roboticsprofessor at the Tokyo Institute of Technology, wrote an essay [1]on how he envisioned peoples reactions to robots that lookedand acted almost like a human. In particular, he hypothesized thata persons response to a humanlike robot would abruptly shiftfrom empathy to revulsion as it approached, but failed to attain, alifelike appearance. This descent into eeriness is known as theuncanny valley. The essay appeared in an obscure Japanese jour-nal called Energy in 1970, and in subsequent years, it receivedalmost no attention. However, more recently, the concept of theuncanny valley has rapidly attracted interest in robotics and other

scientific circles as well as in popular culture. Some researchershave explored its implications for humanrobot interaction andcomputer-graphics animation, whereas others have investigatedits biological and social roots. Now interest in the uncanny valleyshould only intensify, as technology evolves and researchers buildrobots that look human. Although copies of Moris essay have circu-lated among researchers, a complete version hasnt been widelyavailable. The following is the first publication of an English transla-tion that has been authorized and reviewed by Mori. (See TurningPoint in this issue for an interview with Mori.)

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pinker as if it had just come out ofthe bath.

One might say that the prosthetichand has achieved a degree of resem-blance to the human form, perhapson par with false teeth. However, oncewe realize that the hand that looked

real at first sight is actually artificial,we experience an eerie sensation. Forexample, we could be startled during ahandshake by its limp boneless griptogether with its texture and coldness.When this happens, we lose our senseof affinity, and the hand becomesuncanny. In mathematical terms, thiscan be represented by a negative value.Therefore, in this case, the appearanceof the prosthetic hand is quite human-like, but the level of affinity is negative,

thus placing the hand near the bottomof the valley in Figure 1.

I dont think that, on close inspec-tion, abunrakupuppet appears similarto a human being. Its realism in termsof size, skin texture, and so on, does noteven reach that of a realistic prosthetichand. But when we enjoy a puppetshow in the theater, we are seated at acertain distance from the stage. Thepuppets absolute size is ignored, andits total appearance, including handand eye movements, is close to that of ahuman being. So, given our tendencyas an audience to become absorbed inthis form of art, we might feel a highlevel of affinity for the puppet.

From the preceding discussion, thereaders should be able to understand theconcept of the uncanny valley. Now letus consider in detail the relation betweenthe uncanny valley and movement.

The Effect of MovementMovement is fundamental to ani-malsincluding human beingsandthus to robots as well. Its presencechanges the shape of the uncanny val-ley graph by amplifying the peaks and

valleys (Figure 2). For illustration,when an industrial robot is switchedoff, it is just a greasy machine. Butonce the robot is programmed tomove its gripper like a human hand,we start to feel a certain level of affin-

ity for it. (In this case, the velocity,acceleration, and deceleration must

approximate human movement.)Conversely, when a prosthetic handthat is near the bottom of the uncanny

valley starts to move, our sensation ofeeriness intensifies.

Some readers may know that re-cent technology has enabled prosthetic

hands to extend and contract their fin-gers automatically. The best commer-cially available model, shown in Figure3, was developed by a manufacturer inVienna. To explain how it works, evenif a persons forearm is missing, theintention to move the fingers producesa faint current in the arm muscles,

which can be detected by an electro-myogram. When the prosthetic handdetects the cur-rent by means ofelectrodes on theskins surface, itamplifies the sig-

nal to activate asmall motor thatmoves its fingers.As this myoelec-tric hand makesmovements, itcould make healthy people feel uneasy.If someone wearing the hand in a dark

Uncanny Valley Healthy Person

BunrakuPuppet

Prosthetic Hand

Toy Robot

Industrial Robot

Affinity(Shinwakan)

Human Likeness 50% 100%

+

Figure 1. The graph depicts the uncanny valley, the proposed relation between thehuman likeness of an entity, and the perceivers affinity for it. [Translators note: Bunrakuis a traditional Japanese form of musical puppet theater dating to the 17th century. Thepuppets range in size but are typically a meter in height, dressed in elaborate costumes,and controlled by three puppeteers obscured only by their black robes (see front cover).]

Uncanny Valley

StuffedAnimal

Death

Healthy Person

BunrakuPuppet

Prosthetic Hand

Humanoid Robot

Industrial Robot

Affinity(Shinwaka

n)

Human Likeness 50% 100%

+

Ill Person

OkinaMask

Ordinary Doll

Myoelectric Hand

Zombie

Yase OtokoMask (NohPlay)Corpse

Moving

Still

Figure 2. The presence of movement steepens the slopes of the uncanny valley. Thearrows path represents the sudden death of a healthy person. [Translators note: Noh isa traditional Japanese form of musical theater dating to the 14th century in which

actors commonly wear masks. The yase otokomask bears the face of an emaciatedman and represents a ghost from hell. Theokinamask represents an old man.]

