Cody Ross

University of California, Davis | UCD · Department of Anthropology

Natural selection should favour litter sizes that optimise trade-offs between brood-size and offspring viability. Across the primate order, the modal litter size is one, suggesting a deep history of selection favouring minimal litters in primates. Humans, however – despite having the longest juvenile period and slowest life-history of all primates...

Humans cooperate in groups in which mutual monitoring is common, and this provides the possibility of third-party arbitration. Third-party arbitration stabilizes reciprocity in at least two ways: first, when it is accurate, it reduces the frequency of misunderstandings resulting from perception errors, and second, even when it is inaccurate, it pro...

The collection of social network data from small-scale communities has been an increasingly important part of long-form research projects on resilience, adaptation, and well-being. Various tools are available to facilitate such data collection protocols. Roster-based methods yield high quality data, but typically scale poorly with sample size. Name...

Formal theoretical models propose that cooperative networks can be maintained when individuals condition behavior on social standing. Here, we empirically examine the predictions of such models of positive and negative indirect reciprocity using a suite of network-structured economic games in four rural Colombian communities ( N i n d = 496 individ...

Although still prevalent in many human societies, the practice of cousin marriage has precipitously declined in populations undergoing rapid demographic and socioeconomic change. However, it is still unclear whether changes in the structure of the marriage pool or changes in the fitness-relevant consequences of cousin marriage more strongly influen...

While it is commonly assumed that farmers have higher, and foragers lower, fertility compared to populations practicing other forms of subsistence, robust supportive evidence is lacking. We tested whether subsistence activities—incorporating market integration—are associated with fertility in 10,250 women from 27 small-scale societies and found con...

In hunter-gatherer societies, women’s subsistence activities are crucial for food provisioning and children’s social learning but are understudied relative to men’s activities. To understand the structure of women’s foraging networks, we present 230 days of focal-follow data in a BaYaka community. To analyze these data, we develop a stochastic bloc...

Natural selection should favor litter sizes that optimize trade-offs between brood-size and offspring viability. Across the primate order, modal litter size is one, suggesting a deep history of selection favoring minimal litters. Humans, however---despite having the longest juvenile period and slowest life-history of all primates---still produce tw...

There have been recent calls for wider application of generative modelling approaches in applied social network analysis. At present, however, it remains difficult for typical end users—for example, field researchers—to implement generative network models, as there is a dearth of openly available software packages that make application of such mode...

To address claims of human exceptionalism, we determine where humans fit within the greater mammalian distribution of reproductive inequality. We show that humans exhibit lower reproductive skew (i.e., inequality in the number of surviving offspring) among males and smaller sex differences in reproductive skew than most other mammals, while neverth...

In small-scale communities, social-scientists can use photo-rosters to collect social network and dyadic peer-ratings data. In past work, we introduced an R package to automate photo-standardization, survey construction, and data-entry. This R package, however, lacked two key features required for fully-unsupervised data-entry. First, respondent ID...

Social network analysis provides an important framework for studying the causes, consequences, and structure of social ties. However, standard self-report measures-for example, as collected through the popular "name-generator" method-do not provide an impartial representation of such ties, be they transfers, interactions, or social relationships. A...

Social network data are often constructed by incorporating reports from multiple individuals. However, it is not obvious how to reconcile discordant responses from individuals. There may be particular risks with multiply reported data if people’s responses reflect normative expectations—such as an expectation of balanced, reciprocal relationships....

Intergroup and long-distance relationships are both central features of human social life, but because intergroup relationships are emphasized in the literature, long-distance relationships are often overlooked. Here, we make the case that intergroup and long-distance relationships should be studied as distinct, albeit related, features of human so...

Friendship is a recurring feature of human sociality. Extant evidence has highlighted several axes upon which the formation and maintenance of friendships rest, and has emphasised the importance of market-like mechanisms and preferential assortment in such dynamics. Such evidence has emerged from qualitative ethnographic descriptions, and observati...

When interacting with infants, humans often alter their speech and song in ways thought to support communication. Theories of human child-rearing, informed by data on vocal signalling across species, predict that such alterations should appear globally. Here, we show acoustic differences between infant-directed and adult-directed vocalizations acro...

Friendship is a recurring feature of human sociality. Extant evidence has highlighted several axes upon which the formation and maintenance of friendships rest, and has emphasised the importance of market-like mechanisms and preferential assortment in such dynamics. Such evidence has emerged from qualitative ethnographic descriptions, and observati...

There have been recent calls for wider application of generative modelling approaches in applied social network analysis. These calls have been motivated by the limitations of contemporary empirical frameworks, which have generally relied on post hoc permutation methods that do not actively account for interdependence in network data. At present, h...

There are compelling reasons to expect that cognitively representing any active, powerful deity motivates cooperative behavior. One mechanism underlying this association could be a cognitive bias toward generally attributing moral concern to anthropomorphic agents. If humans cognitively represent the minds of deities and humans in the same way, and...

