Social network structure and the spread of complex contagions from a population genetics perspective

Ideas, behaviors, and opinions spread through social networks. If the probability of spreading to a new individual is a non-linear function of the fraction of the individuals' affected neighbors, such a spreading process becomes a "complex contagion". This non-linearity does not typically appear with physically spreading infections, but instead can emerge when the concept that is spreading is subject to game theoretical considerations (e.g. for choices of strategy or behavior) or psychological effects such as social reinforcement and other forms of peer influence (e.g. for ideas, preferences, or opinions). Here we study how the stochastic dynamics of such complex contagions are affected by the underlying network structure. Motivated by simulations of complex epidemics on real social networks, we present a general framework for analyzing the statistics of contagions with arbitrary non-linear adoption probabilities based on the mathematical tools of population genetics. Our framework provides a unified approach that illustrates intuitively several key properties of complex contagions: stronger community structure and network sparsity can significantly enhance the spread, while broad degree distributions dampen the effect of selection. Finally, we show that some structural features can exhibit critical values that demarcate regimes where global epidemics become possible for networks of arbitrary size. Our results draw parallels between the competition of genes in a population and memes in a world of minds and ideas. Our tools provide insight into the spread of information, behaviors, and ideas via social influence, and highlight the role of macroscopic network structure in determining their fate

Higher Education in 1980

Any reasonable prediction of future changes in higher education must necessarily be based on observations and interpretations of what is taking place today in society in general and in education in particular. The following observations provide some of the rationale for points to be made later

Draco -- A Failure of the Tidal Model

We test whether the structural properties of the nearby dwarf spheroidal (dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled with the suggestion that dSphs are unbound tidal remnants with a large depth extent along the line of sight. In order to apply the observational test of this hypothesis suggested by Klessen & Zhao, we use public photometric data from the Sloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontal branch over a range of areas covering 0.06 square degrees to 6.25 square degrees centered on Draco. The SDSS database is the only currently existing database with sufficient depth and area coverage to permit a stringent test of the tidal models. We show that blue horizontal branch stars extend beyond the previously inferred limiting radii of Draco, consistent with the observed absence of a truncated stellar surface density profile of this dSph. We calculate new models for a galaxy without dark matter, using Draco's morphological properties as constraints. The resulting models are unable to reproduce the narrow observed horizontal branch width of Draco, which stays roughly constant regardless of the field of view. We conclude that Draco cannot be the remnant of a tidally disrupted satellite, but is probably strongly dark-matter dominated. (ABSTRACT ABBREVIATED)Comment: 26 pages, 9 figures included, accepted for publication in ApJ, high-resolution version available at http://www.aip.de./~ralf/Publications/p22.abstract.htm

Cardiovascular complications of conventional and targeted adjuvant breast cancer therapy

Adjuvant therapy has improved the survival of women with early breast cancer (BC). Meta-analyses suggest that anthracycline-based regimens reduced the annual BC death rate by ∼40% in women below the age of 50 and 20% in older women. Novel agents designed to modulate abnormal growth factor signaling in and around the BC cell further increase patients' chances of survival. However, both conventional chemotherapeutic agents as well as some of the novel signaling inhibitors can induce important cardiovascular side-effects, potentially attenuating the progress made in recent years. The mechanism of cancer drug-induced cardiovascular complications varies greatly with some compounds inducing irreversible myocardial cell damage, while others lead to temporary cell dysfunction. The challenge of the future will be to prospectively discriminate between irreversible damage which can lead to progressive cardiovascular disease and reversible cardiovascular dysfunctions without further prognostic implications. Since adjuvant therapy for BC is potentially curative, emphasis must be placed on finding treatments combining maximum efficacy with the minimum of long-term side-effects in order to achieve survival with preserved quality of lif

Chemical Evolution in the Carina Dwarf Spheroidal

We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of -1.91 dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph.Comment: 3 pages, 2 figures, to be published in the proceedings of the ESO/Arcetri-workshop on "Chemical Abundances and Mixing in Stars", 13.-17. Sep. 2004, Castiglione della Pescaia, Italy, L. Pasquini, S. Randich (eds.