Reproductive inequality in humans and other mammals.

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 nevertheless falling within the mammalian range. Additionally, female reproductive skew is higher in polygynous human populations than in polygynous nonhumans mammals on average. This patterning of skew can be attributed in part to the prevalence of monogamy in humans compared to the predominance of polygyny in nonhuman mammals, to the limited degree of polygyny in the human societies that practice it, and to the importance of unequally held rival resources to women's fitness. The muted reproductive inequality observed in humans appears to be linked to several unusual characteristics of our species-including high levels of cooperation among males, high dependence on unequally held rival resources, complementarities between maternal and paternal investment, as well as social and legal institutions that enforce monogamous norms.

Why is violence high and persistent in deprived communities? A formal model.

There is massive variation in rates of violence across time and space. These rates are positively associated with economic deprivation and inequality. They also tend to display a degree of local persistence, or 'enduring neighbourhood effects'. Here, we identify a single mechanism that can produce all three observations. We formalize it in a mathematical model, which specifies how individual-level processes generate the population-level patterns. Our model assumes that agents try to keep their level of resources above a 'desperation threshold', to reflect the intuitive notion that one of people's priorities is to always meet their basic needs. As shown in previous work, being below the threshold makes risky actions, such as property crime, beneficial. We simulate populations with heterogeneous levels of resources. When deprivation or inequality is high, there are more desperate individuals, hence a higher risk of exploitation. It then becomes advantageous to use violence, to send a 'toughness signal' to exploiters. For intermediate levels of poverty, the system is bistable and we observe hysteresis: populations can be violent because they were deprived or unequal in the past, even after conditions improve. We discuss implications of our findings for policy and interventions aimed at reducing violence.

Formalizing theories of child development: Introduction to the special section.

Here we introduce a Special Section of Child Development entitled "Formalizing Theories of Child Development." This Special Section features five papers that use mathematical models to advance our understanding of central questions in the study of child development. This landmark collection is timely: it signifies growing awareness that rigorous empirical bricks are not enough; we need solid theory to build the house. By stating theory in mathematical terms, formal models make concepts, assumptions, and reasoning more explicit than verbal theory does. This increases falsifiability, promotes cumulative science, and enables integration with mathematical theory in allied disciplines. The Special Section contributions cover a range of topics: the developmental origins of counting, interactions between mathematics and language development, visual exploration and word learning in infancy, referent identification by toddlers, and the emergence of typical and atypical development. All are written in an accessible manner and for a broad audience.

No effect of hunger on attentional capture by food cues: Two replication studies.

Food cues potently capture human attention, and it has been suggested that hunger increases their propensity to do so. However, the evidence for such hunger-related attentional biases is weak. We focus on one recent study that did show significantly greater attentional capture by food cues when participants were hungry, using an Emotional Blink of Attention (EBA) task [Piech, Pastorino, & Zald, 2010. Appetite, 54, 579-582]. We conducted online (N = 29) and in-person (N = 28) replications of this study with British participants and a Bayesian analytical approach. For the EBA task, participants tried to identify a rotated target image in a Rapid Serial Visual Presentation (RSVP). Targets were preceded by "neutral", "romantic", or "food" distractor images. Participants completed the task twice, 6-11 days apart, once hungry (overnight plus 6h fast) and once sated (after a self-selected lunch in the preceding 1h). We predicted that food images would create a greater attentional blink when participants were hungry than when they were sated, but romantic and neutral images would not. We found no evidence that hunger increased attentional capture by food cues, despite our experiments passing manipulation and quality assurance checks. Our sample and stimuli differed from the study we were replicating in several ways, but we were unable to identify any specific factor responsible for the difference in results. The original finding may not be generalisable. The EBA is more sensitive to the physical distinctiveness of distractors from filler and target images than their emotional valence, undermining the sensitivity of the EBA task for picking up subtle changes in motivational state. Moreover, hunger-related attentional bias shifts may not be substantial over the intensities and durations of hunger typically induced in laboratory experiments.

Food insecurity and patterns of dietary intake in a sample of UK adults.

