---
title: "Monkeys show an uncanny valley response; five macaques looked away from lifelike digital monkey faces"
url: https://badiyaa.com/monkeys-uncanny-valley-macaques-princeton-study/
published: 2026-11-11T08:05:00+00:00
modified: 2026-11-11T08:05:00+00:00
section: Science
publisher: Badiyaa
---

# Monkeys show an uncanny valley response; five macaques looked away from lifelike digital monkey faces

Monkeys show an uncanny valley response: in a Princeton study, five macaques looked least at realistic computer-generated monkey faces.

Show a monkey a cartoonish computer-generated monkey and it will look. Show it a real monkey and it will look. Show it a nearly perfect digital monkey and it looks away. In a 2009 Princeton study, **all five** long-tailed macaques tested spent the least time on the most lifelike synthetic faces, the first evidence that the creepy-robot feeling known as the uncanny valley is not unique to humans.

“To our surprise, monkey visual behavior fell into the uncanny valley,” Shawn A. Steckenfinger and Asif A. Ghazanfar wrote in their [paper in the Proceedings of the National Academy of Sciences](https://pmc.ncbi.nlm.nih.gov/articles/PMC2760490/), published in October 2009.

## What the uncanny valley is

The idea comes from Japanese robotics professor Masahiro Mori, who described it in 1970 as *bukimi no tani*. As a machine grows more humanlike, people warm to it, until it is almost but not quite human. At that point the warmth collapses into unease, then recovers once the copy becomes indistinguishable from the real thing. Plotted on a graph, that dip is the [uncanny valley](https://en.wikipedia.org/wiki/Uncanny_valley). The English term comes from a 1978 book by Jasia Reichardt, which translated the phrase literally.

Hollywood knows the dip well. The Princeton authors pointed to strongly negative audience reactions to computer-animated films such as *The Polar Express* and *Final Fantasy: The Spirits Within* as the kind of anecdotal evidence that preceded lab tests.

## How the monkeys were tested

The researchers built three kinds of monkey face. One was video of a real macaque. One was a realistic computer model with detailed skin texture and a high polygon count. The third was deliberately crude: gray, untextured, with red pupils and about a third as many polygons, to look industrial and robot-like.

Each face appeared in neutral, “coo” and “scream” expressions, as still images and as 2-second video clips. The monkeys sat 65 centimeters from a 17-inch screen while an infrared tracker logged where their eyes landed. They earned apple juice simply for looking anywhere on the screen, so the test measured spontaneous interest, not a trained choice.

The workload adds up quickly. Each session ran 45 trials, and three sessions per animal meant 135 trials per monkey, or 675 trials across the five animals, each showing a face for two seconds.

## What they found

Five outcomes were possible, from preferring real faces to ignoring all of them. The monkeys produced exactly one: more fixations, and longer ones, on the real and crude faces than on the realistic fakes. There was no significant difference between real and crude faces. Every one of the five subjects showed the pattern, and it did not depend on which expression was shown or on screen brightness.

The authors argue that realism itself is not the trigger. “It is not the increased realism that elicits the uncanny valley effect, but rather that the increased realism lowers the tolerance for abnormalities,” they wrote. A near-perfect face invites the brain to judge it as a real member of the species, and small flaws then fail that test.

Movement mattered less than Mori predicted. The monkeys looked longer at moving faces overall, but the dip was only slightly sharper for video than for stills.

Primate studies keep turning up on Badiyaa, from [fossil teeth suggesting monkeys rafted across the Atlantic](https://badiyaa.com/monkeys-rafted-atlantic-ucayalipithecus/) to a [214-species study of why animal pupils come in different shapes](https://badiyaa.com/animal-pupils/).

## Limits

Five male lab-born macaques are a small sample. Looking time measures preference, not feelings, and the authors said they could not tell whether the monkeys felt fear, disgust or simple disinterest. They suggested future work track skin conductance or pupil dilation. Later work has not settled the question: a 2022 study by Carp and colleagues found little support for an uncanny valley in rhesus monkeys, though those animals did favor the eyes of realistic faces over the mouth.

The 2009 paper’s conclusion still frames the debate: the effect “is not unique to humans,” the authors wrote, making evolutionary explanations for it tenable.

## Further reading

- [Steckenfinger & Ghazanfar 2009, PNAS](https://doi.org/10.1073/pnas.0910063106)

- [Wikipedia: Uncanny valley](https://en.wikipedia.org/wiki/Uncanny_valley)

*Image: Basile Morin, CC BY-SA 4.0 via Wikimedia Commons*

*This article was produced with AI assistance and reviewed by our Editorial Desk. [How we work](https://badiyaa.com/editorial-policy/).*