Social network data are often constructed by incorporating reports from multiple individuals. However, it is not obvious how to reconcile discordant responses from individuals. There may be particular risks with multiply-reported data if people's responses reflect normative expectations -- such as an expectation of balanced, reciprocal relationship...

While intergroup relationships (IRs) dominate the literature on human sociality, long-distance relationships (LDRs) are also highly prevalent in human social life; however, they are often conflated with IRs or overlooked entirely. We suggest that by focusing on IRs to the exclusion of LDRs, scholars are painting an incomplete picture of human socia...

Parochial altruism, or in-group favoritism at out-group expense, is popularly believed to be a universal in humans—something that characterizes all societies. However, the empirical literature points to considerable variability in the expression of parochial preferences. We argue that greater emphasis should be placed on understanding this variatio...

There are compelling reasons to expect that representing any active, powerful god or spirit may contribute to cooperation. One possible mechanism underlying this effect is a system that infers that spiritual agents are morally concerned. If individuals cognitive represent deities as agents, and if agents are generally conceptualized as having moral...

Cesario et al. argue that benchmarking the relative counts of killings by police on relative crime rates, rather than relative population sizes, generates a measure of racial disparity in the use of lethal force that is unbiased by differential crime rates. Their publication, however, lacked any formal derivation showing that their benchmarking met...

The intensifying pace of research based on cross-cultural studies in the social sciences necessitates a discussion of the unique challenges of multi-sited research. Given an increasing demand for social scientists to expand their data collection beyond WEIRD (western, educated, industrialized, rich, and democratic) populations, there is an urgent n...

As economic games have spread from experimental economics to other social sciences, so too have critiques of their usefulness for drawing inferences about the 'real world'. What these criticisms often miss is that games can be used to reveal individuals' private preferences in ways that observational and interview data cannot; furthermore, economic...

Human adaptation depends on the integration of slow life history, complex production skills, and extensive sociality. Refining and testing models of the evolution of human life history and cultural learning benefit from increasingly accurate measurement of knowledge, skills, and rates of production with age. We pursue this goal by inferring hunters...

A hypothesis for the evolution of long post-reproductive lifespans in the human lineage involves asymmetries in relatedness between young immigrant females and the older females in their new groups. In these circumstances, inter-generational reproductive conflicts between younger and older females are predicted to resolve in favour of the younger f...

We develop a method of analysis for testing the marginal value theorem (MVT) in natural settings that does not require an independent definition or mapping of patches. We draw on recent theoretical work on area-restricted search (ARS) that links turning-angle and step-size changes to geographically localized encounter-rates. These models allow us t...

A hypothesis for the evolution of long post-reproductive lifespans in the human lineage involves asymmetries in relatedness between young immigrant females and the older females in their new groups. In these circumstances, inter-generational reproductive conflicts between younger and older females are predicted to resolve in favor of the younger fe...

Human adaptation depends upon the integration of slow life history, complex production skills, and extensive sociality. Refining and testing models of the evolution of human life history and cultural learning will benefit from increasingly accurate measurement of knowledge, skills, and rates of production with age. We pursue this goal by inferring...

Active search for prey is energetically costly, so understanding how foragers optimize search has been central to foraging theory. Some theoretical work has suggested that foragers of randomly distributed prey should search using Lévy flights, while work on area-restricted and intermittent search strategies has demonstrated that foragers can use th...

Additional methodological details and results of robustness checks. (PDF)

The sub-section 2.1 of the Supplementary Information file in the original article was incorrect. This whole sub-section has now been removed.

Monogamy appears to have become the predominant human mating system with the emergence of highly unequal agricultural populations that replaced relatively egalitarian horticultural populations, challenging the conventional idea-based on the polygyny threshold model-that polygyny should be positively associated with wealth inequality. To address thi...

A theoretical and applied literature has suggested that foragers search using Lévy flights, since Lévy flights can maximize the efficiency of search in the absence of information on the location of randomly distributed prey. Foragers, however, often have available to them at least some information about the distribution of prey, gained either throu...

Abstract Analyses of racial disparities in police use-of-force against unarmed individuals are central to public policy interventions; however, recent studies have come to apparently paradoxical findings concerning the existence and form of such disparities. Although anti-black racial disparities in U.S. police shootings have been consistently docu...

Researchers have recently proposed that “moralistic” religions—those with moral doctrines, moralistic supernatural punishment, and lower emphasis on ritual—emerged as an effect of greater wealth and material security. One interpretation appeals to life history theory, predicting that individuals with “slow life history” strategies will be more attr...

Descriptive demographic statistics, methods, statistical models, main and supplementary analyses. (PDF)

Digging deeper: population-level racial disparities, exposure to police victimisation and psychological trauma - C. T. Ross

This study presents the results from a survey of landowner interest in a propositional payment modality for Costa Rica's Pagos por Servicios Ambientales (PSA) program that is based on a sliding-scale payment scheme. This payment scheme would transition from higher monetary payments per hectare per year for the first units of land invested, to lower...

