Subsequent research found that the pupils of more intelligent people (as defined by their Scholastic Aptitude Test scores) dilated less in response to cognitive tasks compared with those of lower-scoring participants, indicating more efficient use of brainpower.
Scientists have since used pupillometry to assess everything from sleepiness, introversion and sexual interest to race bias, schizophrenia, moral judgment, autism and depression. And whereas they haven't been reading people's thoughts per se, they've come pretty close.
"Pupil dilation can betray an individual's decision before it is openly revealed," concluded a 2010 study led by Wolfgang Einhäuser-Treyer, a neurophysicist at Philipps University Marburg in Germany. Participants were told to press a button at any point during a 10-second interval, and their pupil sizes correlated with the timing of their decisions. Dilation began about one second before they pressed the button and peaked one to two seconds after.
But are pupils informative outside the lab? Can pupil size be used to "read" a person's intentions and feelings? According to Men's Health magazine a man can tell when it is "time to make your move" by watching his date's pupils, but some skepticism is warranted. "It is unclear to me to what extent this can be exploited in completely unrestrained settings," Einhäuser-Treyer wrote in an e-mail, pointing out that light conditions could easily interfere with amateur attempts at interpersonal pupillometry.
Other efforts to exploit pupil dilations for purposes beyond scientific research have failed. During the Cold War, Canadian government officials tried to develop a device they called the "fruit machine" to detect homosexuality among civil service employees by measuring how the pupils in their eyes responded to racy images of women and men. The machine, which never worked, was to aid the government's purge of gay men and lesbians from the civil service and thereby purportedly reduce vulnerability to Soviet blackmail.
A pupil test for sexual orientation remains as unlikely as it was in the 1960s. Researchers at Cornell University recently showed that sexual orientation correlated with pupil dilation to erotic videos of their preferred gender, but only on average and only for male subjects. Although pupillometry shows promise as a noninvasive measure of sexual response, they concluded, "not every participant’s sexual orientation was correctly classified" and "an observable amount of variability in pupil dilation was unrelated to the participant's sexual orientation."
Pupillometry also became popular in the advertising industry during the 1970s as a way to test consumers' responses to television commercials, says Jagdish Sheth, a marketing professor at Emory University. But the practice was eventually abandoned. "There was no scientific way to establish whether it measured interest or anxiety," Sheth says.