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The Caged Primate: Indoor Living, Nature Deficit, and the Biological Cost of Enclosure

A. C. Applebee, L. N. Combe · 2026

Abstract

Humans in developed nations spend approximately 93% of their time indoors (Klepeis et al., 2001), a figure that has almost certainly increased in the 25 years since it was measured. This historically unprecedented shift from outdoor to indoor living has occurred within two to three generations — a timescale invisible to natural selection but sufficient to produce widespread physiological mismatch. This paper synthesises evidence from five independent research domains — ophthalmology, immunology, environmental psychology, indoor air quality science, and psychoneuroimmunology — to demonstrate that the indoor environment is systematically inadequate for human biological function.

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