Human ability to detect kinship in strangers' faces: effects of the degree of relatedness

Multiple mechanisms for inbreeding avoidance used simultaneously in a wild ape

Mating with close kin can have considerable negative fitness consequences, which are expected to result in selective pressure for inbreeding avoidance mechanisms, such as dispersal, mate choice and post-copulatory biases. Captive studies have suggested that inbreeding avoidance through mate choice is far less widespread than expected and may be absent where other mechanisms already limit inbreeding. However, few studies have examined multiple mechanisms of inbreeding avoidance simultaneously, particularly in the wild. We use 13 years of detailed dispersal, copulation and paternity data from mountain gorillas to examine inbreeding avoidance. We find that partial dispersal of both sexes results in high kinship in multimale groups, but that copulations between close kin occur 40% less than expected. We find strong kin discrimination in mate choice, with significant avoidance of maternal kin but more limited avoidance of paternal kin. We find no evidence for post-copulatory inbreeding avoidance. Our analyses support familiarity-based mechanisms of kin identification and age-based avoidance that limits mating between fathers and daughters in their natal group. Our findings demonstrate that multiple complementary mechanisms for inbreeding avoidance can evolve in a single species and suggest that inbreeding avoidance through mate choice may enable more flexible dispersal systems to evolve.

Cooperation and group similarity in children and young adults in the UK

For cooperation to be beneficial, cooperators should be able to differentiate individuals who are willing to cooperate from free-riders. In the absence of kin or of familiar individuals, phenotypic similarity (e.g. in terms of language) can be used as a cue of how likely two or more individuals are to behave similarly (whether they will cooperate or free-ride). Thus, phenotypic similarity could affect cooperation. However, it is unclear whether humans respond to any type of phenotypic similarity or whether only salient phenotypic traits guide cooperation. We tested whether within-group, non-salient phenotypic similarity affects cooperation in 280, 3 to 10 year old children and in 76 young adults (mean 19.8 years old) in the UK. We experimentally manipulated the degree of phenotypic similarity in three computer-based experiments. We found no evidence of a preference for, or greater cooperation with, phenotypically similar individuals in children, even though children displayed ingroup preference. Conversely, young adults cooperated more with phenotypically similar than with phenotypically diverse individuals to themselves. Our results suggest that response to non-salient phenotypic similarity varies with age and that young adults may pay more attention to non-salient cues of diversity then children.

Investigating the detection of parent-child relationships in early childhood: The role of partiality in resource distributions

By early childhood, children possess clear expectations about how resources should be, and typically are, distributed, expecting and advocating for equal resource distributions to recipients. Moreover, recent evidence suggests that children may be able to use deviations from equality in resource distributions to make inferences about the nature of social relationships. Here, we investigated whether children use partiality in resource distributions displayed by adults toward children in third-party contexts to identify parent-child relationships, whether children anticipate preferential treatment based upon knowledge of third-party parent-child relationships, and whether children anticipate different emotional reactions to impartiality in resource distributions in parent-child interactions compared to neighbor-child interactions. Four-to seven-year-old children were presented with hypothetical vignettes about an adult character who distributed resources to two children either equally, or systematically favoring one child. By the age of 4, children used resource distribution partiality to identify an adult as a child’s parent, and also used these expectations to guide their anticipated emotional reactions to impartiality. By the age of 6, children were also more likely to anticipate partiality to be displayed in parent-child compared to neighbor-child relationships. The findings from the current study reveal that partiality in resource distributions acts as a valuable cue to aid in identifying and understanding social relationships, highlighting the integral role that resources play in children’s understanding of their social world. More broadly, our findings support the claim that children use cues that signal interpersonal investment to specify and evaluate parent-child relationships in third-party contexts.

The sibling familiarity effect: Is within-person facial variability shared across siblings?

Humans are experts at familiar face recognition, but poor at unfamiliar face recognition. Familiarity is created when a face is encountered across varied conditions, but the way in which a person's appearance varies is identity-specific, so familiarity with one identity does not benefit recognition of other individuals. However, the faces of biological siblings share structural similarities, so we explored whether the benefits of familiarity are shared across siblings. Results show that familiarity with one half of a sibling pair improves kin detection (experiment 1), and that unfamiliar face matching is more accurate when targets are the siblings of familiar versus unfamiliar individuals (experiment 2). PCA applied to facial images of celebrities and their siblings demonstrates that faces are generally better reconstructed in the principal components of a same-sex sibling than those of an unrelated individual. When we encounter the unfamiliar sibling of someone we already know, our pre-existing representation of their familiar relation may usefully inform processing of the unfamiliar face. This can benefit both kin detection and identity processing, but the benefits are constrained by the degree to which facial variability is shared.