The aim of this study was to identify the dietary intake correlates of food insecurity (FI) in UK adults. We recruited groups of low-income participants who were classified as food insecure (n 196) or food secure (n 198). Participants completed up to five 24 h dietary recalls. There was no difference in total energy intake by FI status (ßFI = -0·06, 95 % CI - 0·25, 0·13). Food insecure participants consumed a less diverse diet, as evidenced by fewer distinct foods per meal (ßFI = -0·27, 95 % CI - 0·47, -0·07), and had more variable time gaps between meals (ßFI = 0·21, 95 % CI 0·01, 0·41). These associations corresponded closely to those found in a recent US study using similar measures, suggesting that the dietary intake signature of FI generalises across populations. The findings suggest that the consequences of FI for weight gain and health are not due to increased energy intake. We suggest that there may be important health and metabolic effects of temporal irregularity in dietary intake, which appears to be an important component of FI.

Designing trials of Universal Basic Income for health impact: identifying interdisciplinary questions to address.

BACKGROUND: A large body of evidence indicates the importance of upstream determinants to health. Universal Basic Income (UBI) has been suggested as an upstream intervention capable of promoting health by affecting material, biopsychosocial and behavioural determinants. Calls are emerging across the political spectrum to introduce an emergency UBI to address socioeconomic insecurity. However, although existing studies indicate effects on health through cash transfers, UBI schemes have not previously been designed specifically to promote health. METHODS: In this article, we scope the existing literature to set out a set of interdisciplinary research challenges to address in designing a trial of the effectiveness of UBI as a population health measure. RESULTS: We present a theoretical model of impact that identifies three pathways to health impact, before identifying open questions related to regularity, size of payment, needs-based supplements, personality and behaviour, conditionality and duration. CONCLUSIONS: These results set, for the first time, a set of research activities required in order to maximize health impact in UBI programmes.

Measurement of Telomere Length for Longitudinal Analysis: Implications of Assay Precision.

Researchers increasingly wish to test hypotheses concerning the impact of environmental or disease exposures on telomere length (TL), and they use longitudinal study designs to do so. In population studies, TL is usually measured with a quantitative polymerase chain reaction (qPCR)-based method. This method has been validated by calculating its correlation with a gold standard method such as Southern blotting (SB) in cross-sectional data sets. However, in a cross-section, the range of true variation in TL is large, and measurement error is introduced only once. In a longitudinal study, the target variation of interest is small, and measurement error is introduced at both baseline and follow-up. In this paper, we present results from a small data set (n = 20) in which leukocyte TL was measured twice 6.6 years apart by means of both qPCR and SB. The cross-sectional correlations between qPCR and SB were high at both baseline (r = 0.90) and follow-up (r = 0.85), yet their correlation for TL change was poor (r = 0.48). Moreover, the qPCR data but not the SB data showed strong signatures of measurement error. Through simulation, we show that the statistical power gain from performing a longitudinal analysis is much greater for SB than for qPCR. We discuss implications for optimal study design and analysis.

Time perception and patience: individual differences in interval timing precision predict choice impulsivity in European starlings, Sturnus vulgaris.

Impulsivity, in the sense of the extent rewards are devalued as the time until their realization increases, is linked to various negative outcomes in humans, yet understanding of the cognitive mechanisms underlying it is limited. Variation in the imprecision of interval timing is a possible contributor to variation in impulsivity. We use a numerical model to generate predictions concerning the effect of timing imprecision on impulsivity. We distinguish between fixed imprecision (the imprecision that applies even when timing the very shortest time intervals) and proportional imprecision (the rate at which imprecision increases as the interval becomes longer). The model predicts that impulsivity should increase with increasing fixed imprecision, but decrease with increasing proportional imprecision. We present data from a cohort of European starlings (Sturnus vulgaris, n = 28) in which impulsivity had previously been measured through an intertemporal choice paradigm. We tested interval timing imprecision in the same individuals using a tri-peak temporal reproduction procedure. We found repeatable individual differences in both fixed and proportional imprecision. As predicted, birds with greater proportional imprecision in interval timing made fewer impulsive choices, whilst those with greater fixed imprecision tended to make more. Contradictory observations in the literature regarding the direction of association between timing imprecision and impulsivity might be clarified by distinguishing between fixed and proportional components of imprecision.

