Our study provides the first evidence that plasticity of face recognition for different face types is greater in childhood than in adulthood. Our results demonstrate sensitivity to natural social contact during primary school ages (5–12 years) and, equally important, lack of sensitivity to such contact at later ages (particularly adulthood). These findings were replicated across four independent participant samples. The replications also demonstrated generalisation across multiple situations that included: people raised in countries where their own race is the majority; people raised in countries where their own race is the minority; two races of observers; and faces with larger morphological differences between categories (differing in race) and faces with smaller morphological differences between categories (differing in within-race ethnicity).

Our findings have important implications for a range of theoretical and practical issues. We describe these below, after first dealing with a methodological issue concerning the possibility that our results are limited due to our reliance on retrospective report.

Measuring contact via retrospective report

Where the aim is to assess the effects of natural, social contact at different stages of development, methodological options are limited. Longitudinal approaches are not practical; that is, it is not feasible to track hundreds or thousands of children over, say, 15 years (5 yrs old to 20 yrs old), recording their number of other-race classmates and friends every year, and hoping that the final sample will contain primary school contact that is sufficiently dissociated from later-stage contact to allow age-of-contact effects on adult ORE to be assessed. Thus, prior-life contact was necessarily measured via retrospective self-report.

Two issues concerning these self-report measures then arise: reliability and validity. Concerning reliability (i.e., level of noise in the measurements), it is clear that reliability was adequate. First, all contact measures were able to reveal correlations with other variables (ORE/OEE and/or prejudice). Second, we would expect the oldest recollections of contact to be the most noisy (i.e., primary school), yet even these were quite capable of revealing significant correlations. Third, for the Hancock and Rhodes8 contact questionnaire measure, it is possible to calculate internal reliability because there are multiple items on the scale, and this was high (Cronbach’s alphas = 0.74-0.93 for both primary school and high school; Table S9). Fourth, our Time-in-West measure for Western-raised Asians should have essentially no noise (i.e., international students are well aware of when they first arrived in Australia).

Concerning validity, results confirmed our contact measures correlated with prejudice in the predicted direction, i.e., they reproduced the standard finding that greater other-race contact is associated with reduced prejudice. Additionally, further evidence arguing that contact scores were unlikely to be biased — for example, by participants somehow estimating their previous number of other-race classmates or friends based on their current level of fluency with other-race faces — are the facts that: time periods for reporting contact were chosen to be easily separable in memory; the time periods were not too far in the past (e.g., there seems no reason why 18–22 year olds would fail to directly remember the make-up of their school classes, or who their friends were in school); and, most compellingly, any use of estimation based on current fluency with other-race faces, rather than actual recollection, should have biased all contact measures yet our data show other-race recognition ability was associated only with primary and not secondary or adult contact scores.

Resolving the longstanding conflict concerning contact effects: The importance of asking about the age at which contact occurred

The question of whether other-race face recognition ability is influenced by contact is core to many theories of ORE origin. Yet, previous results vary dramatically across studies (e.g.7,8,9, versus1,10,11,12,13). Our results offer resolution of this longstanding conflict, arguing the confusion arises due the presence of a crucial unmeasured variable, namely the age at which the contact occurred.

Additionally, our results fail to support the proposal5 that the conflicting findings might be due to confounds with quality of the contact. Like several previous studies8,26,27, we find no support for the idea that “high-quality contact” (as assessed here by number of other-race friends, and also the Hancock and Rhodes8 questionnaire) is any more useful in reducing the ORE than mere exposure. Importantly, however, we extend these previous findings to show the conclusion holds for all ages at which contact is experienced: high-quality contact and mere exposure are equally effective in childhood, and equally ineffective in adulthood.