Transforming faces to mimic natural kin: A comparison of different paradigms

The ability to detect phenotypic similarity or kinship in third-parties' faces is not perfect, but better than chance. Still, some humans are better than others at this task. Yet researchers in kinship detection have difficulties in building up large and diverse datasets of high-quality pictures of related persons. The current experiments tested a novel method for circumventing this difficulty by using morphing techniques in order to generate a wide array of stimuli derived from a limited number of individual pictures. Six experiments tested various stimuli (standard protocol, mirrored face, other-sex face, other-ethnicity face, other-expression face and antiface). Our benchmarks are the similarity or kinship scores achieved by participants when faced with pictures of real siblings. We show that all stimuli, except the antiface, elicit detection scores similar to those elicited by real pictures of actual siblings. In addition, by exploring different experiment parameters (simultaneous or sequential task, kinship or similarity task) and some individual characteristics, these experiments provide a better understanding of kinship detection in third parties. The validation of our new method will allow widening the range of available stimuli to the research community, and even to develop new ecologically relevant experimental protocols that are hardly or not feasible with veridical images.

Weighted full binary tree-sliced binary pattern: An RGB-D image descriptor

We present an Algorithm to understand Inter-pixel similarity, which shall be observed in images with the help of a data structure Full Binary Tree. The Full Binary Tree has certain properties like every node must have 2 children or none. Based on this property of Binary Tree, the method of Sliced Binary Pattern is proposed. The inter-pixel similarity may be observed by converting any pixel information of an image within a block of size 3 × 3 to its binarized form, as the pixel information, whose similarity with neighboring pixel cannot be exploited, when it is in decimal form. Thus, we convert all pixel information within a block of size 3 × 3 to its binarized form then we compare the binary pattern of a central pixel with its 8-nearest neighbors. If there is a binary pattern match between central pixel and its 8-nearest neighbors of a block, we assign weights to it, where the weights are determined by the position of match that exist between central pixel and 8-other neighboring pixels of an image. This process helps in determining the inter-pixel similarity of 8-nearest neighbors with respect to central pixel of a block. Every block of 3 × 3 pixels is processed with this strategy to obtain the similarity between patterns in an image. The erected Weighted Full Binary Tree-Sliced Binary Pattern analyzes an image in RGB-Dimensions based on patterns of Inter-Pixel Similarity by tracing the similarity path. The proposed RGB-D texture based inter-pixel similarity addresses the verification of facial similarity. Further, the proposed WFBT-SBP has yielded a good classification accuracy of 77.4%, 77.3%, 77.98%, and 77.94% over a relations of F–S, F-D, M-S, M-D of KinfaceW-I and 76.89%, 76.72%, 77.01%, 76.99% over a relations of F–S, F-D, M-S, and M-D of KinfaceW-II respectively.

Same father, same face: Deep learning reveals selection for signaling kinship in a wild primate

Many animals rely on facial traits to recognize their kin; however, whether these traits have been selected specifically for this function remains unknown. Using deep learning for face recognition, we present the first evidence that interindividual facial resemblance has been selected to signal paternal kinship. Mandrills (Mandrillus sphinx) live in matrilineal societies, in which females spend their entire lives not only with maternal half-sisters (MHS) but also with paternal half-sisters (PHS). We show that PHS have more differentiated social relationships compared to nonkin, suggesting the existence of kin recognition mechanisms. We further demonstrate that facial resemblance increases with genetic relatedness. However, PHS resemble each other visually more than MHS do, despite both kin categories sharing similar degrees of genetic relatedness. This paternally derived facial resemblance among PHS indicates selection to facilitate kin recognition. This study also highlights the potential of artificial intelligence to study phenotypic evolution.

