People often view certain ways of classifying people (e.g., by gender, race, or ethnicity) as reflecting real distinctions found in nature. Such categories are viewed as marking meaningful, fundamental, and informative differences between distinct kinds of people. The present paper examines the development of these essentialist intuitive theories of how the social world is structured, along with the developmental consequences of these beliefs. We first examine the processes that give rise to social essentialism, arguing that essentialism emerges as children actively attempt to make sense of their environment by relying on several basic representational and explanatory biases. These developmental processes give rise to the widespread emergence of social essentialist views in early childhood, but allow for vast variability across development and cultural contexts in the precise nature of these beliefs. We then examine what is known and still to be discovered about the implications of essentialism for stereotyping, inter-group interaction, and the development of social prejudice. We conclude with directions for future research, particularly on the theoretical payoff that could be gained by including more diverse samples of children in future developmental investigations.

Why do essentialist beliefs promote prejudice? We proposed that essentialist beliefs increase prejudice toward Black people because they imply that existing social hierarchies reflect a naturally occurring structure. We tested this hypothesis in three studies (N ¼ 621). Study 1 revealed that racial essentialism was associated with increased prejudice toward Blacks among both White and Black adult participants, suggesting that essentialism relates to prejudice according to social hierarchy rather than only to group membership. Studies 2 and 3 experimentally demonstrated that increasing essentialist beliefs induced stronger endorsement of social hierarchies in both Black and White participants, which in turn mediated the effect of essentialism on negative attitudes toward Black people. Together, these findings suggest that essentialism increases prejudice toward low-status groups by increasing endorsement of social hierarchies and existing inequality.

Social hierarchies are ubiquitous and determine a range of developmental outcomes, yet little is known about when children develop beliefs about status hierarchies in their communities. The present studies (3.5-6.9 years; N = 420) found that children begin to use gender and race as cues to status in early childhood, but that gender and race related to different status dimensions and had different consequences for inter-group attitudes. Children expected boys to hold higher status as defined by access to resources and decision-making power (e.g., having more toys and choosing what other people play with) but did not expect boys to have more wealth overall. Gender-related status beliefs did not relate to gender-related social preferences; instead, children preferred members of their own gender, regardless of their status beliefs. In contrast, children expected White people to be wealthier than Black people, and among some populations of children, the belief that White people were higher status (as defined by access to resources and decision-making power) weakly related to pro-White bias. Children's statusexpectations about others were unrelated to beliefs about their own status, suggesting children more readily apply category-based status beliefs to others than to themselves.

It is widely believed that race divides the world into biologically distinct kinds of people-an essentialist belief inconsistent with reality. Essentialist views of race have been described as early emerging, but this study found that young children (n = 203, M = 5.45) hold only the more limited belief that the physical feature of skin color is inherited and stable. Overall, children rejected the causal essentialist view that behavioral and psychological characteristics are constrained by an inherited racial essence. Although average levels of children's causal essentialist beliefs about race were low, variation in these beliefs was related to children's own group membership, exposure to diversity, as well as children's own social attitudes.

Understanding the genetic and neuroendocrine basis of the mother-infant bond is critical to understanding mammalian affiliation and attachment. Functionally similar non-synonymous mu-opioid receptor (OPRM1) SNPs have arisen and been maintained in humans (A118G) and rhesus macaques (C77G). In rhesus macaques, variation in OPRM1 predicts individual differences in infant affiliation for mothers. Specifically, infants carrying the G allele show increased distress on separation from their mothers, and spend more time with them upon reunion, than individuals homozygous for the C allele. In humans, individuals possessing the G allele report higher perceptions of emotional pain on receiving rejection by social partners. We studied maternal behavior over the course of a year among free-ranging female rhesus macaques on Cayo Santiago, Puerto Rico. We then trapped females and collected blood samples, from which we assessed OPRM1 genotype; we also collected CSF samples from which we measured oxytocin (OT) levels. We show that females possessing the G allele restrain their infants more (i.e. prevent infants from separating from them by pulling them back) than females homozygous for the C allele. Females possessing the G allele also show higher OT levels when lactating, and lower OT levels when neither lactating nor pregnant, than females homozygous for the C allele. This is the first study to demonstrate an association between OPRM1 genotype and maternal attachment for infants, and is one of the first studies of any free-ranging primate population to link functional genetic variation to behavior via potentially related neuroendocrine mechanisms.