Bayesian decision making in human collectives with binary choices

Here we focus on the description of the mechanisms behind the process of information aggregation and decision making, a basic step to understand emergent phenomena in society, such as trends, information spreading or the wisdom of crowds. In many situations, agents choose between discrete options. We analyze experimental data on binary opinion choices in humans. The data consists of two separate experiments in which humans answer questions with a binary response, where one is correct and the other is incorrect. The questions are answered without and with information on the answers of some previous participants. We find that a Bayesian approach captures the probability of choosing one of the answers. The influence of peers is uncorrelated with the difficulty of the question. The data is inconsistent with Weber's law, which states that the probability of choosing an option depends on the proportion of previous answers choosing that option and not on the total number of those answers. Last, the present Bayesian model fits reasonably well to the data as compared to some other previously proposed functions although the latter sometime perform slightly better than the Bayesian model. The asset of the present model is the simplicity and mechanistic explanation of the behavior.Comment: 8 pages, 6 figures, 1 tabl

From continuous to discontinuous transitions in social diffusion

Models of social diffusion reflect processes of how new products, ideas or behaviors are adopted in a population. These models typically lead to a continuous or a discontinuous phase transition of the number of adopters as a function of a control parameter. We explore a simple model of social adoption where the agents can be in two states, either adopters or non-adopters, and can switch between these two states interacting with other agents through a network. The probability of an agent to switch from non-adopter to adopter depends on the number of adopters in her network neighborhood, the adoption threshold $T$ and the adoption coefficient $a$, two parameters defining a Hill function. In contrast, the transition from adopter to non-adopter is spontaneous at a certain rate $\mu$. In a mean-field approach, we derive the governing ordinary differential equations and show that the nature of the transition between the global non-adoption and global adoption regimes depends mostly on the balance between the probability to adopt with one and two adopters. The transition changes from continuous, via a transcritical bifurcation, to discontinuous, via a combination of a saddle-node and a transcritical bifurcation, through a supercritical pitchfork bifurcation. We characterize the full parameter space. Finally, we compare our analytical results with Montecarlo simulations on annealed and quenched degree regular networks, showing a better agreement for the annealed case. Our results show how a simple model is able to capture two seemingly very different types of transitions, i.e., continuous and discontinuous and thus unifies underlying dynamics for different systems. Furthermore the form of the adoption probability used here is based on empirical measurements.Comment: 7 pages, 3 figure

Four thousand years in the Blue Nile: Paths to inequality and ways of resistance. Cuatro mil años en el Nilo Azul: Caminos a la desigualdad y vías de resistencia

As general conclusions of the papers of the Blue Nile Project dossier herein, some ideas are suggested about the prehistoric societies investigated by Spanish archaeologists in Central Sudan during the 1990’s. The project included a survey of the Wadi Soba-El Hasib region east of Khartoum and excavations of two Mesolithic sites and one Neolithic site in the Wadi Soba area. Data from different sources are combined in an attempt to construct a historical narrative. Vestiges of some cultural hiatuses were noticed in the region, namely at the beginning and the end of the Mesolithic period, the latter involving the emergence of social stratification and the decline of women status. The archaeological gap at the end of the Neolithic period is interpreted as being a consequence of hindrance to social division. Early cultures of resistance and population movements towards the Ethiopian escarpment as a refuge area are proposed as longue duréeprocesses among Nilo-Saharans of the Eastern Sahel.Como conclusión de los trabajos del dossier ‘Proyecto del Nilo Azul’ en este volumen, se sugieren algunas ideas sobre las sociedades prehistóricas investigadas por arqueólogos españoles en el Sudán central durante los años noventa. El proyecto incluyó la prospección del área de Wadi Soba-El Hasib al este de Jartum y la excavación de dos yacimientos mesolíticos y uno neolítico en la zona de Wadi Soba. Se esboza una narrativa histórica a partir de datos de orígenes diversos. Se advierten indicios de varias discontinuidades culturales en la región, tanto al comienzo como al final del período mesolítico, la última coincidiendo con el comienzo de la estratificación social y la disminución del estatus femenino. El vacío arqueológico de finales del Neolítico se interpreta como consecuencia del rechazo a la división interna. La presencia antigua de culturas de resistencia y movimientos hacia el escarpe etíope como zona de refugio, podrían ser procesos históricos de larga duración entre las poblaciones nilo-saharianas del Sahel Oriental

