Correction: Spatial regulation of Drosophila ovarian Follicle Stem Cell division rates and cell cycle transitions.

[This corrects the article DOI: 10.1371/journal.pgen.1010965.].

Reputations for treatment of outgroup members can prevent the emergence of political segregation in cooperative networks.

Reputation systems promote cooperation and tie formation in social networks. But how reputations affect cooperation and the evolution of networks is less clear when societies are characterized by fundamental, identity-based, social divisions like those centered on politics in the contemporary U.S. Using a large web-based experiment with participants (N = 1073) embedded in networks where each tie represents the opportunity to play a dyadic iterated prisoners' dilemma, we investigate how cooperation and network segregation varies with whether and how reputation systems track behavior toward members of the opposing political party (outgroup members). As predicted, when participants know others' political affiliation, early cooperation patterns show ingroup favoritism. As a result, networks become segregated based on politics. However, such ingroup favoritism and network-level political segregation is reduced in conditions in which participants know how others behave towards participants from both their own party and participants from the other party. These findings have implications for our understanding of reputation systems in polarized contexts.

Spatial regulation of Drosophila ovarian Follicle Stem Cell division rates and cell cycle transitions.

Drosophila ovarian Follicle Stem Cells (FSCs) present a favorable paradigm for understanding how stem cell division and differentiation are balanced in communities where those activities are independent. FSCs also allow exploration of how this balance is integrated with spatial stem cell heterogeneity. Posterior FSCs become proliferative Follicle Cells (FCs), while anterior FSCs become quiescent Escort Cells (ECs) at about one fourth the frequency. A single stem cell can nevertheless produce both FCs and ECs because it can move between anterior and posterior locations. Studies based on EdU incorporation to approximate division rates suggested that posterior FSCs divide faster than anterior FSCs. However, direct measures of cell cycle times are required to ascertain whether FC output requires a net flow of FSCs from anterior to posterior. Here, by using live imaging and FUCCI cell-cycle reporters, we measured absolute division rates. We found that posterior FSCs cycle more than three times faster than anterior FSCs and produced sufficient new cells to match FC production. H2B-RFP dilution studies supported different cycling rates according to A/P location and facilitated live imaging, showing A/P exchange of FSCs in both directions, consistent with the dynamic equilibrium inferred from division rate measurements. Inversely graded Wnt and JAK-STAT pathway signals regulate FSC differentiation to ECs and FCs. JAK-STAT promotes both differentiation to FCs and FSC cycling, affording some coordination of these activities. When JAK-STAT signaling was manipulated to be spatially uniform, the ratio of posterior to anterior division rates was reduced but remained substantial, showing that graded JAK-STAT signaling only partly explains the graded cycling of FSCs. By using FUCCI markers, we found a prominent G2/M cycling restriction of posterior FSCs together with an A/P graded G1/S restriction, that JAK-STAT signaling promotes both G1/S and G2/M transitions, and that PI3 kinase signaling principally stimulates the G2/M transition.

Piecewise Structural Equation Modeling of the Quantity Implicature in Child Language.

We review an array of experimental methodological factors that either contribute to or detract from the measurement of pragmatic implicatures in child language. We carry out a truth value judgment task to measure children's interpretations of the Spanish existential quantifier algunos in implicature-consistent and implicature-inconsistent contexts. Independently, we take measures of children's inhibition, working memory, attention, approximate number ability, phrasal syntax, and lexicon. We model the interplay of these variables using a piecewise structural equation model (SEM), common in the life sciences, but not in the social and behavioral sciences. By 6 years of age, the children in our sample were not statistically different from adults in their interpretations. Syntax, lexicon, and inhibition significantly predict implicature generation, each accounting for unique variance. The approximate number system and inhibition significantly predict lexical development. The statistical power of the piecewise SEM components, with a sample of 64 children, is high, in comparison to a traditional, globally estimated SEM of the same data.

Inequality and cooperation in social networks.

