This study investigated whether pupil contagion to pupil change occurs in the faces of other races. In Experiment 1, to investigate this point, we measured 5–6-month-old infants’ pupil-diameter response to the pupil-change direction (dilating/constricting) of upright and inverted other-race faces. We hypothesized that if pupil contagion did not occur in other-race faces, the pupil-diameter response between dilating and contracting pupils would not differ. The results of Experiment 1 showed that 5–6-month-old infants’ pupil-diameter response to dilating pupils was larger than the pupil-diameter response to constricting pupils of the upright and inverted other-race faces. Therefore, 5–6-month-old infants’ pupil contagion to pupil changing occurred on both other-race faces. However, this finding did not support our hypotheses. As pupil contagion to other-race faces occurred in Experiment 1, we further investigated in Experiment 2 whether older infants might show pupil contagion to other-race faces. The results highlighted that 7–8-month-old infants’ diameter response to dilating pupils was larger than the pupil-diameter response to constricting pupils of the upright other-race faces. Hence, 7–8-month-old infants’ pupil contagion to pupil changing had occurred. The outcomes of Experiments 1 and 2 did not support our hypothesis. Additionally, 7–8-month-old infants’ pupil-diameter response between dilating and constricting pupils did not differ in the inverted other-race face. To explain the difference in pupil-diameter response to the inverted other-race face between 5–6-month-old and 7–8-month-old, we conducted a three-way ANOVA, including age groups (5–6-month-old and 7–8-month-old) as a between-subject factor. The findings outlined that in the upright other-race face, 5–6-month-old and 7–8-month-old infants’ diameter response to dilating pupils was larger than the pupil-diameter response to constricting pupils (i.e., pupil contagion had occurred). Regarding inverted other-race faces, 5–6-month-old infants’ diameter response to dilating pupils was larger than the pupil-diameter response to constricting pupils, but 7–8-month-old infants’ diameter response between dilating and constricting pupils did not differ. Thus, 5–6-month-old infants’ pupil contagion occurred in response to inverted other-race faces, but 7–8-month-old infants’ pupil contagion did not occur in response to inverted other-race faces.
In this study, using faces of other faces, 5–6-month-old infants’ pupil contagion occurred in upright and inverted faces. Conversely, 7–8-month-old infants’ pupil contagion did not occur in the inverted face but in the upright face. Our previous study of pupil contagion using own-race faces showed that 5–6-month-old infants’ pupil contagion did not occur in the inverted face but in the upright face, similar to our current 7–8-month-old infants’ results. These findings indicate that pupil contagion depended on face orientation (i.e., pupil contagion did not occur with an inverted face but with an upright face). These results indicate that pupil contagion depended on face orientation; in other words, pupil contagion did not occur with an inverted face but with an upright face. Generally, in the face-inversion effect, task performance is worse for inverted faces. In our previous study, we insisted that the face-inversion effect occurred in pupil contagion to own-race face among 5–6-month-old infants. In the current study, the face-inversion effect occurred in pupil contagion to other-race faces among 7–8-month-old infants. These studies indicated that the face-inversion effect in pupil contagion was observed at different ages between own-race faces (observed in 5–6-month-old) and other-race faces (observed in 7–8-month-old). This age difference suggests that, in pupil contagion, the other-race effect might occur in the face-inversion effect.
Our previous study found that 5–6-month-old infants exhibited pupil contagion to own-race faces when the faces were upright, but not when they were inverted. However, the current study found that 7–8-month-old infants demonstrated pupil contagion to other-race faces when the faces were upright, but not when they were inverted. This indicates that face processing was involved in pupil contagion to own-race faces among 5–6-month-old infants and other-race faces among 7–8-month-old infants. Conversely, face processing did not occur during 5–6-month-old infants’ pupil contagion to other-race faces. Comparing the current study with previous studies, we found that face processing was involved in pupil contagion to own-race faces but not to other-race faces among 5–6-month-old infants. Generally, the task performance of other-race faces is typically poorer than that involving own-race faces. In summary, the results of this study provide evidence of the other-race effect on pupil contagion among 5–6-month-old infants but not among 7–8-month-olds.