Rationalization: Why, when, and what for?

In this commentary, we ask when rationalization is most likely to occur and to not occur, and about where to expect, and how to measure, its benefits.

The evolution of life-history theory: a bibliometric analysis of an interdisciplinary research area.

The term 'life-history theory' is a familiar label in several disciplines. Life-history theory has its roots in evolutionary models of the fitness consequences of allocating energy to reproduction, growth and self-maintenance across the life course. Increasingly, the term is also used in the conceptual framing of psychological and social-science studies. As a scientific paradigm expands its range, its parts can become conceptually isolated from one another, even to the point that it is no longer held together by a common core of shared ideas. Here, we investigate the literature invoking the term 'life-history theory' using quantitative bibliometric methods based on patterns of shared citation. Results show that the literature up to and including 2010 was relatively coherent: it drew on a shared body of core references and had only weak cluster divisions running along taxonomic lines. The post-2010 literature is more fragmented: it has more marked cluster boundaries, both between the human and non-human literatures, and within the human literature. In particular, two clusters of human research based on the idea of a fast-slow continuum of individual differences are bibliometrically isolated from the rest. We also find some evidence suggesting a decline over time in the incidence of formal modelling. We point out that the human fast-slow continuum literature is conceptually closer to the non-human 'pace-of-life' literature than it is to the formal life-history framework in ecology and evolution.

Developmental history, energetic state and choice impulsivity in European starlings, Sturnus vulgaris.

Impulsivity-the extent to which a reward is devalued by the amount of time until it is realized-can be affected by an individual's current energetic state and long-term developmental history. In European starlings (Sturnus vulgaris), a previous study found that birds that were lighter for their skeletal size, and birds that had undergone greater shortening of erythrocyte telomeres over the course of development, were more impulsive as adults. Here, we studied the impulsivity of a separate cohort of 29 starlings hand-reared under different combinations of food amount and begging effort. The task involved repeated choice between a key yielding one pellet after 3 s and another key yielding two pellets after 8 s. Impulsivity was operationalised as the proportion of choices for the short-delay option. We found striking variation in impulsivity. We did not replicate the results of the previous study concerning developmental telomere attrition, though combining all the evidence to date in a meta-analysis did support that robustness of that association. We also found that early-life conditions and mass for skeletal size interacted in predicting impulsivity. Specifically, birds that had experienced the combination of high begging effort and low food amount were less impulsive than other groups, and the usual negative relationship between impulsivity and body mass was abolished in birds that had experienced high begging effort. We discuss methodological differences between our study and studies that measure impulsivity using an adjusting-delay procedure.

Developmental history and stress responsiveness are related to response inhibition, but not judgement bias, in a cohort of European starlings (Sturnus vulgaris).

Judgement bias tasks are designed to provide markers of affective states. A recent study of European starlings (Sturnus vulgaris) demonstrated modest familial effects on judgement bias performance, and found that adverse early experience and developmental telomere attrition (an integrative marker of biological age) both affected judgement bias. Other research has shown that corticosterone levels affect judgement bias. Here, we investigated judgement bias using a modified Go/No Go task in a new cohort of starlings (n = 31) hand-reared under different early-life conditions. We also measured baseline corticosterone and the corticosterone response to acute stress in the same individuals. We found evidence for familial effects on judgement bias, of a similar magnitude to the previous study. We found no evidence that developmental treatments or developmental telomere attrition were related to judgement bias per se. We did, however, find that birds that experienced the most benign developmental conditions, and birds with the greatest developmental telomere attrition, were significantly faster to probe the learned unrewarded stimulus. We also found that the birds whose corticosterone levels were faster to return towards baseline after an acute stressor were slower to probe the learned unrewarded stimulus. Our results illustrate the potential complexities of relationships between early-life experience, stress and affectively mediated decision making. For judgement bias tasks, they demonstrate the importance of clearly distinguishing factors that affect patterns of responding to the learned stimuli (i.e. response inhibition in the case of the Go/No Go design) from factors that influence judgements under ambiguity.