Social networks are fundamental to the broad scale cooperation observed in human populations. But by structuring the flow of benefits from cooperation, networks also create and sustain macro-level inequalities. Here we ask how two aspects of inequality shape the evolution of cooperation in dynamic social networks. Results from a crowdsourced experiment (N = 1080) show that inequality alters the distribution of cooperation within networks such that participants engage in more costly cooperation with their wealthier partners in order to maintain more valuable connections to them. Inequality also influences network dynamics, increasing the tendency for participants to seek wealthier partners, resulting in structural network change. These processes aggregate to alter network structures and produce greater system-level inequality. The findings thus shed critical light on how networks serve as both boon and barrier to macro-level human flourishing.

Opposing JAK-STAT and Wnt signaling gradients define a stem cell domain by regulating differentiation at two borders.

Many adult stem cell communities are maintained by population asymmetry, where stochastic behaviors of multiple individual cells collectively result in a balance between stem cell division and differentiation. We investigated how this is achieved for Drosophila Follicle Stem Cells (FSCs) by spatially-restricted niche signals. FSCs produce transit-amplifying Follicle Cells (FCs) from their posterior face and quiescent Escort Cells (ECs) to their anterior. We show that JAK-STAT pathway activity, which declines from posterior to anterior, dictates the pattern of divisions over the FSC domain, promotes more posterior FSC locations and conversion to FCs, while opposing EC production. Wnt pathway activity declines from the anterior, promotes anterior FSC locations and EC production, and opposes FC production. The pathways combine to define a stem cell domain through concerted effects on FSC differentiation to ECs and FCs at either end of opposing signaling gradients, and impose a pattern of proliferation that matches derivative production.

Adult organisms contain a variety of cells that are routinely replaced using adult stem cells which can generate the cells of a specific tissue. These stem cells are often clustered into small groups, where combinations of chemical signals from nearby cells can encourage each stem cell to divide or 'differentiate' into another type of cell. These different signals must somehow balance stem cell division and differentiation to maintain the size and shape of the community. The ovary of an adult fruit fly contains a group of adult stem cells called follicle stem cells, or FSCs for short. FSCs support the continual production of eggs by supplying two types of cell from opposite faces of the stem cell cluster: dividing follicle cells emerge from the back of the cluster and guide late egg development, while non-dividing escort cells come from the front and guide early egg development. Two of the signals that control FSCs are graded over the cluster. JAK-STAT signaling is strongest in the follicle cell territory and gradually declines towards the front, while Wnt signaling is strongest in escort cells and absent from early follicle cells. However, it was unclear how the gradients of these two signals maintain the FSC population and control the formation of follicle and escort cells. To answer this question, Melamed and Kalderon used genetic engineering to modify the strength of these two signals. The experiments measured how this affected the rate at which FSCs divide and are converted into follicle or escort cells. Melamed and Kalderon found that the strength of JAK-STAT signaling dictated division rates, which may explain why the rate cells divide varies across the FSC cluster and escort cells do not divide at all. JAK-STAT signaling also stimulated FSCs to become follicle cells and opposed their conversion to escort cells. Conversely, stronger Wnt signaling favored the production of escort cells and inhibited FSCs from transitioning to follicle cells. This suggests that the relative strength of these two opposing signals helps maintain thecorrect number of FSCs while also balancing the formation of follicle and escort cells. JAK-STAT, Wnt and other signals guide the development of many organisms, including humans, and have also been linked to cancer. Therefore, the principles and mechanisms uncovered may apply to other types of stem cells. Furthermore, this work highlights genetic changes that can allow a mutant stem cell to amplify and take over an entire stem cell community, which may play a role in cancer and other illnesses.

The robustness of reciprocity: Experimental evidence that each form of reciprocity is robust to the presence of other forms of reciprocity.

