891 research outputs found
The Web of Human Sexual Contacts
Many ``real-world'' networks are clearly defined while most ``social''
networks are to some extent subjective. Indeed, the accuracy of
empirically-determined social networks is a question of some concern because
individuals may have distinct perceptions of what constitutes a social link.
One unambiguous type of connection is sexual contact. Here we analyze data on
the sexual behavior of a random sample of individuals, and find that the
cumulative distributions of the number of sexual partners during the twelve
months prior to the survey decays as a power law with similar exponents for females and males. The scale-free nature of the web of human
sexual contacts suggests that strategic interventions aimed at preventing the
spread of sexually-transmitted diseases may be the most efficient approach.Comment: 7 pages with 2 eps figures. Latex file. For more details or for
downloading the PDF file of the published article see
http://polymer.bu.edu/~amaral/WebofContacts.html . For more results on teh
structure of complex networks see http://polymer.bu.edu/~amaral/Networks.htm
Error and attack tolerance of complex networks
Many complex systems, such as communication networks, display a surprising
degree of robustness: while key components regularly malfunction, local
failures rarely lead to the loss of the global information-carrying ability of
the network. The stability of these complex systems is often attributed to the
redundant wiring of the functional web defined by the systems' components. In
this paper we demonstrate that error tolerance is not shared by all redundant
systems, but it is displayed only by a class of inhomogeneously wired networks,
called scale-free networks. We find that scale-free networks, describing a
number of systems, such as the World Wide Web, Internet, social networks or a
cell, display an unexpected degree of robustness, the ability of their nodes to
communicate being unaffected by even unrealistically high failure rates.
However, error tolerance comes at a high price: these networks are extremely
vulnerable to attacks, i.e. to the selection and removal of a few nodes that
play the most important role in assuring the network's connectivity.Comment: 14 pages, 4 figures, Late
The velocity distribution of nearby stars from Hipparcos data II. The nature of the low-velocity moving groups
The velocity distribution of nearby stars contains many "moving groups" that
are inconsistent with the standard assumption of an axisymmetric,
time-independent, and steady-state Galaxy. We study the age and metallicity
properties of the low-velocity moving groups based on the reconstruction of the
local velocity distribution in Paper I of this series. We perform stringent,
conservative hypothesis testing to establish for each of these moving groups
whether it could conceivably consist of a coeval population of stars. We
conclude that they do not: the moving groups are not trivially associated with
their eponymous open clusters nor with any other inhomogeneous star formation
event. Concerning a possible dynamical origin of the moving groups, we test
whether any of the moving groups has a higher or lower metallicity than the
background population of thin disk stars, as would generically be the case if
the moving groups are associated with resonances of the bar or spiral
structure. We find clear evidence that the Hyades moving group has higher than
average metallicity and weak evidence that the Sirius moving group has lower
than average metallicity, which could indicate that these two groups are
related to the inner Lindblad resonance of the spiral structure. Further we
find weak evidence that the Hercules moving group has higher than average
metallicity, as would be the case if it is associated with the bar's outer
Lindblad resonance. The Pleiades moving group shows no clear metallicity
anomaly, arguing against a common dynamical origin for the Hyades and Pleiades
groups. Overall, however, the moving groups are barely distinguishable from the
background population of stars, raising the likelihood that the moving groups
are associated with transient perturbations. [abridged
Mesoscopic structure conditions the emergence of cooperation on social networks
We study the evolutionary Prisoner's Dilemma on two social networks obtained
from actual relational data. We find very different cooperation levels on each
of them that can not be easily understood in terms of global statistical
properties of both networks. We claim that the result can be understood at the
mesoscopic scale, by studying the community structure of the networks. We
explain the dependence of the cooperation level on the temptation parameter in
terms of the internal structure of the communities and their interconnections.
