Network-Based Criterion for the Success of Cooperation in an Evolutionary Prisoner\u27s Dilemma

We consider an evolutionary prisoner\u27s dilemma on a random network. We introduce a simple quantitative network-based parameter and show that it effectively predicts the success of cooperation in simulations on the network. The criterion is shown to be accurate on a variety of networks with degree distributions ranging from regular to Poisson to scale free. The parameter allows for comparisons of random networks regardless of their underlying topology. Finally, we draw analogies between the criterion for the success of cooperation introduced here and existing criteria in other contexts

Evolution of Cooperation through the Heterogeneity of Random Networks

We use the standardized variance (nu_{st}) of the degree distribution of a random network as an analytic measure of its heterogeneity. We show that nu_{st} accurately predicts, quantitatively, the success of cooperators in an evolutionary prisoner\u27s dilemma. Moreover, we show how the generating functional expression for nu_{st} suggests an intrinsic interpretation for the heterogeneity of the network that helps explain local mechanisms through which cooperators thrive in heterogeneous populations. Finally, we give a simple relationship between nu_{st} , the cooperation level, and the epidemic threshold of a random network that reveals an appealing connection between epidemic disease models and the evolutionary prisoner\u27s dilemma

Reply To Comment on \u27Cooperation in an Evolutionary Prisoner\u27s Dilemma on Networks with Degree-Degree Correlations\u27

We respond to the comment of Zhu et al. [Phys. Rev. E 82, 038101 (2010)] and show that the results in question are not misleading

Cooperation in an Evolutionary Prisoner\u27s Dilemma on Networks with Degree-Degree Correlations

We study the effects of degree-degree correlations on the success of cooperation in an evolutionary prisoner\u27s dilemma played on a random network. When degree-degree correlations are not present, the standardized variance of the network\u27s degree distribution has been shown to be an accurate analytical measure of network heterogeneity that can be used to predict the success of cooperation. In this paper, we use a local-mechanism interpretation of standardized variance to give a generalization to graphs with degree-degree correlations. Two distinct mechanisms are shown to influence cooperation levels on these types of networks. The first is an intrinsic measurement of base-line heterogeneity coming from the network\u27s degree distribution. The second is the increase in heterogeneity coming from the degree-degree correlations present in the network. A strong linear relationship is found between these two parameters and the average cooperation level in an evolutionary prisoner\u27s dilemma on a network

Hepatitis C prevention education needs to be grounded in social relationships

Most hepatitis C transmission occurs through the sharing of equipment used for injecting drugs, and in many settings, the majority of equipment sharing occurs between sexual partners. Despite this, few health promotion materials directly address sexual partnerships, couples or social relationships in general. This blindspot is one example of the ways in which prevention education in the area of drug use would benefit from careful rethinking. Focusing on the case of Australia, we argue that hepatitis C prevention education insufficiently acknowledges or mobilize social relationships, social dynamics and social contexts in its efforts to prevent hepatitis C transmission. This can lead it to reproduce the conditions for the very problems it seeks to solve. We further argue that hepatitis C prevention education is insufficiently attentive to its own social location, drawing too little on stakeholder expertise. Its effectiveness relies upon its social context, including the collaborative input and engagement of affected communities and other stakeholders. Better recognizing this would produce a stronger foundation for developing prevention strategies. As we conclude, this social foundation for hepatitis C prevention could be articulated into national, collaboratively developed guidelines on effective communication in hepatitis C and injecting drug use risk

"Don't think I'm going to leave you over it": Accounts of changing hepatitis C status among couples who inject drugs.

While the health-related benefits of intimate partnership are well documented, little attention has been paid to couples exposed to high levels of social stigma and exclusion. In this project we investigated an important site of stigma for partnerships by collecting accounts of changing hepatitis C (HCV) status ("sero-change") among couples that inject drugs. We explored what these accounts reveal about the meaning of HCV for these couples, and how this understanding contributes to our collective efforts at prevention and care. Drawing from a large dataset of qualitative interviews with couples, we focussed on those containing reports of sero-change. By adopting a methodology that positioned partnerships rather than individuals as the primary unit of analysis, we addressed the commonplace tendency to either overlook or discount as dysfunctional, the sexual relationships of people who inject drugs. While some couples sought greater biomedical understanding as a means of coming to terms with sero-change, others drew on alternative logics or "rationalities" that sat firmly outside conventional biomedical discourse (privileging notions of kinship, for example). Regardless of which explanatory framework they drew on, participants ultimately prioritised the security of their relationship over the dangers of viral infection. Effectively engaging couples in HCV prevention and care requires acknowledging and working with the competing priorities and complex realities of such partnerships beyond simply the identification of viral risk. The "new era" of direct acting antiviral treatments will provide ongoing opportunities to learn to integrate biomedical information within more socially sophisticated, relationally aware approaches