Pleasant body odours, but not genetic similarity, influence trustworthiness in a modified trust game

Identifying trustworthy partners is an important adaptive challenge for establishing mutually cooperative relationships. Previous studies have demonstrated a marked relationship between a person's attractiveness and his apparent trustworthiness (beauty premium). Kin selection theory, however, suggests that cues to kinship enhance trustworthiness. Here we directly tested predictions of the beauty premium and kin selection theory by using body odours as cues to trustworthiness. Body odours reportedly portray information about an individuals' genotype at the human leucocyte antigen system (HLA) and thus olfactory cues in body odours serve as a promising means for kin recognition. Ninety men played trust games in which they divided uneven sums of monetary units between two male trustees represented by their body odour and rated each body odour for pleasantness. Half of the odours came from HLA-similar men (suggesting closer kin) and half from HLA dissimilar men (suggesting non-kin). We found that the amount of money the players transferred was not related to HLA-similarity, but to the pleasantness of the trustee's body odour. By showing that people with more pleasant body odours are trusted more than people with unpleasant body odour we provide evidence for a "beauty-premium" that overrides any putative effect of kin.

On the Complexity of Visual Judgement of Kinship

Discrimination of close relatives is a basic ability of humans, with demonstrated and important consequences in social and sexual behaviours. In this article, we investigate the visual judgement of kinship, that is the process of discriminating relatives based on visual cues and, in particular, on facial resemblance. Starting from triplets of face stimuli, we focus on a simple two-alternative forced choice protocol and we ask participants to evaluate kinship, similarity, or dissimilarity. Response times of the participants performing these visual judgements are recorded and further analysed. The analysis can also benefit from previous findings on the adopted face data set; in particular, results are compared with reference to an independently generated and statistically reliable similarity index, which is available for each possible considered pair of images. Our results confirm previous findings stating that kinship and similarity judgements are closely related and take longer, on average, than dissimilarity judgement. Moreover, they confirm that similarity and dissimilarity cannot be considered just as opposite concepts, and strongly support the existence of different pathways for similarity and dissimilarity judgements. Concerning kinship judgements, results confirm the assumption, inherent in previous models, of a close relationship between cues signalling for kinship and cues signalling for similarity but suggest the existence of a more complex process, where dissimilarity cues need to be explicitly included in order to model measured effects. Our results reinforce the idea that modulation mechanisms between similarity and dissimilarity measures could explain selective suppression or enhancement effects reported in previous works. A new framework is thus proposed hypothesising that kinship recognition is the result of a balanced evaluation of both similar or dissimilar pathways.

Do mothers also "manipulate" grandparental care?

Paternity uncertainty has proven to be a robust ultimate hypothesis for predicting the higher investment in grandchildren observed among maternal grandparents compared to that of the paternal grandparents. Yet the proximate mechanisms for generating such preferred biases in grandparental investment remain unclear. Here we address two different questions for better understanding the proximate mechanisms leading to the observed bias in grandparental investments: (i) is there a larger emphasis on resemblance descriptions (between grandchildren and grandparent) among daughters than among sons, and (ii) do mothers really believe that their offspring more resemble their parents, that is, the children's grandparents, than fathers do? From questioning grandparents, we find that daughters more often and more intensely than sons express opinions about grandchild-grandparent resemblance. Moreover, daughters also seem to believe that their children more resemble their grandmother than sons do. The latter is, however, not the case for beliefs about children's resemblance to grandfathers. In sum, our results suggest that even in a population of Norwegians, strongly influenced by ideas concerning gender equality, there exist a sexual bias among parents in opinions and descriptions about grandchild-grandparent resemblance. This resemblance bias, which echoes that of mothers biasing resemblance descriptions of newborns to putative fathers, does not seem to represent a conscious manipulation. Yet it could be instrumental for influencing grandparental investments. We believe that a "manipulative mother hypothesis" might parsimoniously account for many of the results relating to biased alloparenting hitherto not entirely explained by "the paternity uncertainty hypothesis."

