Although face recognition is governed by low-level neural detection mechanisms1, it is also affected by seemingly incidental high-level social processes. A good example is the effect of social in- and out-groups. A social in-group is a collection of people with whom a person identifies as a member whereas an out-group is outside one’s identity. Social in- and out-groups can occur along multiple dimensions including race, sexual orientation, and age2. Hugenberg and Corneille3 demonstrated that social groups affect the processing of faces using the composite face paradigm. In this paradigm, participants are shown two faces that are the same at the top (eyes, forehead etc) but different at the bottom (mouth, chin etc). When asked to determine whether the top-halves are identical, participants erroneously indicate that the top halves are different. This effect is thought to reflect a holistic processing strategy where the bottom- and top-halves are processed as a composite. Composite processing can be disrupted by simply shifting the bottom- and top-halves so that they are misaligned – preventing holistic processing.

Hugenberg and Corneille3 used the composite face paradigm to examine holistic processing for faces that were either cast as an in-group (same university) or an out-group (different university). Same and different halves of faces were shown that were either aligned or misaligned from each of the groups. Results revealed a stronger composite face effect (i.e., holistic processing) for the in-group compared to the out-group and it was concluded that in-group faces are processed using a more holistic strategy. In addition to different styles of processing, Sporer4 suggests that group belonging also affects the depth of processing. In this model, in-group faces are thought to automatically receive in-depth processing and are categorised as individuals whereas out-group faces receive relatively limited processing resources and are not individuated.

The effect of in- and out-groups may be particularly relevant to the processing of faces of different ages. A common finding within the field is the own-age bias, where recognition memory is better for faces closer to one’s own age compared to other ages5. A meta-analysis by Rhodes and Anastasi6 revealed superior discrimination for own-age faces in both younger and older participants – though the advantage for own-age faces was weaker for older participants. The weaker effect for older participants is often explained by an exposure effect where older participants are exposed to a wider variety of ages compared to younger participants7. Rhodes and Anastasia6 suggest that these perceptual/learning effects should be integrated with social in- and out-group models to provide a comprehensive explanation of the own-age effect.

While a link between age-related social processes and facial recognition has been established, the level of consciousness at which this link occurs is less clear. The own-age bias for face recognition is classically demonstrated using a learning, retention and recognition paradigm6. In such tasks, participants are aware of the social categories and this knowledge may allow conscious high-level social processes to affect low-level face processing mechanisms (see: Ratner and Amodio8). The level of consciousness required for processing in- and out-group faces has been explored by Van Bavel, Packer and Cunningham9. They used fMRI to measure brain activity as participants made group-identity classifications for faces. On some trials, participants made explicit judgements related to group identity whereas judgements were made on an orthogonal dimension of race for other trials. Results revealed that the same neural centres were engaged irrespective of whether the group identity was overt or covert – suggesting an automatic, sub-conscious process.

Perceptual tasks, such as ambiguous figures, can also be used to investigate the level of conscious processing. Indeed, because reversals between different interpretations of a figure occur at different locations and levels of complexity within the visual processing system, they may be ideally suited to investigate the interplay between different levels of processing10. Balcetis and Dunning11 used ambiguous figures to investigate whether motivational states affect preconscious processing of visual stimuli. They assigned the two alternate perceptions of an ambiguous figure (whether a ‘I3’ is viewed as a ‘B’ or as ‘13’) with a positive or negative outcome and found that participants subconsciously perceived the version of the image that produced a positive outcome. Similarly, Van de Cruys, Schouten and Wagemans12 presented participants with ambiguous human light-point walkers, whose movement can be interpreted as either facing towards or away from the observer. They found that socially anxious participants were more likely to report the figure as facing away from them and therefore suggested that trait emotion can bias perception (also see Brugger & Brugger13).

Ambiguous figures provide an ideal means for exploring whether social/cognitive states can affect perception at a subconscious level. Given this utility, the current study used ambiguous stimuli to examine whether own-age social biases affect the basic sensory processing of faces. To do this, we used a well-known ambiguous image. The “my wife and my mother-in-law” illusion was introduced to the psychological literature by Boring14 and alternates between the perception of a young or an old woman (See Fig. 1). The young/old woman illusion was administered to a large sample of people of differing ages using Mechanical Turk. To ensure that conscious processes would have little impact on the perceptual task, the stimulus was only shown once to each participant for half a second. Participants were subsequently asked how old the woman was and it was predicted that participants would report their respective in-groups, with younger participants more predisposed to report a young woman and older participants more predisposed to report an old woman. It is likely, however, that the difference between younger and older respondents will not be bimodal. There is an overall bias to report the younger woman15 and it is also likely that older respondents will often report seeing a younger woman, in line with the weaker own-age effect for older participants reported by Rhodes and Anastasia6. It is therefore expected that the range of reported ages will be larger for older compared to younger respondents.

Figure 1 My Wife and My Mother-In-Law, by the cartoonist W. E. Hill, 1915. This media file is in the public domain in the United States. This applies to U.S. works where the copyright has expired, often because its first publication occurred prior to January 1, 1923. Full size image

The initial sample from Mechanical Turk included participants from around the World, with the largest numbers coming from the USA and India, who are both prolific users of Mechanical Turk16. Initial checks of task compliance (see methods for details) revealed that a significant proportion of participants (55%) from India were not able to complete the task. The sample was therefore limited to respondents from the USA. The advantage of this sample is that it is relatively homogenous and also that Americans generally have more negative views toward ageing compared to Indians – especially in relation to socio-emotional processes17. In a study examining cultural differences in children’s attitudes towards the elderly, Zandi, Mirle and Jarvis18 wrote: “In the Western world, old age has often been conceived of as a period of life without meaningful roles” (p. 163). Bearing these socio-cultural practices in mind, we chose to use a sample from the USA, which would be most likely to yield an own-age effect.