Opportunistic food consumption in relation to childhood and adult food insecurity: An exploratory correlational study.

Food insecurity is associated with high body weight for women but not men in affluent Western societies. However, it is not currently known what behavioural or psychological mechanisms drive this association. Moreover, it is also unknown whether only current experience of food insecurity in adulthood is important, or there are lasting effects of childhood experience. We carried out a mock 'taste test' where 126 adult volunteers had the opportunity to consume and rate senergy-dense snack foods. Current food insecurity was measured using the standard USDA measure, and in addition, we used a novel measure that also captures childhood experience of food insecurity. As well as the expected gender-specific association between current food insecurity and body weight, we found some evidence for associations between food insecurity and calorie consumption in the taste test, and liking of one of the foods, chocolate. However, associations between current food insecurity and the outcomes were moderated by childhood experience of food insecurity, with greater childhood food insecurity enhancing the positive effect of current food insecurity on body weight, but attenuating the positive effect of food insecurity on calorie consumption and liking for chocolate. These findings are exploratory, but they suggest that any effects of food insecurity in adulthood on eating and the hedonic value of foods may be moderated by childhood experience.

Echoes of Early Life: Recent Insights From Mathematical Modeling.

In the last decades, developmental origins of health and disease (DOHaD) has emerged as a central framework for studying early-life effects, that is, the impact of fetal and early postnatal experience on adult functioning. Apace with empirical progress, theoreticians have built mathematical models that provide novel insights for DOHaD. This article focuses on three of these insights, which show the power of environmental noise (i.e., imperfect indicators of current and future conditions) in shaping development. Such noise can produce: (a) detrimental outcomes even in ontogenetically stable environments, (b) individual differences in sensitive periods, and (c) early-life effects tailored to predicted future somatic states. We argue that these insights extend DOHaD and offer new research directions.

External-environmental and internal-health early life predictors of adolescent development.

A wealth of evidence documents associations between various aspects of the rearing environment and later development. Two evolutionary-inspired models advance explanations for why and how such early experiences shape later functioning: (a) the external-prediction model, which highlights the role of the early environment (e.g., parenting) in regulating children's development, and (b) the internal-prediction model, which emphasizes internal state (i.e., health) as the critical regulator. Thus, by using data from the NICHD Study of Early Child Care and Youth Development, the current project draws from both models by investigating whether the effect of the early environment on later adolescent functioning is subject to an indirect effect by internal-health variables. Results showed a significant indirect effect of internal health on the relation between the early environment and adolescent behavior. Specifically, early environmental adversity during the first 5 years of life predicted lower quality health during childhood, which then led to problematic adolescent functioning and earlier age of menarche for girls. In addition, for girls, early adversity predicted lower quality health that forecasted earlier age of menarche leading to increased adolescent risk taking. The discussion highlights the importance of integrating both internal and external models to further understand the developmental processes that effect adolescent behavior.

Adaptive principles of weight regulation: Insufficient, but perhaps necessary, for understanding obesity.

We reflect on the major issues raised by a thoughtful and diverse set of commentaries on our target article. We draw attention to the need to differentiate between ultimate and proximate explanation; the insurance hypothesis (IH) needs to be understood as an ultimate-level argument, although we welcome the various suggestions made about proximate mechanisms. Much of this response is concerned with clarifying the interrelationships between adaptationist explanations like the IH, constraint explanations, and dysfunction explanations, in understanding obesity. We also re-examine the empirical evidence base, concurring that it is equivocal and only partially supportive. Several commentators offer additional supporting evidence, whereas others propose alternative explanations for the evidence we reviewed and suggest ways that our current knowledge could be strengthened. Finally, we take the opportunity to clarify some of the assumptions and predictions of our formal model.

Strengths, altered investment, risk management, and other elaborations on the behavioural constellation of deprivation.