Parent-offspring facial resemblance increases with age in rhesus macaques

Kin recognition is a key ability which facilitates the acquisition of inclusive fitness benefits and enables optimal outbreeding. In primates, phenotype matching is considered particularly important for the recognition of patrilineal relatives, as information on paternity is unlikely to be available via social familiarity. Phenotypic cues to both paternal and maternal relatedness exist in the facial features of humans and other primates. However, theoretical models suggest that in systems with uncertainty parentage it may be adaptive for offspring to conceal such cues when young, in order to avoid potential costs of being discriminated against by unrelated adults. Using experienced human raters, we demonstrate in a computer-based task that detection of parent-offspring resemblances in the faces of rhesus macaques (Macaca mulatta) increases significantly with offspring age. Moreover, this effect is specific to information about kinship, as raters were extremely successful at discriminating individuals even among the youngest animals. To our knowledge, this is the first evidence in non-humans for the age-dependent expression of visual cues used in kin recognition.

Supervised Mixed Norm Autoencoder for Kinship Verification in Unconstrained Videos

Identifying kinship relations has garnered interest due to several applications such as organizing and tagging the enormous amount of videos being uploaded on the Internet. Existing research in kinship verification primarily focuses on kinship prediction with image pairs. In this research, we propose a new deep learning framework for kinship verification in unconstrained videos using a novel Supervised Mixed Norm regularization Autoencoder (SMNAE). This new autoencoder formulation introduces class-specific sparsity in the weight matrix. The proposed three-stage SMNAE based kinship verification framework utilizes the learned spatio-temporal representation in the video frames for verifying kinship in a pair of videos. A new kinship video (KIVI) database of more than 500 individuals with variations due to illumination, pose, occlusion, ethnicity, and expression is collected for this research. It comprises a total of 355 true kin video pairs with over 250,000 still frames. The effectiveness of the proposed framework is demonstrated on the KIVI database and six existing kinship databases. On the KIVI database, SMNAE yields video-based kinship verification accuracy of 83.18% which is at least 3.2% better than existing algorithms. The algorithm is also evaluated on six publicly available kinship databases and compared with best reported results. It is observed that the proposed SMNAE consistently yields best results on all the databases.

Hierarchical Representation Learning for Kinship Verification

Kinship verification has a number of applications such as organizing large collections of images and recognizing resemblances among humans. In this paper, first, a human study is conducted to understand the capabilities of human mind and to identify the discriminatory areas of a face that facilitate kinship-cues. The visual stimuli presented to the participants determine their ability to recognize kin relationship using the whole face as well as specific facial regions. The effect of participant gender and age and kin-relation pair of the stimulus is analyzed using quantitative measures such as accuracy, discriminability index d' , and perceptual information entropy. Utilizing the information obtained from the human study, a hierarchical kinship verification via representation learning (KVRL) framework is utilized to learn the representation of different face regions in an unsupervised manner. We propose a novel approach for feature representation termed as filtered contractive deep belief networks (fcDBN). The proposed feature representation encodes relational information present in images using filters and contractive regularization penalty. A compact representation of facial images of kin is extracted as an output from the learned model and a multi-layer neural network is utilized to verify the kin accurately. A new WVU kinship database is created, which consists of multiple images per subject to facilitate kinship verification. The results show that the proposed deep learning framework (KVRL-fcDBN) yields the state-of-the-art kinship verification accuracy on the WVU kinship database and on four existing benchmark data sets. Furthermore, kinship information is used as a soft biometric modality to boost the performance of face verification via product of likelihood ratio and support vector machine based approaches. Using the proposed KVRL-fcDBN framework, an improvement of over 20% is observed in the performance of face verification.

Self-Resemblance and Social Rejection

Humans perceive and treat self-resembling others in ways that suggest that self-resemblance is a cue of kinship. However, we know little about how individuals respond to treatment by self-resembling others. Here we approach this problem by connecting facial self-resemblance to social rejection. Given that individuals should expect to cooperate with kin, we hypothesized that (1) social inclusion by perceived kin should elicit lesser feelings of rejection and (2) social exclusion by perceived kin should elicit greater feelings of rejection relative to inclusion or exclusion, respectively, by nonkin. To test these hypotheses, we recruited 90 participants to play two games of Cyberball, a virtual ball-tossing game, with separate pairs of ostensible partners. In one game, the ostensible partners were programed to fully include the participants in group play and, in the other game, they were programed to exclude the participants after a few rounds; the order of inclusion and exclusion was counterbalanced across participants. Partner faces were digitally manipulated to be either self- or nonself-resembling, and these conditions were also counterbalanced. Rejection feelings differed significantly as a function of self-resemblance between the inclusion and exclusion conditions, but only for participants who experienced inclusion first. Moreover, for these individuals, inclusion by self-resembling partners led to significantly lesser feelings of rejection than did inclusion by nonself-resembling partners. To explain this effect, we explore potential mechanisms of kin recognition and social rejection. Although nuanced, our results suggest that perceptions of kinship can moderate psychological responses to the actions of others.