We present formal evolutionary models for the origins and persistence of the practice of Female Genital Modification (FGMo). We then test the implications of these models using normative cross-cultural data on FGMo in Africa and Bayesian phylogenetic methods that explicitly model adaptive evolution. Empirical evidence provides some support for the...

The main objective of our target article was to sketch the empirical case for the importance of selection at the level of groups on cultural variation. Such variation is massive in humans but modest or absent in other species. Group selection processes acting on this variation is a framework for developing explanations of the unusual level of coope...

Evolutionary ecological models of human fertility predict that (1) parents will bias investment toward the sex with the highest fitness prospects in a particular socio-ecological context; (2) fertility is subject to quantity-quality trade-offs; and (3) fertility decisions will be sensitive to both predictable and stochastic mortality risk and the r...

Human cooperation is highly unusual. We live in large groups composed mostly of non-relatives. Evolutionists have proposed a number of explanations for this pattern, including cultural group selection and extensions of more general processes such as reciprocity, kin selection, and multi-level selection acting on genes. Evolutionary processes are co...

We present a quantitative account based on ethnographic and documentary research of the prevalence of female genital modification (FGMo) in the African diaspora and indigenous populations of Colombia. We use these data to test hypotheses concerning the cultural evolutionary drivers of costly trait persistence, attenuation, and intergroup transmissi...

This essay serves as a response to Hirschfield's (2015) article, “Lethal Policing: Making Sense of American Exceptionalism,” published in the December 2015 issue of this journal.

A geographically-resolved, multi-level Bayesian model is used to analyze the data presented in the U.S. Police-Shooting Database (USPSD) in order to investigate the extent of racial bias in the shooting of American civilians by police officers in recent years. In contrast to previous work that relied on the FBI's Supplemental Homicide Reports that...

Using complementary behavioural and population ecological models, we explore the role of production risk, normal surplus and inter-annual food storage in the adaptations of societies dependent on seasonal agriculture. We find that (a) household-level, risk-sensitive adaption to unpredictable environmental variation in annual agricultural yields is...

Tuberculosis (TB) pathologic lesions in rhesus macaques resemble those in humans. The expression levels of several host TB candidate genes in the peripheral blood mononuclear cells (PBMCs) of six rhesus macaques experimentally infected with Mycobacterium tuberculosis were quantified pre-infection and at several dates post-infection. Quantitative me...

Drawing on Skellam's (1958) work on sampling animal populations using transects, we derive a behavioral ecological model of the choice between sit-and-wait and active-search hunting. Using simple, biologically-based assumptions about the characteristics of predator and prey, we show how an empirically definable parameter space favoring active-searc...

Objectives: We conduct a revaluation of the Thornhill and Fincher research project on parasites using finely-resolved geographic data on parasite prevalence, individual-level sociocultural data, and multilevel Bayesian modeling. In contrast to the evolutionary psychological mechanisms linking parasites to human behavior and cultural characteristic...

We use the Reciprocal Smallest Distance (RSD) algorithm to identify amino acid sequence orthologs in the Chinese and Indian rhesus macaque draft sequences and estimate the evolutionary distance between such orthologs. We then use GOanna to map gene function annotations and human gene identifiers to the rhesus macaque amino acid sequences. We conclu...

Inducible nitric oxide synthase (iNOS) is an enzyme that plays a key role in intracellular immune response against respiratory infections. Since various species of nonhuman primates exhibit different levels of susceptibility to infectious respiratory diseases, and since variation in regulatory regions of genes is thought to play a key role in expre...

In this review, we discuss the dynamic linkages between culture and the genetic evolution of the human species. We begin by briefly describing the framework of gene-culture coevolutionary (or dual-inheritance) models for human evolutionary change. Until recently, the literature on gene-culture coevolution was composed primarily of mathematical mode...

Humans learn and share information on a massive scale through the use of culture. In this paper, we will outline the mechanisms of cultural evolutionary change, evaluate their role in ecologically destructive feedback loops, and conclude by describing how we might harness the mechanisms of cultural evolution to favor ecologically and socially benef...

We compare the effectiveness of short tandem repeat (STR) and single nucleotide polymorphism (SNP) genotypes for estimating pairwise relatedness, using molecular data and pedigree records from a captive Chinese rhesus macaque population at the California National Primate Research Center. We find that a panel of 81 SNPs is as effective at estimating...

We genotyped a Chinese and an Indian-origin rhesus macaque using the Affymetrix Genome-Wide Human SNP Array 6.0 and cataloged 85,473 uniquely mapping heterospecific SNPs. These SNPs were assigned to rhesus chromosomes according to their probe sequence alignments as displayed in the human and rhesus reference sequences. The conserved gene order (syn...

Some breeding facilities in the United States have crossbred Chinese and Indian rhesus macaque (Macaca mulatta) founders either purposefully or inadvertently. Genetic variation that reflects geographic origins among research subjects has the potential to influence experimental outcomes. The use of animals from different geographic regions, their hy...