We then test our results on community-structured, specifically designed
artificial networks, finding perfect agreement with the observations in the
real networks. Our results support the conclusion that studies of evolutionary
games on model networks and their interpretation in terms of global properties
may not be sufficient to study specific, real social systems. In addition, the
community perspective may be helpful to interpret the origin and behavior of
existing networks as well as to design structures that show resilient
cooperative behavior.Comment: Largely improved version, includes an artificial network model that
fully confirms the explanation of the results in terms of inter- and
intra-community structur
Analysis of promoter regions of co-expressed genes identified by microarray analysis
BACKGROUND: The use of global gene expression profiling to identify sets of genes with similar expression patterns is rapidly becoming a widespread approach for understanding biological processes. A logical and systematic approach to study co-expressed genes is to analyze their promoter sequences to identify transcription factors that may be involved in establishing specific profiles and that may be experimentally investigated. RESULTS: We introduce promoter clustering i.e. grouping of promoters with respect to their high scoring motif content, and show that this approach greatly enhances the identification of common and significant transcription factor binding sites (TFBS) in co-expressed genes. We apply this method to two different dataset, one consisting of micro array data from 108 leukemias (AMLs) and a second from a time series experiment, and show that biologically relevant promoter patterns may be obtained using phylogenetic foot-printing methodology. In addition, we also found that 15% of the analyzed promoter regions contained transcription factors start sites for additional genes transcribed in the opposite direction. CONCLUSION: Promoter clustering based on global promoter features greatly improve the identification of shared TFBS in co-expressed genes. We believe that the outlined approach may be a useful first step to identify transcription factors that contribute to specific features of gene expression profiles
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Mitigation of Moral Hazard and Adverse Selection in Venture Capital Financing: The Influence of the Country’s Institutional Setting
A venture capitalist (VC) needs to trade off benefits and costs when attempting to mitigate agency problems in their investor-investee relationship. We argue that signals of ventures complement the VC’s capacity to screen and conduct a due diligence during the pre-investment phase, but its attractiveness may diminish in institutional settings supporting greater transparency. Similarly, whereas a VC may opt for contractual covenants to curb potential opportunism by ventures in the post-investment phase, this may only be effective in settings supportive of shareholder rights enforcement. Using an international sample of VC contracts, our study finds broad support for these conjectures. It delineates theoretical and practical implications for how investors can best deploy their capital in different institutional settings whilst nurturing their relationships with entrepreneurs
Systematic identification of conserved motif modules in the human genome
<p>Abstract</p> <p>Background</p> <p>The identification of motif modules, groups of multiple motifs frequently occurring in DNA sequences, is one of the most important tasks necessary for annotating the human genome. Current approaches to identifying motif modules are often restricted to searches within promoter regions or rely on multiple genome alignments. However, the promoter regions only account for a limited number of locations where transcription factor binding sites can occur, and multiple genome alignments often cannot align binding sites with their true counterparts because of the short and degenerative nature of these transcription factor binding sites.</p> <p>Results</p> <p>To identify motif modules systematically, we developed a computational method for the entire non-coding regions around human genes that does not rely upon the use of multiple genome alignments. First, we selected orthologous DNA blocks approximately 1-kilobase in length based on discontiguous sequence similarity. Next, we scanned the conserved segments in these blocks using known motifs in the TRANSFAC database. Finally, a frequent pattern mining technique was applied to identify motif modules within these blocks. In total, with a false discovery rate cutoff of 0.05, we predicted 3,161,839 motif modules, 90.8% of which are supported by various forms of functional evidence. Compared with experimental data from 14 ChIP-seq experiments, on average, our methods predicted 69.6% of the ChIP-seq peaks with TFBSs of multiple TFs. Our findings also show that many motif modules have distance preference and order preference among the motifs, which further supports the functionality of these predictions.</p> <p>Conclusions</p> <p>Our work provides a large-scale prediction of motif modules in mammals, which will facilitate the understanding of gene regulation in a systematic way.</p
The role of nominating committees and director reputation in shaping the labor market for directors: an empirical assessment
Research Question/Issue: Do the presence and independence of nominating committees within boards of directors affect the extent of rewards and sanctions provided by the labor market to directors with a reputation for being active in monitoring management? Research Findings/Insights: Results drawn from a longitudinal sample of directors sitting on the board of 200 public French firms suggest that the stronger a director's reputation for being active in increasing control over management, the larger the number of his/her subsequent appointments to (1) boards with a nominating committee, (2) to boards with a nominating committee which excludes the CEO and (3) to boards with a nominating committee dominated by non-executive directors. In contrast, we found that a director's reputation of being active in increasing control over management does not impact the number of his/her subsequent appointments (1) to boards without a nominating committee, (2) to boards with a nominating committee which includes the CEO and (3) to boards with a nominating committee dominated by executive directors. Theoretical/Academic Implications: This study shows that the outcome of the power struggle between the CEO and incumbent directors during the candidate selection process determines the profile of directors who will ultimately obtain the board appointment. On the one hand, independent nominating committees are likely to reduce the influence of CEOs over the process of a director's appointment, and therefore are likely to increase the recruitment of directors with reputations for being active in exercising control over managers. On the other hand, nonexistence of nominating committees or presence of weak nominating committees under the influence of the CEO decouple directors' reputations for being active in controlling management from the likelihood of obtaining new appointments. Practitioner/Policy Implications: This study offers insights to policy makers interested in increasing the efficiency of the labor market for directors. More specifically, it highlights the conditions under which directors with a reputation of being active in increasing control over management are likely to be rewarded by the labor market for directors. These conditions include (1) the creation of a nominating committee; (2) exclusion of the CEO from this committee and (3) domination of this committee by outside directors
Rhesus Monkeys See Who They Hear: Spontaneous Cross-Modal Memory for Familiar Conspecifics
Rhesus monkeys gather much of their knowledge of the social world through visual input and may preferentially represent this knowledge in the visual modality. Recognition of familiar faces is clearly advantageous, and the flexibility and utility of primate social memory would be greatly enhanced if visual memories could be accessed cross-modally either by visual or auditory stimulation. Such cross-modal access to visual memory would facilitate flexible retrieval of the knowledge necessary for adaptive social behavior. We tested whether rhesus monkeys have cross-modal access to visual memory for familiar conspecifics using a delayed matching-to-sample procedure. Monkeys learned visual matching of video clips of familiar individuals to photographs of those individuals, and generalized performance to novel videos. In crossmodal probe trials, coo-calls were played during the memory interval. The calls were either from the monkey just seen in the sample video clip or from a different familiar monkey. Even though the monkeys were trained exclusively in visual matching, the calls influenced choice by causing an increase in the proportion of errors to the picture of the monkey whose voice was heard on incongruent trials. This result demonstrates spontaneous cross-modal recognition. It also shows that viewing videos of familiar monkeys activates naturally formed memories of real monkeys, validating the use of video stimuli in studies of social cognition in monkeys
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