Prosocial behavior is paradoxical because it often entails a cost to one's own welfare to benefit others. Theoretical models suggest that prosociality is driven by several forms of reciprocity. Although we know a great deal about how each of these forms operates in isolation, they are rarely isolated in the real world. Rather, the topological features of human social networks are such that people are often confronted with multiple types of reciprocity simultaneously. Does our current understanding of human prosociality break down if we account for the fact that the various forms of reciprocity tend to co-occur in nature? Results of a large experiment show that each basis of human reciprocity is remarkably robust to the presence of other bases. This lends strong support to existing models of prosociality and puts theory and research on firmer ground in explaining the high levels of prosociality observed in human social networks.

Status and competitive choice.

In this paper, we extend the logic of existing sociological theory on status to explain how status processes can inform selection in competitive choice situations. We argue that in the absence of knowledge about the specific abilities of others and assuming a desire to win, when given the opportunity to "pick their battles," people will draw on overt status differences as a basis for selecting a competitor from a pool of possible competitors. Results from three studies indicate that, as predicted, status differences affect competitor selection, with individuals choosing to compete against those who are relatively lower status based on diffuse characteristics. Moreover, consistent with expectation state theories, results from two studies show that the expectations that people form for their potential competitors based on status differences mediate this relationship. We conclude by discussing the implications of this research.

Precluding rare outcomes by predicting their absence.

Forecasting extremely rare events is a pressing problem, but efforts to model such outcomes are often limited by the presence of multiple causes within classes of events, insufficient observations of the outcome to assess fit, and biased estimates due to insufficient observations of the outcome. We introduce a novel approach for analyzing rare event data that addresses these challenges by turning attention to the conditions under which rare outcomes do not occur. We detail how configurational methods can be used to identify conditions or sets of conditions that would preclude the occurrence of a rare outcome. Results from Monte Carlo experiments show that our approach can be used to systematically preclude up to 78.6% of observations, and application to ground-truth data coupled with a bootstrap inferential test illustrates how our approach can also yield novel substantive insights that are obscured by standard statistical analyses.

The strength of dynamic ties: The ability to alter some ties promotes cooperation in those that cannot be altered.

Dynamic networks, where ties can be shed and new ties can be formed, promote the evolution of cooperation. Yet, past research has only compared networks where all ties can be severed to those where none can, confounding the benefits of fully dynamic networks with the presence of some dynamic ties within the network. Further, humans do not live in fully dynamic networks. Instead, in real-world networks, some ties are subject to change, while others are difficult to sever. Here, we consider whether and how cooperation evolves in networks containing both static and dynamic ties. We argue and find that the presence of dynamic ties in networks promotes cooperation even in static ties. Consistent with previous work demonstrating that cooperation cascades in networks, our results show that cooperation is enhanced in networks with both tie types because the higher rate of cooperation that occurs following the dynamics process "spills over" to those relations that are more difficult to alter. Thus, our findings demonstrate the critical role that dynamic ties play in promoting cooperation by altering behavioral outcomes even in non-dynamic relations.

Division-independent differentiation mandates proliferative competition among stem cells.

Cancer-initiating gatekeeper mutations that arise in stem cells would be especially potent if they stabilize and expand an affected stem cell lineage. It is therefore important to understand how different stem cell organization strategies promote or prevent variant stem cell amplification in response to different types of mutation, including those that activate proliferation. Stem cell numbers can be maintained constant while producing differentiated products through individually asymmetrical division outcomes or by population asymmetry strategies in which individual stem cell lineages necessarily compete for niche space. We considered alternative mechanisms underlying population asymmetry and used quantitative modeling to predict starkly different consequences of altering proliferation rate: A variant, faster proliferating mutant stem cell should compete better only when stem cell division and differentiation are independent processes. For most types of stem cells, it has not been possible to ascertain experimentally whether division and differentiation are coupled. However, Drosophila follicle stem cells (FSCs) provided a favorable system with which to investigate population asymmetry mechanisms and also for measuring the impact of altered proliferation on competition. We found from detailed cell lineage studies that division and differentiation of an individual FSC are not coupled. We also found that FSC representation, reflecting maintenance and amplification, was highly responsive to genetic changes that altered only the rate of FSC proliferation. The FSC paradigm therefore provides definitive experimental evidence for the general principle that relative proliferation rate will always be a major determinant of competition among stem cells specifically when stem cell division and differentiation are independent.