The other-species effect1 and other-race effect3, 4 of adults’ pupil mimicry were identified by using only eye region images as stimuli. The other-species effect, also known as the cross-species effect, was observed in the pupil mimicry of both humans and chimpanzees using only eye region images as stimuli1. The researchers measured changes in pupil diameter in response to constricting or dilating pupils and found that pupil sizes synchronized with those of the same species, but not with those of the other species. In subsequent studies, the researchers found evidence of the other-race effect in adults’ pupil mimicry3, 4. In these studies, the researchers measured the pupil size of participants while they played incentivized trust games with virtual partners whose pupils dilated, remained static, or constricted, and the researchers used images of the eyes of both the same and other ethnic groups. The results showed that participants synchronized their pupil size with the dilating pupils, but only when the eyes belonged to someone of the same ethnic group.
Aktar et al.9 investigated the other-race effect in 9-14-month-old infants’ pupil mimicry using eye region images as stimuli, however, they did not find the effect. They measured infants’ and their parents’ pupil diameter response to own-race and other-race eye regions with static, constricting, or dilating pupils. The results showed that both infants and parents responded more to dilating pupils than to static and constricting pupils, regardless of the race of the individual. This indicates that the other-race effect was not observed in the pupil mimicry of both 9-14-month-old infants and their parents in response to the eye region. Our study investigated the other-race effect in 5–6-month-old and 7–8-month-old infants’ pupil contagion by using face images as stimuli. The study identified a significant other-race effect in 5–6-month-old infants’ pupil contagion. The study measured the infants’ pupil diameter response to dilating or constricting pupils in both upright or inverted other-race faces. As indicated above, the results showed that 5–6-month-old infants did not exhibit the face-inversion effect of pupil contagion for other-race faces, unlike 7–8-month-old infants. In contrast, our previous study demonstrated that 5–6-month-old infants did exhibit the face-inversion effect of pupil contagion for own-race faces. The difference in the face-inversion effect of pupil contagion between the own-race and the other-race faces is likely due to the other-race effect of the face. This study is the first to demonstrate the other-race effect of the face on 5–6-month-old infants’ pupil contagion by using face images as stimuli.
The differences in results between our study and Aktar’s study9 may be attributed to differences in stimuli and the information contained in each stimulus. Our study demonstrated that using whole-face images as stimuli induced the other-race effect of pupil contagion in 5–6 month-old infants, while Aktar’s study9 reported that using only the eye region images as stimuli did not induce the other-race effect of pupil contagion in 9–14-month-old infants. Our face image stimuli included multiple morphological features, such as eyes, nose, and mouth, whereas their stimuli only included the single morphological feature of eyes. Since our study’s whole-face stimuli included more morphological features than their eyes region stimuli, it is possible that our stimuli containing greater facial information may have induced the other-race effect of pupil contagion.
We speculate that the other-race effect of pupil contagion in 5–6-month-old infants may have been affected by face processing. Both our and Aktar et al.’s studies9 provided similar results, where the other-race effect of pupil contagion did not occur despite using different stimuli. However, our results indicate that the other-race effect of the face was present. This study investigated the face-inversion effect on pupil contagion and compared the pupil contagion between upright and inverted faces. The results indicated that in upright faces, pupil contagion occurred regardless of the race of the face, in 5–6-month-old infants. However, in inverted faces, pupil contagion did not occur for own-race faces, but it did for other-race faces. This suggests that the face-inversion effect of pupil contagion did not occur in other-race faces and highlights the other-race effect of pupil contagion, which could be influenced by face processing. Previous studies on face recognition24 and identification25 have reported the other-race effect in infants, with better performance observed for own-race faces than other-race faces18. The other-race effect of the face has been observed as early as three months of age18,24 and as late as six months of age25, and the other-race effect of pupil contagion is found almost the same age. In the current study, the other-race effect of pupil contagion disappeared at 7–8 months, unlike the acquisition of the other-race effect of the face.