In darkness the resting pupil size showed a MZ intraclass correlation coefficient of 0.85, almost double that of DZ (0.44), suggesting strong additive genetic effects, with the most parsimonious model estimating a heritability of 86% with 14% (95% CI 10–21%) explained by unique environmental factors. Pupil size and PLR were not associated with iris colour, intraocular pressure or refractive error, but were associated with age (diameter β = −0.02, p = 0.016, constriction amplitude β = −0.01, p < 0.001, velocity β = 0.03, p < 0.001, and latency β = 0.98, p < 0.001). Pupil light reaction (PLR) had a mean (SD) amplitude of 1.38 mm (0.27) and latency of 250.34 milliseconds (28.58). Results: Mean (SD) pupil diameter in darkness was 5.29 mm (0.81), decreasing to 3.24 mm (0.57) in bright light. Maximum-likelihood structural equation models estimated the proportion of trait variance attributable to genetic factors. Methods: Pupillometry was performed on 326 female twins (mean age 64 years) from the TwinsUK Adult Twin Registry, assessing resting pupil diameter in darkness and increasing levels of ambient light, alongside dynamic pupillary characteristics. Purpose: A classic twin study to evaluate the relative contributions of genetic and environmental factors to resting pupil size and reactivity. 3Department of Neuro-Ophthalmology, National Hospital for Neurology and Neurosurgery, London, United Kingdom.2Department of Ophthalmology and Epidemiology, University Medical Centre Groningen, University of Groningen, Groningen, Netherlands.1Section of Academic Ophthalmology, School of Life Course Sciences, Faculty of Life Sciences & Medicine, King's College London, London, United Kingdom.Abdus Samad Ansari 1 Jelle Vehof 1,2 Christopher J.
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