Cooperation, clustering, and assortative mixing in dynamic networks.

Humans' propensity to cooperate is driven by our embeddedness in social networks. A key mechanism through which networks promote cooperation is clustering. Within clusters, conditional cooperators are insulated from exploitation by noncooperators, allowing them to reap the benefits of cooperation. Dynamic networks, where ties can be shed and new ties formed, allow for the endogenous emergence of clusters of cooperators. Although past work suggests that either reputation processes or network dynamics can increase clustering and cooperation, existing work on network dynamics conflates reputations and dynamics. Here we report results from a large-scale experiment (total n = 2,675) that embedded participants in clustered or random networks that were static or dynamic, with varying levels of reputational information. Results show that initial network clustering predicts cooperation in static networks, but not in dynamic ones. Further, our experiment shows that while reputations are important for partner choice, cooperation levels are driven purely by dynamics. Supplemental conditions confirmed this lack of a reputation effect. Importantly, we find that when participants make individual choices to cooperate or defect with each partner, as opposed to a single decision that applies to all partners (as is standard in the literature on cooperation in networks), cooperation rates in static networks are as high as cooperation rates in dynamic networks. This finding highlights the importance of structured relations for sustained cooperation, and shows how giving experimental participants more realistic choices has important consequences for whether dynamic networks promote higher levels of cooperation than static networks.

Alternative direct stem cell derivatives defined by stem cell location and graded Wnt signalling.

Adult stem cells provide a renewable source of differentiated cells for a wide variety of tissues and generally give rise to multiple cell types. Basic principles of stem cell organization and regulation underlying this behaviour are emerging. Local niche signals maintain stem cells, while different sets of signals act outside the niche to diversify initially equivalent stem cell progeny. Here we show that Drosophila ovarian follicle stem cells (FSCs) produced two distinct cell types directly. This cell fate choice was determined by the anterior-posterior position of an FSC and by the magnitude of spatially graded Wnt pathway activity. These findings reveal a paradigm of immediate diversification of stem cell derivatives according to stem cell position within a larger population, guided by a graded niche signal. We also found that FSCs strongly resemble mammalian intestinal stem cells in many aspects of their organization, including population asymmetry and dynamic heterogeneity.

Prosocial Orientation Alters Network Dynamics and Fosters Cooperation.

Dynamic networks have been shown to increase cooperation, but prior findings are compatible with two different mechanisms for the evolution of cooperation. It may be that dynamic networks promote cooperation even in networks composed entirely of egoists, who strategically cooperate to attract and maintain profitable interaction partners. Alternatively, drawing on recent insights into heterogeneous social preferences, we expect that dynamic networks will increase cooperation only when nodes are occupied by persons with more prosocial preferences, who tend to attract and keep more cooperative partners relative to egoists. Our experiment used a standard procedure to classify participants a priori as egoistic or prosocial and then embedded them in homogeneous networks of all prosocials or all egoists, or in heterogeneous networks (50/50). Participants then interacted in repeated prisoner's dilemma games with alters in both static and dynamic networks. In both heterogeneous and homogeneous networks, we find dynamic networks only promote cooperation among prosocials. Resulting from their greater cooperation, prosocials' relations are more stable, yielding substantially higher fitness compared to egoists in both heterogeneous and homogeneous dynamic networks. Our results suggest that a key to the evolution and stability of cooperation is the ability of those with prosocial preferences to alter their networks.

The Effects of Stability and Presentation Order of Rewards on Justice Evaluations.

