Poets say that the eyes are the windows to the soul. Researchers are finding that they are also windows to our sexual identity. The dilation of pupils in response to erotic stimuli may be the most accurate objective measure of an individual’s sexuality, researchers reported Monday.

The findings confirm a long-held belief among sexual researchers that has apparently not been studied in any depth before. The results provide new insight into the evolutionary development of human sexual responses, suggesting that women may have evolved a more responsive sexuality to help them cope with forced copulation.

Established ways of assessing sexual identity are more invasive. For men, it involves wrapping the penis in a device that measures expansion in response to erotic stimuli. For women, it is the insertion of a vaginal probe that assesses changes in the amplitude of the vaginal pulse.

Many potential participants in studies object to such invasive measures, and some who do participate may actually be seeking illicit pleasure. Some people, moreover, are apparently able to suppress such reactions, clouding results.


Some early researchers had briefly studied pupil dilation as a sexual response, and the Canadian government had even attempted to use it in the 1950s and 1970s to identify homosexuals, who were thought to be a threat to national security.

But scientists have identified a variety of problems with such studies: pupillary measurements were performed by hand, there was no standardized distance between eye and camera, and the brightness of the images viewed was variable, which could easily affect pupil size.

Gerulf Rieger and Ritch C. Savin-Williams of the department of human development at Cornell University in Ithaca, N.Y., decided to use more modern technology to study the effect, relying on an infrared gaze tracker to monitor pupil size. They recruited a total of 165 men and 160 women who identified their sexuality on a seven-point scale. Each was shown two one-minute videos obtained from the Internet, one showing an attractive man masturbating and one an attractive woman doing the same.

Rieger and Savin-Williams reported in the journal PLoS One that heterosexual men showed a much stronger dilation in response to the female image. Heterosexual women, however, showed a response to both images, as did bisexual men, indicating a flexible sexual desire. Homosexual women showed responses more typical of heterosexual men.


The findings support the idea that sexual response has different biological functions in men and women. For men, an important function is to facilitate erection and penetration. For women, the function is to stimulate lubrication and prevent genital injury in case of penetration -- a response that may have developed early in evolutionary history in response to episodes of rape by males.

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