Pupillometry is the measurement of pupil diameter in psychology. This method established itself after its use in three seminal studies. The method examines humans (including infants) and animals. Pupillary responses occur from birth and are involuntary. Pupil dilation of 0.5 mm can occur when elicited by psychological stimuli.
Hess and Polt (1960) presented pictures of semi-naked adults and babies to adults (four men and two women). Pupils of both sexes dilated after seeing pictures of people of the opposite sex. In females, the difference in pupil size occurred also after seeing pictures of babies and mothers with babies. This examination showed that pupils react not only to the changes of intensity of light (pupillary light reflex) but also reflect arousal or emotions.
In 1965 Hess, Seltzer and Shlien examined pupillary responses in heterosexual and homosexual males. Results showed a greater pupil dilation to pictures of the opposite sex for heterosexuals and to pictures of the same sex for homosexuals .
According to T.M. Simms (1967), pupillary responses of males and females were greater when they were exposed to pictures of the opposite sex. In another study, Nunnally and colleagues (1967) found that seeing slides rated as 'very pleasant' was associated with greater pupil dilation as seeing slides rated as neutral or very unpleasant.
Infants showed greater pupil size when they saw pictures of faces than when they saw geometric shapes, and greater dilation after seeing pictures of the infant's mother than pictures of a stranger.
Pupillary responses can reflect activation of the brain allocated to cognitive tasks. Greater pupil dilation is associated with increased processing in the brain. Vacchiaco and colleagues (1968) found that pupillary responses were associated with visual exposure to words with high, neutral or low value. Presented low-value words were associated with dilation, and high-value words with constriction of a pupil. In decision-making tasks dilation increased before the decision as a function of cognitive load. In an experiment about short-term serial memory, students heard strings of words and were asked to repeat them. Greater pupil diameter was observed after the items were heard (depending on how many items were heard), and decreased after items were repeated. The more difficult the task, the greater pupil diameter observed from the time preceding the solution  until the task was completed.
- Laeng B., Sirois S., Gredebäck G. (2012) "Pupillometry: A Window to the Preconscious?" Perspectives on Psychological Science January 2012 vol. 7 no. 1 18-27
- Hess E. H., Polt J. M. (1960) "Pupil size as related to interest value of visual stimuli" Science, 132, 349-350.
- Hess E. H., Seltzer A. L., Shlien J.M. (1965) "Pupil response of hetero- and homosexual males to pictures of men and women: A pilot study" Journal of Abnormal Psychology, 70, 165-168
- Simms T. M. (1967) "Pupillary response of male and female subjects to pupillary difference in male and female picture stimuli" Perception and Psychophysics, 2, 553-555.
- Goldwater B. C. (1972) "Psychological significance of pupillary movements" Psychological Bulletin 77(5):340-55. Cite error: Invalid
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- Nunally J. C., Knott P. D., Duchnowski A., Parker R. (1967) "Pupillary response as a general measure of activation." Perception and Psychophysics, 2, 149-155
- Fitzgerald H. E. (1968), "Autonomic pupillary reflex activity during early infancy and its relation to social and nonsocial visual stimuli" Journal of Experimental Child Psychology, 6, 470-482
- Fitzgerald H. E., Lintz L. M., Brackbill Y., Adams G. (1967), "Time perception and conditioning an autonomic response in human infants" Perceptual and Motor Skills, 24, 479-486
- Granholm E., Steinhauer S. R. (2004) "Pupillometric measures of cognitive and emotional processes" International Journal of Psychophysiology, Vol 52(1), 1-6. 
- Vacchiano R. B., Strauss P. S., Ryan S., Hochman L. (1968) "Pupillary response to value-lined words" Perceptual and Motor Skills, 27, 207-210.
- Simpson H. M., Hale S. M., (1969) "Pupillary Changes During a Decision-Making Task" Perceptual and Motor Skills, 29, 495-498.
- Kahneman D., Beatty J. (1967) "Pupillary Response in a Pitch Discrimination Task" Perception and Psychophysics, 2, 101-105.
- Kahneman D., Beatty J. (1966) "Pupil Diameter and Load on Memory" Science, 154, 1583-1585.
- Hess E. H., Polt J. H. (1964) "Pupil Size in Relation to Mental Activity During Simple Problem Solving" Science, 143, 1190-1192
- Bradshaw J. L. (1968), "Pupil size and problem solving" Quarterly Journal of Experimental Psychology, Volume 20, Issue 2, 116-122.
- Kafkas, A., & Montaldi, D. (2011). Recognition memory strength is predicted by pupillary responses at encoding while fixation patterns distinguish recollection from familiarity. The Quarterly Journal of Experimental Psychology, 64(10), 1971–1989. http://www.tandfonline.com/doi/abs/10.1080/17470218.2011.588335
- Kafkas, A., & Montaldi, D. (2012). Familiarity and recollection produce distinct eye movement and pupil and medial temporal lobe responses when memory strength is matched. Neuropsychologia, 50(13), 3080–93. doi:10.1016/j.neuropsychologia.2012.08.001
- Tryon, W. W. (1975), "Pupillometry: A Survey of Sources of Variation" Psychophysiology, 12: 90–93. 
- Verney, S. P., Granholm, E. and Dionisio, D. P. (2001), "Pupillary responses and processing resources on the visual backward masking task" Psychophysiology, 38: 76–83. 
- Beatty J. (1977), "Pupillometric Measurement of Cognitive Workload"