Justice research has evolved by elucidating the factors that affect justice evaluations, as well as their consequences. Unfortunately, few researchers have paid attention to the pattern of rewards over time as a predictor of justice evaluations. There are two main objectives of this research. First, it aims to test the effect of reward stability on justice evaluations. Based on justice theory and prospect theory, we assume that an under-reward at one time cannot be fully offset by an equivalent over-reward at another time. Therefore, in unstable reward systems the asymmetry of the effect of unjust rewards with opposite directions will produce a lower level of justice evaluations over time. The second objective of this research is to show the moderating effect of the presentation order (primacy vs. recency) of unstable rewards on justice evaluations. The results from a controlled experiment with five conditions, which presents the instability of rewards in different orders, confirm both the negative effect of unstable rewards and the stronger effect of primacy on justice evaluations.

Status, Faction Sizes, and Social Influence: Testing the Theoretical Mechanism.

With two experiments the authors test and find support for the argument that in small, collectively oriented task groups, status affects social influence the most when the distribution of opinions reduces the least uncertainty. Moreover, they demonstrate that people use the distribution of both status and opinions to reduce uncertainty about the task on which they are working and that this, in turn, promotes social influence. Experiment 1 illustrates that, regardless of the group's sex composition, basis for status differentiation, or size of the group, uncertainty reduction mediates a significant share of the effect of status and opinions on social influence. Experiment 2 confirms that the effect of the distribution of both status and opinions on social influence is weaker as the task becomes more certain. These findings inform discussion about how status affects certainty in task groups and what this potentially means for organizational settings and sociological theory more generally.

Community structures in bipartite networks: a dual-projection approach.

Identifying communities or clusters in networked systems has received much attention across the physical and social sciences. Most of this work focuses on single layer or one-mode networks, including social networks between people or hyperlinks between websites. Multilayer or multi-mode networks, such as affiliation networks linking people to organizations, receive much less attention in this literature. Common strategies for discovering the community structure of multi-mode networks identify the communities of each mode simultaneously. Here I show that this combined approach is ineffective at discovering community structures when there are an unequal number of communities between the modes of a multi-mode network. I propose a dual-projection alternative for detecting communities in multi-mode networks that overcomes this shortcoming. The evaluation of synthetic networks with known community structures reveals that the dual-projection approach outperforms the combined approach when there are a different number of communities in the various modes. At the same time, results show that the dual-projection approach is as effective as the combined strategy when the number of communities is the same between the modes.

Predicting social influence with faction sizes and relative status.

Building on a recent theoretical development in the field of sociological social psychology, we develop a formal mathematical model of social influence processes. The extant theoretical literature implies that factions and status should have non-linear effects on social influence, and yet these theories have been evaluated using standard linear statistical models. Our formal model of influence includes these non-linearities, as specified by the theories. We evaluate the fit of the formal model using experimental data. Our results indicate that a one-parameter mathematical model fits the experimental data. We conclude with the implications of our research and a discussion of how it may be used as an impetus for further work on social influence processes.

Do magnitudes of difference on status characteristics matter for small group inequalities?

The theory of status characteristics and expectation states (SCT) explains how macro-level dimensions of stratification and specific abilities come to organize small group processes. The theory argues that people generate expectation states for each other based on relative standings on dimensions of stratification such that people with the more culturally valued states of the characteristics have higher expectations. Subsequently social influence, participation rates and evaluations of participation are purported to be directly related to expectation states. The result of this process is that large-scale inequalities are perpetuated in small group interactions, and individuals higher on abilities receive systematic advantages in small groups. SCT has received substantial experimental support for over 40years. However, the theory assumes that only states of relatively high and relatively low matter. That is, the theory and its applications assume that the magnitude of difference separating individuals on a dimension of stratification or ability is irrelevant. Recently, though, extensions to both the theory and its mathematics have been introduced that allow the magnitude of difference to be incorporated into the theory's predictions, supposedly yielding more precise predictions. This paper offers an experimental test of these procedures, showing that including the magnitude of difference into the theoretical predictions yields more precise estimates that explain more status-based inequalities.