Extinction-induced community reorganization in bipartite networks

We study how the community structure of bipartite mutualistic networks changes in a dynamic context. First, we consider a real mutualistic network and introduce extinction events according to several scenarios. We model extinctions as node or interaction removals. For node removal, we consider random, directed and sequential extinctions; for interaction removal, we consider random extinctions. The bipartite network reorganizes showing an increase of the effective modularity and a fast decrease of the persistence of the species in the original communities with increasing number of extinction events. Second, we compare extinctions in a real mutualistic network with the growth of a bipartite network model. The modularity reaches a stationary value and nodes remain in the same community after joining the network. Our results show that perturbations and disruptive events affect the connectivity pattern of mutualistic networks at the mesoscale level. The increase of the effective modularity observed in some scenarios could provide some protection to the remaining ecosystem

Resource location based on precomputed partial random walks in dynamic networks

The problem of finding a resource residing in a network node (the \emph{resource location problem}) is a challenge in complex networks due to aspects as network size, unknown network topology, and network dynamics. The problem is especially difficult if no requirements on the resource placement strategy or the network structure are to be imposed, assuming of course that keeping centralized resource information is not feasible or appropriate. Under these conditions, random algorithms are useful to search the network. A possible strategy for static networks, proposed in previous work, uses short random walks precomputed at each network node as partial walks to construct longer random walks with associated resource information. In this work, we adapt the previous mechanisms to dynamic networks, where resource instances may appear in, and disappear from, network nodes, and the nodes themselves may leave and join the network, resembling realistic scenarios. We analyze the resulting resource location mechanisms, providing expressions that accurately predict average search lengths, which are validated using simulation experiments. Reduction of average search lengths compared to simple random walk searches are found to be very large, even in the face of high network volatility. We also study the cost of the mechanisms, focusing on the overhead implied by the periodic recomputation of partial walks to refresh the information on resources, concluding that the proposed mechanisms behave efficiently and robustly in dynamic networks.Comment: 39 pages, 25 figure

Teoría del discurso y paradigmas arqueológicos

A theoretical survey is presented of the current theory and methodology of discourse analysis, focusing on the political and scientific discourses. The present paper is explicitly ascribed to Ernesto Laclau’s and the Essex school’s poststructuralist ‘theory of discourse’. After a short introduction to the recent theoretical positions in Spanish archaeology, a quantitative ‘content analysis’ is made of four theoretical papers written in Spanish. The application of the Lacanian ‘nodal point’ (point de capiton) analysis to the texts shows that the more empiricist paradigms (processual and Marxist) need their signifiers to be more strongly fixed (through the nodal points ‘economy’ and ‘production’) than the structuralist and post-processual discourses, which allow for a greater thematic dispersion and flotation of meanings.Se presenta una revisión de la teoría y metodología actuales de análisis del discurso, tanto general como más concretamente de los discursos político y científico, optando por la posición postestructuralista de la “teoría del discurso” de Ernesto Laclau y la “Escuela de Essex”. Tras analizar las posturas teóricas de la arqueología española actual, se hace un “análisis de contenido” cuantitativo de cuatro artículos en castellano, representativos de los paradigmas teóricos actuales más extendidos. Empleando la idea lacaniana de los “puntos nodales” (point de capiton) que dan sentido a los discursos, se advierte cómo los paradigmas más empiricistas (procesualismo, marxismo) necesitan de una mayor fijación del sentido de sus significantes (por los puntos nodales de “economía” y “producción”) que los paradigmas estructuralista y posprocesual, los cuales admiten una mayor dispersión temática e indeterminación de los significantes