We are grateful to have received so many insightful commentaries from interested colleagues regarding our proposed behavioural constellation of deprivation (BCD) and our thoughts on its causes and consequences. In this response article, we offer some clarifications regarding our perspective and tackle some common misperceptions, including, for example, assumptions that the BCD is adaptive and that it should include all behaviours that vary with socioeconomic status. We then welcome some excellent proposals for extensions and modifications of our ideas, such as the conceptualisation of the BCD as a risk-management strategy and the calls for a greater focus on strengths and differential investment rather than deficits and disinvestment. Finally, we highlight some insightful explorations of the implications of our ideas for ethics, policy, and practice.

The behavioural constellation of deprivation: Causes and consequences.

Socioeconomic differences in behaviour are pervasive and well documented, but their causes are not yet well understood. Here, we make the case that a cluster of behaviours is associated with lower socioeconomic status (SES), which we call "the behavioural constellation of deprivation." We propose that the relatively limited control associated with lower SES curtails the extent to which people can expect to realise deferred rewards, leading to more present-oriented behaviour in a range of domains. We illustrate this idea using the specific factor of extrinsic mortality risk, an important factor in evolutionary theoretical models. We emphasise the idea that the present-oriented behaviours of the constellation are a contextually appropriate response to structural and ecological factors rather than a pathology or a failure of willpower. We highlight some principles from evolutionary theoretical models that can deepen our understanding of how socioeconomic inequalities can become amplified and embedded. These principles are that (1) small initial disparities can lead to larger eventual inequalities, (2) feedback loops can embed early-life circumstances, (3) constraints can breed further constraints, and (4) feedback loops can operate over generations. We discuss some of the mechanisms by which SES may influence behaviour. We then review how the contextually appropriate response perspective that we have outlined fits with other findings about control and temporal discounting. Finally, we discuss the implications of this interpretation for research and policy.

Food insecurity as a driver of obesity in humans: The insurance hypothesis.

Integrative explanations of why obesity is more prevalent in some sectors of the human population than others are lacking. Here, we outline and evaluate one candidate explanation, the insurance hypothesis (IH). The IH is rooted in adaptive evolutionary thinking: The function of storing fat is to provide a buffer against shortfall in the food supply. Thus, individuals should store more fat when they receive cues that access to food is uncertain. Applied to humans, this implies that an important proximate driver of obesity should be food insecurity rather than food abundance per se. We integrate several distinct lines of theory and evidence that bear on this hypothesis. We present a theoretical model that shows it is optimal to store more fat when food access is uncertain, and we review the experimental literature from non-human animals showing that fat reserves increase when access to food is restricted. We provide a meta-analysis of 125 epidemiological studies of the association between perceived food insecurity and high body weight in humans. There is a robust positive association, but it is restricted to adult women in high-income countries. We explore why this could be in light of the IH and our theoretical model. We conclude that although the IH alone cannot explain the distribution of obesity in the human population, it may represent a very important component of a pluralistic explanation. We also discuss insights it may offer into the developmental origins of obesity, dieting-induced weight gain, and anorexia nervosa.

Variants at serotonin transporter and 2A receptor genes predict cooperative behavior differentially according to presence of punishment.

Punishment of free-riding has been implicated in the evolution of cooperation in humans, and yet mechanisms for punishment avoidance remain largely uninvestigated. Individual variation in these mechanisms may stem from variation in the serotonergic system, which modulates processing of aversive stimuli. Functional serotonin gene variants have been associated with variation in the processing of aversive stimuli and widely studied as risk factors for psychiatric disorders. We show that variants at the serotonin transporter gene (SLC6A4) and serotonin 2A receptor gene (HTR2A) predict contributions to the public good in economic games, dependent upon whether contribution behavior can be punished. Participants with a variant at the serotonin transporter gene contribute more, leading to group-level differences in cooperation, but this effect dissipates in the presence of punishment. When contribution behavior can be punished, those with a variant at the serotonin 2A receptor gene contribute more than those without it. This variant also predicts a more stressful experience of the games. The diversity of institutions (including norms) that govern cooperation and punishment may create selective pressures for punishment avoidance that change rapidly across time and space. Variant-specific epigenetic regulation of these genes, as well as population-level variation in the frequencies of these variants, may facilitate adaptation to local norms of cooperation and punishment.