Prototype-Based Discriminative Feature Learning for Kinship Verification

In this paper, we propose a new prototype-based discriminative feature learning (PDFL) method for kinship verification. Unlike most previous kinship verification methods which employ low-level hand-crafted descriptors such as local binary pattern and Gabor features for face representation, this paper aims to learn discriminative mid-level features to better characterize the kin relation of face images for kinship verification. To achieve this, we construct a set of face samples with unlabeled kin relation from the labeled face in the wild dataset as the reference set. Then, each sample in the training face kinship dataset is represented as a mid-level feature vector, where each entry is the corresponding decision value from one support vector machine hyperplane. Subsequently, we formulate an optimization function by minimizing the intraclass samples (with a kin relation) and maximizing the neighboring interclass samples (without a kin relation) with the mid-level features. To better use multiple low-level features for mid-level feature learning, we further propose a multiview PDFL method to learn multiple mid-level features to improve the verification performance. Experimental results on four publicly available kinship datasets show the superior performance of the proposed methods over both the state-of-the-art kinship verification methods and human ability in our kinship verification task.

Geometric and Textural Cues for Automatic Kinship Verification

Automatic Kinship verification aims at recognizing the degree of kinship of two individuals from their facial images and it has possible applications in image retrieval and annotation, forensics and historical studies. This is a recent and challenging problem, which must deal with different degrees of kinship and variations in age and gender. Our work explores the computer identification of parent–child pairs using a combination of (i) features of different natures, based on geometric and textural data, (ii) feature selection and (iii) state-of-the-art classifiers. Experiments show that the proposed approach provides a valuable solution to the kinship verification problem, as suggested by its comparison with different methods on the same data and the same experimental protocols. We further show the good generalization capabilities of our method in several cross-database experiments.

Human kin detection

Natural selection has favored the evolution of behaviors that benefit not only one's genes, but also their copies in genetically related individuals. These behaviors include optimal outbreeding (choosing a mate that is neither too closely related, nor too distant), nepotism (helping kin), and spite (hurting non-kin at a personal cost), and all require some form of kin detection or kin recognition. Yet, kinship cannot be assessed directly; human kin detection relies on heuristic cues that take into account individuals' context (whether they were reared by our mother, or grew up in our home, or were given birth by our spouse), appearance (whether they smell or look like us), and ability to arouse certain feelings (whether we feel emotionally close to them). The uncertainties of kin detection, along with its dependence on social information, create ample opportunities for the evolution of deception and self-deception. For example, babies carry no unequivocal stamp of their biological father, but across cultures they are passionately claimed to resemble their mother's spouse; to the same effect, 'neutral' observers are greatly influenced by belief in relatedness when judging resemblance between strangers. Still, paternity uncertainty profoundly shapes human relationships, reducing not only the investment contributed by paternal versus maternal kin, but also prosocial behavior between individuals who are related through one or more males rather than females alone. Because of its relevance to racial discrimination and political preferences, the evolutionary pressure to prefer kin to non-kin has a manifold influence on society at large.

Monkeys spontaneously discriminate their unfamiliar paternal kin under natural conditions using facial cues

Kin recognition can enhance inclusive fitness via nepotism and optimal outbreeding. Mechanisms allowing recognition of patrilineal relatives are of particular interest in species in which females mate promiscuously, leading to paternity uncertainty. Humans are known to detect facial similarities between kin in the faces of third parties, and there is some evidence for continuity of this ability in nonhuman primates . However, no study has yet shown that this propensity translates into an ability to detect one's own relatives, one of the key prerequisites for gaining fitness benefits. Here we report a field experiment demonstrating that free-ranging rhesus macaques (Macaca mulatta) spontaneously discriminate between facial images of their paternal half-siblings and unrelated individuals, when both animals are unfamiliar to the tested individual. Specifically, subjects systematically biased their inspection time toward nonkin when the animals pictured were of their own sex (potential threats), relative to when they were of the opposite sex (potential mates). Our results provide strong evidence for visual phenotype matching and the first demonstration in any primate that individuals can spontaneously detect their own paternal relatives on the basis of facial cues under natural conditions.

