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Gender Differences in Illusion Response: The Influence of Spatial Strategy and Sex Ratio - Statistical Data Included

Sex Roles: A Journal of Research, Feb, 2001 by R. J. Miller

R. J. Miller [1]

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Two experiments explored factors related to gender differences in Ponzo illusion susceptibility. In Experiment 1, 54 male and 54 female (predominantly white, middle class) undergraduates were administered Witkin's Embedded Figures Test (EFT) and, on 2 separate occasions, a form of the Ponzo illusion. Results showed the Ponzo to be quite reliable over several days. Females were significantly more field dependent (as shown by slower responses to the EFT), and significantly more susceptible to the Ponzo illusion, than males. Furthermore, EFT performance correlated significantly with Ponzo susceptibility for females, but not for males, suggesting that the difference between males and females in Ponzo response may be due not to differences in field independence per se, but rather to differences in the strategies used to solve the illusion task. In Experiment 2, 111 male and 148 female (predominantly white, middle class) undergraduates were administered the Ponzo illusion twice, the 2 administrations separated by a bout 90 mm. Again, the illusion task showed good reliability, and females were significantly more susceptible to the illusion. Furthermore, the magnitude of the difference between males and females was systematically related to the sex ratio (the ratio of the number of males to the number of females) of the particular session in which each subject happened to be participating. It is suggested that social factors such as sex ratio might affect the strategies participants use when doing illusion tasks, and perhaps other spatial skills tasks as well.

Several recent investigations are concerned with the impact of pictorial depth and flatness cues on size constancy (Miller, 1997, 1998, 1999). Much of this research has incorporated the Ponzo illusion, two forms of which are illustrated in Fig. 1. A consistent observation has been that females are, on average, significantly more susceptible than males to the complex Ponzo. That is, females overestimate the length of the top line to a significantly greater extent than do males. This finding was unexpected when it first appeared, as several previous studies had shown no gender differences in Ponzo response (e.g., Brislin & Keating, 1976; Porac, Coren, Girgus, & Verde, 1979; Quina & Pollack, 1972). The apparent discrepancy was resolved by a set of experiments showing that although the gender differences found in the complex form of the illusion are reliable, there are in fact no gender differences to be found using the simple Ponzo, the form used by those other investigators (Miller, 1999).

To explain these findings, one must begin with a consideration of why anyone, male or female, would be fooled by the Ponzo stimulus. Various theories have been advanced, including those based on low-pass filtering (Ginsburg, 1984), assimilation (e.g., Pressey & Epp, 1992), relative size comparisons (Kunnapas, 1955), and tilt constancy (Prinzmetal, Shimamura, & Mikolinski, 2001), among others. One of the most frequently discussed explanations is that the illusion results, at least in part, from depth information contained in the illusion array (e.g., Coren & Girgus, 1978; Gillam, 1980; Leibowitz, Brislin, Perlmutter, & Hennessy, 1969; Miller, 1997; Ward, Porac, Coren, & Girgus, 1977). The best known modern form of this explanation is the inappropriate constancy scaling theory of Gregory (e.g., 1967), a variant of earlier conceptions described by Thiery (1896) and Tausch (1954). The radiating lines are assumed to provide linear perspective, so the top horizontal line is perceived (although not necessarily consc iously) as more distant than the bottom line. The result is an overestimation of the length of the top ("far") line relative to the bottom ("near") line due to misapplication of size constancy. Following this logic, one would assume that the greater the amount of depth information present in the array, the higher the susceptibility observers would show to the illusion. This assumption has been confirmed by several experiments (e.g., Miller, 1997, 1999).

One potential explanation for gender differences in Ponzo susceptibility is that females process depth information differently from males (Miller, 1999). In the simple Ponzo, where there is relatively little depth information to process (i.e., only two radiating lines), males and females do not differ. However, in the complex Ponzo, where there are more radiating lines (and thus more depth information), the difference between males and females in the influence of depth cues emerges, resulting in gender differences in illusion susceptibility.

An alternative explanation is anchored on the idea that the Ponzo illusion is in part a spatial task, and its perception is a reflection of one or more spatial skills. One such skill is field independence, the ability to attend to, unembed, or otherwise perceptually separate simple visual forms that are embedded in a perceptually compelling, more complex visual field. Among the most frequently used measures of field independence are embedded figures tests. Such tests consist of complex geometric figures in which are camouflaged simpler figures. An example of the sort of figure included in such tests is shown in Fig. 2. The participant's task is to ignore the perceptually more demanding complex array and find the simple figure as quickly as possible. The more quickly the participant can find the simple figure, the more field independent he/she is said to be.

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Gender Differences in Illusion Response: The Influence of Spatial Strategy and Sex Ratio - Statistical Data Included

Sex Roles: A Journal of Research, Feb, 2001 by R. J. Miller

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