Ambiguous figures are figures that contain two or more different images and therefore can be perceived in distinct ways. The classic duck-rabbit ambiguous figure [1], for instance, contains an image of a rabbit facing right and an image of a duck facing left. People see only one of these images at a time. Ambiguous figures create amusing visual effects and are therefore often found in popular media to attract and entertain readers. However, they are also used in psychological experiments to study the role of both conscious and subconscious processes in their perception [2, 3, 4, 5].
Nicholls, Churches, and Loetscher [6] used an ambiguous figure to investigate whether high-level social processes subconsciously affect face perception. Following Bar [7], they hypothesized that early visual brain regions pass on a partially analyzed version of a face to the prefrontal cortex where it is subject to social expectations that may influence the percept that is formed in the temporal cortex and any subsequent behavior that is directed at it. To test this hypothesis, Nicholls et al. presented their participants with the ambiguous ‘my wife/mother-in-law’ figure (Fig. 1). This figure, which was originally introduced as a psychological tool by Boring [8], can be perceived as either a young woman facing away from the observer, or as an older lady facing more toward the observer. After a presentation time of 500 ms, which was deemed too short for conscious processes to be of influence, participants were asked to estimate the age of the woman they saw. Nicholls et al. found that older participants estimated the woman in the figure to be significantly older than young people did. On average, participants who were older than 30 estimated the woman to be 6.32 years older than did participants who were 30 years or younger. Nicholls et al. also established a reliable positive relationship between the estimated age and the participants’ own age. Across 393 participants, the correlation between the two variables measured .24.
The findings by Nicholls et al. suggest that the way a face is processed is subconsciously affected by the perceiver’s age. But how does one’s age influence face perception exactly? Nicholls and colleagues interpret their finding in terms of a social group bias. The in-group vs. out-group distinction is a common one in the social sciences. One’s in-group consists of people who one shares similar characteristics with. These characteristics are not shared with members of the out-group. Studies have found that people exhibit a recognition bias toward in-group members. That is, people are better able to recognize individuals from the same ethnicity [9], gender [10], or age [11, 12]. For example, Wright and Stroud [13] showed that an eyewitness is more accurate at identifying the suspect of a crime if they belong to the same age group. The increased recognition accuracy for similarly aged faces, also known as the own-age bias in face perception, is believed to be due to increased contact and familiarity with people of one’s own age group [14, 15, 16], as well as more in depth processing of the faces of the members of one’s age in-group [17; 18, 19, 20]. According to a meta-analysis on the own-age bias in face recognition [21], the way participants study and encode faces does not influence the size of the group difference, suggesting that the bias presents automatically, which is in line with the unconscious age effects on face perception observed by Nicholls and colleagues.
If superior recognition of faces from one’s own age group is at the basis of the finding by Nicholls et al., this would imply that an unconscious social group bias lead participants to recognize the woman in the ambiguous ‘my wife/mother-in-law’ figure who most closely matches their age group, resulting in a higher age estimate by older than by young participants. That is, participants would perceive the ambiguous figure differently, depending on their age. Young participants would be likely to see the young woman facing away from the observer, while older participants would be more inclined to perceive the older lady facing the observer.
A second interpretation does not require the figure that is seen by the participants to differ systematically between age groups. What Nicholls et al. observed might be explained alternatively by the own-age anchor effect. According to Tversky and Kahneman [22], anchor effects occur when people’s estimates assimilate toward a salient value (the anchor) causing their estimates to be inaccurate. In the case of the own-age anchor effect, people use their own age as an anchor, causing age estimates to be biased toward their own age. An early study on the own-age anchor effect conducted by Mintz [23], for example, demonstrated that children estimate the age of the cartoon character Peter Pan to be similar to their own age. The observation that older participants estimate the ambiguous ‘my wife/mother-in-law’ figure to be older than younger participants do, could thus be the result of assimilation of the age estimates to participants’ own age, regardless of the woman that is perceived. That is, both when viewing the young woman and the old lady, participants would make an age estimate that is anchored to their own age, resulting in a mean estimate difference between age groups, as well as a positive correlation between participant age and estimated age.
People’s tendency to assimilate others’ characteristics to their own has received less attention than the own-age social group bias, but empirical evidence supporting the own-age anchor effect is increasing [24, 25, 26, 27, 28]. The basis for the effect is not yet well understood. One reason why one’s own age is selected as a yard stick might be because people usually choose what is highly accessible in memory as their standard [29]. In that case, the own-age anchor effect might have a similar origin as the own-age bias in face recognition: both biases could result from increased familiarity with people of one’s own age [30]. Alternatively, the effect may be the result of a social cost reduction strategy, whereby participants overestimate the age of younger faces and underestimate the age of older faces in order to err on the safe side [28]. The own-age anchor effect is also reminiscent of egocentricity biases, whereby people consider themselves rather than others as a reference point [31], making them judge others to be more similar to themselves than the other way around [32] or project their own properties onto others [33]. Anchoring estimates to salient or normative values might be particularly adaptive in highly uncertain situations, where there is insufficient other information to base one’s judgment on [30]. The brief presentation of an ambiguous figure might therefore constitute a situation in which the own-age anchor effect is likely to present.
The own-age social group bias is different from the own-age anchor effect in that the former is usually considered to be a perceptual bias, while the latter tends to be characterized as a cognitive bias. Extensive experience with faces of our own age group is believed to facilitate their perceptual processing according to the own-age social group bias, while in the case of the own-age anchor effect it is believed to skew our judgments. The own-age social group bias thus predicts that young and older participants are respectively more inclined to see the young woman and the older lady in the ambiguous ’my wife and my mother-in-law’ figure, and that this difference in perception is what is driving the higher age estimates by older than by young participants. Were the relationship between own age and age estimates due to the own-age anchor effect, the ambiguous figure would not need to be perceived systematically different by young and older participants. Participants would assimilate the age of the perceived women to their own, regardless of whether the young woman or older lady is perceived.
We present a study that aims to examine whether the findings by Nicholls et al. can be best explained by the own-age social group bias or the own-age anchor effect. In order to do this, we will replicate the original study as closely as possible, but in addition ask participants to indicate whether they saw the young woman or the older lady. If participants’ age were to affect their perception of the ambiguous ‘my wife and my mother-in-law’ figure, this would constitute evidence that older participants’ higher age estimates of the ambiguous figure result from an own-age social group bias. If not, the own-age anchor effect better explains the difference in the estimates of the ambiguous figure’s age.