I predict that it is

possible to createa

safe levelof affinitybydeliberately pursuing

a nonhumandesign.

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place shook a womans hand with it,the woman would assuredly shriek.

Since the negative effects of move-ment are apparent even with a pro-sthetic hand, to build a whole robotwould magnify the creepiness. This is

just one robot. Imagine a craftsmanbeing awakened suddenly in the deadof the night. He searches downstairs forsomething among a crowd of manne-quins in his workshop. If the manne-quins started to move, it would be likea horror story.

Movement-related effects could beobserved at the 1970 World Exposi-tion in Osaka, Japan. Plans for theevent had prompted the constructionof robots with some highly sophisti-cated designs. For example, one robothad 29 pairs of artificial muscles in

the face (the same number as a humanbeing) to make it smile in a humanlikefashion. According to the designer, asmile is a dynamic sequence of facialdeformations, and the speed of thedeformations is crucial. When thespeed is cut in half in an attempt tomake the robot bring up a smile moreslowly, instead of looking happy, itsexpression turns creepy. This showshow, because of a variation in move-ment, something that has come to

appear close to human

like a robot,puppet, or prosthetic handcould

easily tumble down into the uncannyvalley.

Escape by DesignWe hope to design and build robotsand prosthetic hands that will not fallinto the uncanny valley. Thus, because

of the risk inherent in trying toincrease their degree of human like-ness to scale the second peak, I recom-mend that designers instead take thefirst peak as their goal, which results ina moderate degree of human likenessand a considerable sense of affinity. Infact, I predict that it is possible to cre-ate a safe level of affinity by deliber-ately pursuing a nonhuman design. Iask designers to ponder this. To illus-trate the principle, consider eyeglasses.

Eyeglasses do not resemble real eye-balls, but one could say that theirdesign has created a charming pair ofnew eyes. So we should follow thesame principle in designing prosthetichands. In doing so, instead of pitiful-looking realistic hands, stylish oneswould likely become fashionable.

As another example, consider thismodel of a human hand created by awoodcarver who sculpts statues ofBuddhas (Figure 4). The fingers bendfreely at the joints. The hand lacks fin-gerprints, and it retains the naturalcolor of the wood, but its roundnessand beautiful curves do not elicit anyeerie sensation. Perhaps this woodenhand could also serve as a referencefor design.

An Explanation of the UncannyAs healthy persons, we are repre-sented at the second peak in Figure 2

(moving). Then when we die, we areunable to move; the body goes cold,and the face becomes pale. Therefore,our death can be regarded as a move-ment from the second peak (moving)to the bottom of the uncanny valley(still), as indicated by the arrows pathin Figure 2. We might be glad that thisarrow leads down into the still valleyof the corpse and not the valley ani-mated by the living dead.

I think this descent explains the

secret lying deep beneath the uncannyvalley. Why were we equipped with

this eerie sensation? Is it essential forhuman beings? I have not yet consid-ered these questions deeply, but I haveno doubt it is an integral part of ourinstinct for self-preservation. (Note:The sense of eeriness is probably aform of instinct that protects us fromproximal, rather than distal, sourcesof danger. Proximal sources of dangerinclude corpses, members of differentspecies, and other entities we can closelyapproach. Distal sources of dangerinclude windstorms and floods.)

We should begin to build an accu-rate map of the uncanny valley so thatthrough robotics research we can beginto understand what makes us human.This map is also necessary to createusing nonhuman designsdevices towhich people can relate comfortably.(Notes given in parentheses are foot-

notes in the original article.)

Reference[1] M. Mori, The uncanny valley, Energy,

vol. 7, no. 4, pp. 3335, 1970 (in Japanese).

Karl F. MacDorman is an associateprofessor at the School of Informatics,Indiana University.

Norri Kageki is a business and tech-nology writer based in Silicon Valley

and the founder of GetRobo.com.

Figure 4. A model of a hand created by a

woodcarver of Buddha statues. Reprintedfrom [1] with permission.

Figure 3. A human arm (top left)

controls arealisticmyoelectric hand,called the Vienna Hand. Reprinted from[1] with permission.

100 IEEE ROBOTICS& AUTOMATIONMAGAZINE JUNE 2012