How does Masahiro Mori’s uncanny valley theory explain reactions to humanoid robots?

Masahiro Mori’s uncanny valley theory explains that as a robot’s design becomes more humanlike, a viewer’s affinity for it initially increases. However, this positive response only lasts until the robot reaches a point of being nearly—but not perfectly—lifelike.123

At this stage, the viewer’s affinity drops sharply, replaced by feelings of eeriness, revulsion, or discomfort. Mori theorized that this "valley" occurs because the object is close enough to human to trigger expectations of life, yet subtle imperfections in appearance or movement reveal it as artificial, creating a disturbing ambiguity. If the design continues to improve until it achieves a perfect, indistinguishable human likeness, the viewer's affinity rises again.123

Mori suggested that movement intensifies this effect, making a moving robot more unsettling than a still one. While the theory remains a foundational concept in robotics, scientific consensus on its cause is inconclusive, with researchers proposing explanations ranging from primal instincts regarding disease to the unsettling illusion of human consciousness.1