Neighborhood repulsed metric learning for kinship verification

Kinship verification from facial images is an interesting and challenging problem in computer vision, and there are very limited attempts on tackle this problem in the literature. In this paper, we propose a new neighborhood repulsed metric learning (NRML) method for kinship verification. Motivated by the fact that interclass samples (without a kinship relation) with higher similarity usually lie in a neighborhood and are more easily misclassified than those with lower similarity, we aim to learn a distance metric under which the intraclass samples (with a kinship relation) are pulled as close as possible and interclass samples lying in a neighborhood are repulsed and pushed away as far as possible, simultaneously, such that more discriminative information can be exploited for verification. To make better use of multiple feature descriptors to extract complementary information, we further propose a multiview NRML (MNRML) method to seek a common distance metric to perform multiple feature fusion to improve the kinship verification performance. Experimental results are presented to demonstrate the efficacy of our proposed methods. Finally, we also test human ability in kinship verification from facial images and our experimental results show that our methods are comparable to that of human observers.

Female rhesus macaques discriminate unfamiliar paternal sisters in playback experiments: support for acoustic phenotype matching

Widespread evidence exists that when relatives live together, kinship plays a central role in shaping the evolution of social behaviour. Previous studies showed that female rhesus macaques (Macaca mulatta) recognize familiar maternal kin using vocal cues. Recognizing paternal kin might, however, be more difficult as rhesus females mate promiscuously during the possible conception period, most probably concealing paternity. Behavioural observations indicate that semi free-ranging female rhesus macaques prefer to associate with their paternal half-sisters in comparison to unrelated females within the same group, particularly when born within the same age cohort. However, the cues and mechanism/s used in paternal kin discrimination remain under debate. Here, we investigated whether female rhesus macaques use the acoustic modality to discriminate between paternal half-sisters and non-kin, and tested familiarity and phenotype matching as the underlying mechanisms. We found that test females responded more often to calls of paternal half-sisters compared with calls of unrelated females, and that this discrimination ability was independent of the level of familiarity between callers and test females, which provides, to our knowledge, the first evidence for acoustic phenotype matching. Our study strengthens the evidence that female rhesus macaques can recognize their paternal kin, and that vocalizations are used as a cue.

The male advantage in child facial resemblance detection: behavioral and ERP evidence

Males have been suggested to have advantages over females in reactions to child facial resemblance, which reflects the evolutionary pressure on males to solve the adaptive paternal uncertainty problem and to identify biological offspring. However, previous studies showed inconsistent results and the male advantage in child facial resemblance perception, as a kin detection mechanism, is still unclear. Here, we investigated the behavioral and brain mechanisms underlying the self-resembling faces processing and how it interacts with sex and age using event-related potential (ERP) technique. The results showed a stable male advantage in self-resembling child faces processing, such that males have higher detectability to self-resembling child faces than females. For ERP results, males showed smaller N2 and larger late positive component (LPC) amplitudes for self-resembling child faces, which may reflect face-matching and self-referential processing in kin detection, respectively. Further source analysis showed that the N2 component and LPC were originated from the anterior cingulate cortex and medial frontal gyrus, respectively. Our results support the male advantage in self-resembling child detection and further indicate that such distinctions can be found in both early and late processing stages in the brain at different regions.

Is beauty in the face of the beholder?

Opposing forces influence assortative mating so that one seeks a similar mate while at the same time avoiding inbreeding with close relatives. Thus, mate choice may be a balancing of phenotypic similarity and dissimilarity between partners. In the present study, we assessed the role of resemblance to Self's facial traits in judgments of physical attractiveness. Participants chose the most attractive face image of their romantic partner among several variants, where the faces were morphed so as to include only 22% of another face. Participants distinctly preferred a "Self-based morph" (i.e., their partner's face with a small amount of Self's face blended into it) to other morphed images. The Self-based morph was also preferred to the morph of their partner's face blended with the partner's same-sex "prototype", although the latter face was ("objectively") judged more attractive by other individuals. When ranking morphs differing in level of amalgamation (i.e., 11% vs. 22% vs. 33%) of another face, the 22% was chosen consistently as the preferred morph and, in particular, when Self was blended in the partner's face. A forced-choice signal-detection paradigm showed that the effect of self-resemblance operated at an unconscious level, since the same participants were unable to detect the presence of their own faces in the above morphs. We concluded that individuals, if given the opportunity, seek to promote "positive assortment" for Self's phenotype, especially when the level of similarity approaches an optimal point that is similar to Self without causing a conscious acknowledgment of the similarity.

