Human population genetic structure and inference of group membership

A major goal of biomedical research is to develop the capability to provide highly personalized health care. To do so, it is necessary to understand the distribution of interindividual genetic variation at loci underlying physical characteristics, disease susceptibility, and response to treatment. Variation at these loci commonly exhibits geographic structuring and may contribute to phenotypic differences between groups. Thus, in some situations, it may be important to consider these groups separately. Membership in these groups is commonly inferred by use of a proxy such as place-of-origin or ethnic affiliation. These inferences are frequently weakened, however, by use of surrogates, such as skin color, for these proxies, the distribution of which bears little resemblance to the distribution of neutral genetic variation. Consequently, it has become increasingly controversial whether proxies are sufficient and accurate representations of groups inferred from neutral genetic variation. This raises three questions: how many data are required to identify population structure at a meaningful level of resolution, to what level can population structure be resolved, and do some proxies represent population structure accurately? We assayed 100 Alu insertion polymorphisms in a heterogeneous collection of approximately 565 individuals, approximately 200 of whom were also typed for 60 microsatellites. Stripped of identifying information, correct assignment to the continent of origin (Africa, Asia, or Europe) with a mean accuracy of at least 90% required a minimum of 60 Alu markers or microsatellites and reached 99%-100% when >/=100 loci were used. Less accurate assignment (87%) to the appropriate genetic cluster was possible for a historically admixed sample from southern India. These results set a minimum for the number of markers that must be tested to make strong inferences about detecting population structure among Old World populations under ideal experimental conditions. We note that, whereas some proxies correspond crudely, if at all, to population structure, the heuristic value of others is much higher. This suggests that a more flexible framework is needed for making inferences about population structure and the utility of proxies.     

On the perception of religious group membership from faces

Background

The study of social categorization has largely been confined to examining groups distinguished by perceptually obvious cues. Yet many ecologically important group distinctions are less clear, permitting insights into the general processes involved in person perception. Although religious group membership is thought to be perceptually ambiguous, folk beliefs suggest that Mormons and non-Mormons can be categorized from their appearance. We tested whether Mormons could be distinguished from non-Mormons and investigated the basis for this effect to gain insight to how subtle perceptual cues can support complex social categorizations.

Methodology/Principal Findings

Participants categorized Mormons'' and non-Mormons'' faces or facial features according to their group membership. Individuals could distinguish between the two groups significantly better than chance guessing from their full faces and faces without hair, with eyes and mouth covered, without outer face shape, and inverted 180°; but not from isolated features (i.e., eyes, nose, or mouth). Perceivers'' estimations of their accuracy did not match their actual accuracy. Exploration of the remaining features showed that Mormons and non-Mormons significantly differed in perceived health and that these perceptions were related to perceptions of skin quality, as demonstrated in a structural equation model representing the contributions of skin color and skin texture. Other judgments related to health (facial attractiveness, facial symmetry, and structural aspects related to body weight) did not differ between the two groups. Perceptions of health were also responsible for differences in perceived spirituality, explaining folk hypotheses that Mormons are distinct because they appear more spiritual than non-Mormons.

Conclusions/Significance

Subtle markers of group membership can influence how others are perceived and categorized. Perceptions of health from non-obvious and minimal cues distinguished individuals according to their religious group membership. These data illustrate how the non-conscious detection of very subtle differences in others'' appearances supports cognitively complex judgments such as social categorization.     

Racial group membership is associated to gaze-mediated orienting in Italy

Viewing a face with averted gaze results in a spatial shift of attention in the corresponding direction, a phenomenon defined as gaze-mediated orienting. In the present paper, we investigated whether this effect is influenced by social factors. Across three experiments, White and Black participants were presented with faces of White and Black individuals. A modified spatial cueing paradigm was used in which a peripheral target stimulus requiring a discrimination response was preceded by a noninformative gaze cue. Results showed that Black participants shifted attention to the averted gaze of both ingroup and outgroup faces, whereas White participants selectively shifted attention only in response to individuals of their same group. Interestingly, the modulatory effect of social factors was context-dependent and emerged only when group membership was situationally salient to participants. It was hypothesized that differences in the relative social status of the two groups might account for the observed asymmetry between White and Black participants. A final experiment ruled out an alternative explanation based on differences in perceptual familiarity with the face stimuli. Overall, these findings strengthen the idea that gaze-mediated orienting is a socially-connoted phenomenon.     

Oxytocin promotes attention to social cues regardless of group membership

The social saliency account proposes that oxytocin (OT) plays a major role in modulating attentional shifts toward social cues at early stages of processing. We investigated how OT promotes early attention toward nonsocial and social stimuli and explored differences between in-group- and out-group-related social cues. After participants intranasally self-administered OT or placebo, they were eye-tracked while observing a nonsocial and social cues that were assigned to the in- or out-group by a minimal group paradigm. Participants under placebo did not differ in their fixation durations between stimuli, whereas participants administered OT increased gaze durations toward social but not nonsocial stimuli. In this early stage of processing, no in-group bias occurred: in-group- and out-group-related social cues were fixated equally long. These findings support that OT works by a simple illumination of social cues that seem to be processed regardless of social identity aspects at early stages of attention.     