Addressing injecting related risks among people who inject both opioids and stimulants: Findings from an Australian survey of people who inject drugs

Background: Opioids and stimulants are the most commonly injected illicit drugs worldwide and in Australia. While some people who inject drugs (PWID) prefer either opioids or stimulants, others regularly use both opioids and stimulants. Limited available research indicates that those who use opioids and stimulants together, either in combination or alternating between the two, may engage in injection-related practices which potentially place them at greater health risk and could lead to poorer health outcomes. Methods: Participants were recruited nationally through member organizations of the Australian Injecting and Illicit Drug Users League (AIVL); these organizations represent PWID in each Australian state and territory. This study compared a sample of PWID (N = 535) who reported past-month injection of opioids only (N = 173), stimulants only (N = 208), or both (N = 154) on a range of health and wellbeing outcomes. PWID completed a survey assessing drugs injected, frequency of injecting, receptive equipment sharing, psychological distress, self-reported hepatitis C (HCV) status, experienced and internalized stigma, drug use salience, and community attachment. Results: People who injected both opioids and stimulants reported more frequent injecting, more experiences of stigma, and greater reported HCV diagnosis than people who injected stimulants or opioids alone. They also showed greater attachment to a community of PWID and greater salience of drug use to their identity. Conclusions: The findings of increased injecting and broader harms associated with injecting both stimulants and opioids are important for tailoring harm reduction and intervention designs for people who use both opioid and stimulant drugs, including prioritizing peer-based approaches

Scaling of Entropic Shear Rigidity

The scaling of the shear modulus near the gelation/vulcanization transition is explored heuristically and analytically. It is found that in a dense melt the effective chains of the infinite cluster have sizes that scale sub-linearly with their contour length. Consequently, each contributes k_B T to the rigidity, which leads to a shear modulus exponent d\nu. In contrast, in phantom elastic networks the scaling is linear in the contour length, yielding an exponent identical to that of the random resistor network conductivity, as predicted by de Gennes'. For non-dense systems, the exponent should cross over to d\nu when the percolation length becomes much larger than the density-fluctuation length.Comment: 4 pages, 2 eps figure

Goldstone fluctuations in the amorphous solid state

Goldstone modes in the amorphous solid state, resulting from the spontaneous breaking of translational symmetry due to random localisation of particles, are discussed. Starting from a microscopic model with quenched disorder, the broken symmetry is identified to be that of relative translations of the replicas. Goldstone excitations, corresponding to pure shear deformations, are constructed from long wavelength distortions of the order parameter. The elastic free energy is computed, and it is shown that Goldstone fluctuations destroy localisation in two spatial dimensions, yielding a two-dimensional amorphous solid state characterised by power-law correlations.Comment: 7 pages, 2 figure

Smectic-C tilt under shear in Smectic-A elastomers

Stenull and Lubensky [Phys. Rev. E {\bf 76}, 011706 (2007)] have argued that shear strain and tilt of the director relative to the layer normal are coupled in smectic elastomers and that the imposition of one necessarily leads to the development of the other. This means, in particular, that a Smectic-A elastomer subjected to a simple shear will develop Smectic-C-like tilt of the director. Recently, Kramer and Finkelmann [arXiv:0708.2024, Phys. Rev. E {\bf 78}, 021704 (2008)] performed shear experiments on Smectic-A elastomers using two different shear geometries. One of the experiments, which implements simple shear, produces clear evidence for the development of Smectic-C-like tilt. Here, we generalize a model for smectic elastomers introduced by Adams and Warner [Phys. Rev. E {\bf 71}, 021708 (2005)] and use it to study the magnitude of Smectic-C-like tilt under shear for the two geometries investigated by Kramer and Finkelmann. Using reasonable estimates of model parameters, we estimate the tilt angle for both geometries, and we compare our estimates to the experimental results. The other shear geometry is problematic since it introduces additional in-plane compressions in a sheet-like sample, thus inducing instabilities that we discuss.Comment: 8 pages, 5 figure