Visual phenotype matching: cues to paternity are present in rhesus macaque faces

The ability to recognize kin and thus behaviourally discriminate between conspecifics based on genetic relatedness is of importance both in acquiring inclusive fitness benefits and to enable optimal inbreeding. In primates, mechanisms allowing recognition of paternal relatives are of particular interest, given that in these mating systems patrilineal information is unlikely to be available via social familiarity. Humans use visual phenotype matching based on facial features to identify their own and other's close relatives, and recent studies suggest similar abilities may be present in other species. However it is unclear to what extent familial resemblances remain detectable against the background levels of relatedness typically found within demes in the wild - a necessary condition if facial cues are to function in kin recognition under natural conditions. Here, we experimentally investigate whether parent-offspring relationships are discernible in rhesus macaque (Macaca mulatta) faces drawn from a large free-ranging population more representative of the latter scenario, and in which genetic relatedness has been well quantified from pedigrees determined via molecular markers. We used the human visual system as a means of integrating multiple types of facial cue simultaneously, and demonstrate that paternal, as well as maternal, resemblance to both sons and daughters can be detected even by human observers. Experts performed better than participants who lacked previous experience working with nonhuman primates. However the finding that even naïve individuals succeeded at the task underlines the strength of the phenotypic cues present in faces.

Fathers see stronger family resemblances than non-fathers in unrelated children's faces

Even after they have taken all reasonable measures to decrease the probability that their spouses cheat on them, men still face paternal uncertainty. Such uncertainty can lead to paternal disinvestment, which reduces the children's probability to survive and reproduce, and thus the reproductive success of the fathers themselves. A theoretical model shows that, other things being equal, men who feel confident that they have fathered their spouses' offspring tend to enjoy greater fitness (i.e., leave a larger number of surviving progeny) than men who do not. This implies that fathers should benefit from exaggerating paternal resemblance. We argue that the self-deceiving component of this bias could be concealed by generalizing this resemblance estimation boost to (1) family pairs other than father-child and (2) strangers. Here, we tested the prediction that fathers may see, in unrelated children's faces, stronger family resemblances than non-fathers. In Study 1, 70 men and 70 women estimated facial resemblances between children paired, at three different ages (as infants, children, and adolescents), either to themselves or to their parents. In Study 2, 70 men and 70 women guessed the true parents of the same children among a set of adults. Men who were fathers reported stronger similarities between faces than non-fathers, mothers, and non-mothers did, but were no better at identifying childrens' real parents. We suggest that, in fathers, processing of facial resemblances is biased in a manner that reflects their (adaptive) wishful thinking that fathers and children are related.

Evidence for kinship information contained in the rhesus macaque (Macaca mulatta) face

The ability to recognize kin is an important social skill for primates. Humans are adept at using facial similarity to recognize likely kin, and there is evidence that nonhuman primates are also able to use facial similarity to make judgments about kinship. However, if and how nonhuman primate faces actually contain kinship information remains unclear. To test whether there is objectively measurable facial similarity in related nonhuman primates, we compared facial measurements from related (paternal half-sisters) and unrelated adult female rhesus macaques (Macaca mulatta). Facial measurements were first summarized into 5 factors using a principal component analysis. Differences in these factors between the faces of related macaques were compared with differences between the faces of random unrelated macaques and of age-matched unrelated macaques. The difference in facial measurements between related macaques was significantly smaller than the difference in facial measurements of either group of unrelated macaques, constituting an objective measure of facial similarity in macaque kin. These results indicate that kinship information is contained in the rhesus macaque face and suggest that nonhuman primates may rely in part on facial similarity to distinguish kin.