A critical period for face recognition

Comparing our age-of-contact results to previous literature on face plasticity, the question of whether plasticity might reduce as children grow into adults has recently been recognised as being of major theoretical importance15,30. For other-race faces, while it is established that post-infancy childhood plasticity exists (the ORE can be removed by childhood adoption to an other-race country18,19) and that there is some flexibility remaining in adulthood when explicit training is provided (trial-by-trial feedback across several thousand trials can produce some improvement in recognising other-race faces31), it has been difficult to compare the relative degree of plasticity between developmental stages, particularly for natural social exposure which is likely to induce implicit, non-effortful, learning and does not involve explicit training. This issue has arisen partly because previous studies of contact have not recorded contact at different ages. More fundamentally, however, it has arisen because many demographic settings around the world are unsuitable for addressing the age-of-contact question, because contact remains stable across a participant’s lifetime. A key aspect of the present study was the access to many participants for whom the balance of face types to which they are exposed has changed between childhood and adulthood; this allowed us to examine, and reveal differential effects of, contact at different developmental stages.

Our results clearly demonstrate that other-race face recognition is sensitive to contact in primary school, before the age of approximately 12 years. Equally clearly, our results demonstrate that other-race face recognition is not sensitive to contact as an adult after 18 years (even with up to 5 years exposure in a country where the other-race forms the majority of people, Fig. S1). This combination demonstrates a critical period during childhood for easy acquisition of face recognition ability via natural, social exposure in everyday life. It also implies that, by adulthood, any improvement in other-race face coding ability will be difficult and require a different type of learning (e.g., explicit training-with-feedback31).

When does the critical period for face recognition end? Contact in the teenage years

Our data leave open exactly when the sensitive window for natural social contact closes. It is possible that full closure does not occur until sometime during the teenage years. Certainly, the effect of teenage contact on the ORE/OEE was much weaker than the effect of pre-teen contact (Fig. 4), particularly noting that the pre-teen contact correlations are expected to be underestimates of the true strength due to the skewed contact distributions for primary school. However, a small teenage-contact effect cannot be completely ruled out. With N = 223, the total evidence for a contact effect in Fig. 4 sat at p = 0.063. Thus, while we can conclude that by far the greatest sensitivity to natural social contact is prior to 12 years of age, it is also possible that some minor sensitivity remains during the teenage years that might be statistically significant with very large sample sizes (as, indeed, has recently been demonstrated with extremely large sample sizes for language16).

Parallels between faces and language across the full developmental course

Turning to the developmental plasticity of face recognition versus language, our results demonstrate strong similarity between these domains. Putting our own results together with previously-established findings, we conclude face recognition now mirrors language across the full developmental course shown in Fig. 1 (Boxes a–d). In both domains, this pattern can be summarised as: initial broad ability (Box a, e.g., early-infancy discrimination of individual faces for races never experienced); perceptual narrowing across infancy (Box b, loss of this initial discrimination ability without post-birth exposure, e.g., Chinese babies’ loss of early ability to individuate Caucasian faces)32; retained plasticity to easily reverse perceptual narrowing in childhood (Box c); and greatly reduced plasticity in teenage and adult years (Box d).

Note we are not suggesting the time course of sensitive periods is necessarily exactly the same in face recognition as in language; indeed evidence suggests different durations for different aspects of language within infancy14. However, we argue the face recognition system and the language system share a key property of a period of enhanced plasticity, in response to natural real-world experience, earlier in development compared to the adult state.

What ties faces and language together? A shared role in social communication

Why does this similarity between face recognition and language occur? Answering this question requires knowing, first, whether the developmental pattern in Fig. 1 is limited to these domains, or is a general phenomenon that occurs for all visual and auditory stimuli. Evidence argues against domain generality. Domain generality would predict that every box in the Fig. 1 developmental pattern would be found for any stimulus type. Yet, clear violations exist. First, concerning Box a, young-infants’ ability to tell apart individual exemplars of the category even with no previous experience of the category (which is found for phonemes and upright faces) does not occur for upside down faces33, nor for baby strollers34 (babies must be explicitly trained with feedback to tell apart individual strollers34). Further, like faces, humans have extensive everyday visual exposure to hands throughout both infancy (Box b) and childhood (Box c), yet this natural social exposure fails to result in adults developing good ability to discriminate other people from their hands35. Finally, we note that reading—an evolutionarily recent cultural invention which, unlike spoken language, needs to be explicitly taught—produces similar cortical changes regardless of whether it is learned in childhood or as an adult36.