Behavioral effects of changing group membership among captive lowland gorillas

Group membership was modified among 4 gorilla groups at Zoo Atlanta in an attempt to increase compatibility and sexual behavior. The modifications ultimately moved 7 adult females among the groups. The moves were made by individually introducing the new group members to the old members, resulting in 18 total introductions. Several individual and social behaviors differed in the two conditions. The most obvious change was a temporary increase in aggressive display behaviors, as well as an increase in contact and social examination, in the postmovement condition. Additionally, the individual behaviors of travelling and eating increased after the introductions, while resting decreased. Other self-maintenance behaviors did not change. Finally, there was an increase in time mothers and offspring spent closer to one another following the moves. Previous research and anecdotal evidence suggest that gorillas respond to changes in group membership with aggression; these data support and extend those data to different age/sex groups, postmovement trends in behavior, and other aspects of gorilla behavior. © 1996 Wiley-Liss, Inc.     

Behavioral and energetic costs of group membership in a coral reef fish

Animals in social aggregations often spend more time foraging than solitary conspecifics. This may be a product of the relative safety afforded by aggregations: group members can devote more time to foraging and less time to antipredator behaviors than solitary animals (the “risk reduction” effect). All else being equal, risk reduction should result in higher food intake for grouped animals. However, intragroup competition may force group members to spend more time foraging in order to obtain the same food ration as solitary individuals (the “resource competition” effect). We compared these opposing explanations of foraging time allocation in a coral reef fish, bluehead wrasse (Thalassoma bifasciatum). Aggregations of juvenile bluehead wrasse experience safety-in-numbers, and preliminary observations suggested that juveniles in aggregations spent more time foraging for copepods in the water column than solitary juveniles. However, the risk reduction and resource competition hypotheses are indistinguishable on the basis of behavioral observations alone. Therefore, we collected behavioral, dietary, and growth data (using otolith growth rings) for bluehead wrasse at multiple reefs around a Caribbean island. Despite spending more time foraging in the water column, grouped fish did not capture more prey items and had slower growth rates than solitary fish. Thus, the increased foraging time of grouped fish appears to reflect resource competition, not risk reduction. This competition may limit the size and frequency of aggregations among juvenile bluehead wrasse, which have been shown to experience reduced mortality rates in larger groups. Bluehead wrasse recruits also spent less time foraging but grew faster at sites where planktonic copepod prey were more abundant. This suggests the possibility that large-scale spatiotemporal variability in the abundance of planktonic copepods over coral reefs may produce corresponding variability in the dynamics of reef fish populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predicting the temporal dynamics of reproductive skew and group membership in communal breeders

Reproductive skew models attempt to predict the fraction ofreproduction contributed by each individual that participatesin a communal brood. One potential limitation of these modelsis that individuals make a single, fixed decision about groupmembership and reproductive allocation at the beginning of thebreeding period. While this is appropriate for animals thatreproduce in a synchronous bout, many cooperative breeders produceoffspring over a prolonged period of time. It is likely thatthese species adjust reproductive allocation and group membershipover time in response to temporal shifts in group productivityand ecological constraints. In this paper we adapt transactionalmodels of reproductive skew to a continuous form, generatingtime-dependent functions of reproductive allocation. We derivea general method for predicting temporal changes in group membershipas well as a general expression for reproductive skew acrossthe regions over which a group is stable. Using a linear approximationfor time-dependent reproduction, we derive new expressions forreproductive skew in cases where the subordinate departs duringthe breeding period. In this case we find that the traditionalmodel always overestimates the subordinate's share of reproductionwhen dominants are in control of both reproductive shares andgroup membership (i.e., concessions models). Conversely, wefind that the traditional model always underestimates the subordinate'sshare of reproduction when subordinates are in control of reproductiveshares (i.e., constraint models). We discuss the implicationsof these new calculations in relation to the traditional skewmodels and more recent empirical tests of reproductive skewin animal societies.     

Limited indirect fitness benefits of male group membership in a lekking species

In group living species, individuals may gain the indirect fitness benefits characterizing kin selection when groups contain close relatives. However, tests of kin selection have primarily focused on cooperatively breeding and eusocial species, whereas its importance in other forms of group living remains to be fully understood. Lekking is a form of grouping where males display on small aggregated territories, which females then visit to mate. As females prefer larger aggregations, territorial males might gain indirect fitness benefits if their presence increases the fitness of close relatives. Previous studies have tested specific predictions of kin selection models using measures such as group‐level relatedness. However, a full understanding of the contribution of kin selection in the evolution of group living requires estimating individuals' indirect fitness benefits across multiple sites and years. Using behavioural and genetic data from the black grouse (Tetrao tetrix), we show that the indirect fitness benefits of group membership were very small because newcomers joined leks containing few close relatives who had limited mating success. Males' indirect fitness benefits were higher in yearlings during increasing population density but marginally changed the variation in male mating success. Kin selection acting through increasing group size is therefore unlikely to contribute substantially to the evolution and maintenance of lekking in this black grouse population.     

The psychology of deterrence explains why group membership matters for third-party punishment

Humans regularly intervene in others' conflicts as third-parties. This has been studied using the third-party punishment game: A third-party can pay a cost to punish another player (the “dictator”) who treated someone else poorly. Because the game is anonymous and one-shot, punishers are thought to have no strategic reasons to intervene. Nonetheless, punishers often punish dictators who treat others poorly. This result is central to a controversy over human social evolution: Did third-party punishment evolve to maintain group norms or to deter others from acting against one's interests? This paper provides a critical test. We manipulate the ingroup/outgroup composition of the players while simultaneously measuring the inferences punishers make about how the dictator would treat them personally. The group norm predictions were falsified, as outgroup defectors were punished most harshly, not ingroup defectors (as predicted by ingroup fairness norms) and not outgroup members generally (as predicted by norms of parochialism). The deterrence predictions were validated: Punishers punished the most when they inferred that they would be treated the worst by dictators, especially when better treatment would be expected given ingroup/outgroup composition.