Kinship recognition by unrelated observers depends on implicit and explicit cognition

Previous studies have shown that neutral observers are able to identify kinship in strangers by matching photographs of children with their parents. We asked whether this ability depended on implicit and/or explicit cognitive processes. Fifty unrelated male observers viewed triads of photographs (one woman in her early 20's and two older women) and had to select which of the two older women was the mother, and rate their confidence in their decision. Observers identified 62.5% of mother-daughter pairs correctly (p < .001). Signal detection analyses showed that confidence was related to accuracy (d' = .28) and observers could report the cues they utilized. However, those who failed to show a relationship between confidence and accuracy (d' ≤ 0) still performed significantly above chance, and both confidence and d' decreased over trials whereas accuracy did not. Results show that neutral observers spontaneously used both explicit and implicit cognitive processes in the task. Recognition of kinship by neutral observers may be a task which allows the interplay between explicit and implicit cognition for a system relevant to ancestral social environments to be observed in the laboratory.

Kinship Recognition by Unrelated Observers Depends on Implicit and Explicit Cognition

Previous studies have shown that neutral observers are able to identify kinship in strangers by matching photographs of children with their parents. We asked whether this ability depended on implicit and/or explicit cognitive processes. Fifty unrelated male observers viewed triads of photographs (one woman in her early 20's and two older women) and had to select which of the two older women was the mother, and rate their confidence in their decision. Observers identified 62.5% of mother-daughter pairs correctly ( p < .001). Signal detection analyses showed that confidence was related to accuracy ( d' = .28) and observers could report the cues they utilized. However, those who failed to show a relationship between confidence and accuracy ( d' ≤ 0) still performed significantly above chance, and both confidence and d' decreased over trials whereas accuracy did not. Results show that neutral observers spontaneously used both explicit and implicit cognitive processes in the task. Recognition of kinship by neutral observers may be a task which allows the interplay between explicit and implicit cognition for a system relevant to ancestral social environments to be observed in the laboratory.

Facial attractiveness: evolutionary based research

Face preferences affect a diverse range of critical social outcomes, from mate choices and decisions about platonic relationships to hiring decisions and decisions about social exchange. Firstly, we review the facial characteristics that influence attractiveness judgements of faces (e.g. symmetry, sexually dimorphic shape cues, averageness, skin colour/texture and cues to personality) and then review several important sources of individual differences in face preferences (e.g. hormone levels and fertility, own attractiveness and personality, visual experience, familiarity and imprinting, social learning). The research relating to these issues highlights flexible, sophisticated systems that support and promote adaptive responses to faces that appear to function to maximize the benefits of both our mate choices and more general decisions about other types of social partners.

Firstborns’ Disadvantage in Kinship Detection

The ability to assess genetic ties is critical to defining one’s own family and, in a broader context, to understanding relationships in groups of strangers. To recognize younger siblings as such, human firstborns can rely on the perinatal association of the mother with her new baby. Later-borns, who cannot rely on such an association, will by necessity actuate alternate strategies, such as recognition of facial clues set aside by firstborns. The effects of such differential early experiences deserve consideration; the development of matching abilities may be used throughout an individual’s lifetime to detect other kinship types outside the family. In simple cognitive tasks based on matching face pictures, later-borns surpassed firstborns in detecting kinship among strangers; this pattern was found in populations of different ages and in two countries. This birth-order effect contrasts with the traditional cognitive advantage of firstborns. Inclusive fitness theory explains how early life history promotes specific strategies that can, in turn, permanently enhance human performance in certain domains.

Perceptual Factors Affecting the Ability to Assess Facial Resemblance between Parents and Newborns in Humans

People undeniably pay attention to faces, and facial resemblance may act as a kinship cue. However, previous studies have shown that the ability to detect kinship through facial resemblance is limited, and it has been suggested that this may be due to several types of perceptual factors. To further understand the processes that underpin kinship judgment, it is important to investigate which perceptual factors predict the probability of parent–child pairs being detected as related. To this end, we performed two experiments. In the first, we evaluated the ability of human observers to match newborns with one of their parents. In the second, we explored three perceptual factors that may have influenced kinship detection (gender discrimination, facial attractiveness, and perceptual similarity). Results showed that the participants were able to match newborns with one of their parents, even though the task was perceived as difficult. Moreover, our study goes further than previous findings, showing that the perceptual factors investigated may significantly contribute to kinship detection.