We suggest the developmental similarity between faces and language arises from their shared role in social communication. Successful communication between people requires perceiving who (faces) is signalling what (e.g., via language) to whom (faces again). Critically, all stimuli related to these abilities have been found to follow the infant developmental pattern in Fig. 1. Specifically, innate early-infancy discrimination (Box a) followed by perceptual narrowing (Box b) has been demonstrated for: phonemes14; speech tones14; static faces14; silent talking faces14; visual sign language14; discrimination of individuals by their voice37; and cross-modal matching of individuals’ voices to their faces14. The strong theoretical importance of these infant developmental links has previously been noted14,38. Our present results extend the similarity between face processing and language beyond infancy, to across childhood and adulthood (i.e., Boxes c and d in Fig. 1).

What might help to maintain similarity of development over such a long time-frame? We suggest several factors relevant to social communication might contribute. First, faces and speech share a location in physical space; that is, the voice emerges from part of the face, and so attending to speakers and attending to facial information require attending to the same physical location. Second, like faces, humans use voices as a key way to uniquely identify individual people39. Third, again like faces, humans use speech (e.g., notably accent), as a major way we categorise people into social groups40.

Implications for theories of the ORE

Our findings also have important implications for theories of the ORE. First, our results support a core role for perceptual experience in the origin of the ORE (also see, for example7,8,9,41,42). This rules out pure social motivational theories, that is, any theory in which the ORE is attributed only to social outgrouping, prejudice (explicit or implicit), attitudinal bias, lack of effort, or attention to race-category facial characteristics (e.g., light skin tone, thin lips, and prominent-noses-in-profile for Caucasians) rather than within-race individuating characteristics. Instead, our results argue that only theories that include at least some role for perceptual experience are in the running.

Second, our results argue that all previous theories involving perceptual experience (e.g.5,6,7) require a major modification, namely that it is not lack of perceptual experience per se that results in an ORE, but specifically a lack of childhood perceptual experience.

Third and more broadly, evidence across the literature overall favours a theoretical approach in which the total size of the ORE may be a sum of social-motivation and childhood experience contributions; this would be similar to our Wan et al.7 dual-factors model, with the modification that the experience-only route becomes a route for childhood perceptual experience. In our present cultural setting — namely Asians and Caucasians in a country where these groups are of equal socioeconomic status — findings argue for no social-motivational component to the ORE(7,13,43; plus present result that prejudice did not predict the ORE), meaning that only childhood perceptual experience will contribute. However, in a different cultural setting — USA White observers looking at African-American faces (i.e., groups with large differences in socioeconomic status) — a social-motivation contribution to the ORE has been reported6. In that cultural context, we argue the ORE will include contributions from both social-motivation and childhood experience, meaning that, although the ORE would still be reduced by childhood (and not adult) experience, removing the ORE altogether may require also increasing motivation to individuate the other-race faces.

Practical implications: Encourage early-life exposure, or require difficult, time-consuming training as an adult

In many countries in the modern world, successful social interaction requires being able to recognise any individual, regardless of race44,45. As perceivers, how can we best ensure that we are able to successfully recognise other-race people?

Our results imply that, as with language, by far the easiest method is to obtain everyday exposure as a child in natural social environments. Our results confirm that natural exposure as a young child can produce excellent recognition of other-race individuals. Indeed, with enough exposure within the course of everyday social experience a Caucasian child can become effectively a “native recogniser” of Chinese faces (and vice versa), just as, in language, a young English-speaking child who moves to France can become a native speaker of French.

Equally, however, our results imply that the developmental window for this easy acquisition closes (or at least narrows very substantially) by approximately 12 years of age. Where childhood exposure was low, overcoming poor other-race recognition as an adult is not easy. Importantly, our results show it cannot be achieved simply via increasing everyday exposure, even “high quality” exposure to other-race friends. Instead, as with second-language learning, other-race face learning as an adult is likely to require explicit, time-intensive, and expensive training procedures31.

Thus, to reduce the many negative consequences of poor other-race face recognition — which include implicit racism, wrongful convictions, social interaction difficulties and security failings — other-race experience in childhood should be encouraged.