NIR spectroscopy of the most massive open cluster in the Galaxy: Westerlund 1

Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars in the central 2'x2' of the cluster Westerlund 1 (Wd 1) with a resolution of R~9000 at a central wavelength of 2.30 micron. This allowed us to determine radial velocities of ten post-main-sequence stars, and from these values a velocity dispersion. Assuming virial equilibrium, the dispersion of sigma=8.4 km/s leads to a total dynamical cluster mass of 1.25x10^5 solar masses, comparable to the photometric mass of the cluster. There is no extra-virial motion which would have to be interpreted as a signature of cluster expansion or dissolution.Comment: To appear in the proceedings of IAU 246: "Dynamical Evolution of Dense Stellar Systems" (E. Vesperini, M. Giersz, A. Sills, eds.

Never change a winning team: The effect of substitutions on success in football tournaments

We identify a strong and significant negative effect of substitutions in (irrelevant) early games in worldcup and olympic football tournaments on performance in later rounds. We argue that this effect is due to the psychological consequences of such a decision and evaluate alternative possible explanations.Economics (Jel: A)

A MODEL OF IMMIGRATION, INTEGRATION AND CULTURAL TRANSMISSION OF SOCIAL NORMS

I present and study an evolutionary model of immigration and culturaltransmission of social norms in a set-up where agents are repeatedly matchedto play a one-shot interaction prisoner´s dilemma. Matching can be non-randomdue to limited integration (or population viscosity). The latter refers to atendency of individuals to have a higher rate of interaction with individuals oftheir type than with similar numbers of other agents. I derive a culturaltransmission mechanism in order to examine the influence of viscosity and ofother institutional characteristics of society on the evolutionary selection of prosocialnorms. The main findings are that strict norms, sustained by stronginternal punishment, need either viscosity or strong institutional pressures topersist, while norms of intermediate strength persist under a variety ofinstitutional characteristics. Endogenizing norm strength allows to identify twoscenarios in which pro-social norms survive: One of rigidity in whichseparation (high viscosity) leads to monomorphic equilibria with strict normsfor cooperation. And one of integration (low viscosity) where intermediatenorms persist in polymorphic equilibria. Furthermore, with endogenous norms,viscosity and cooperation are not linked in a monotone way.Cultural Evolution, Game Theory, Social Norms, Cooperation, Population Viscosity.

A Trichotomy in the Complexity of Counting Answers to Conjunctive Queries

Conjunctive queries are basic and heavily studied database queries; in relational algebra, they are the select-project-join queries. In this article, we study the fundamental problem of counting, given a conjunctive query and a relational database, the number of answers to the query on the database. In particular, we study the complexity of this problem relative to sets of conjunctive queries. We present a trichotomy theorem, which shows essentially that this problem on a set of conjunctive queries is either tractable, equivalent to the parameterized CLIQUE problem, or as hard as the parameterized counting CLIQUE problem; the criteria describing which of these situations occurs is simply stated, in terms of graph-theoretic conditions

The Logic of Counting Query Answers

We consider the problem of counting the number of answers to a first-order formula on a finite structure. We present and study an extension of first-order logic in which algorithms for this counting problem can be naturally and conveniently expressed, in senses that are made precise and that are motivated by the wish to understand tractable cases of the counting problem

Starburst triggered by compressive tides in galaxy mergers

The tidal field of galaxies is known generally to be disruptive. However, in the case of galaxy mergers, a compressive mode of tidal wave may develop and last long enough to cocoon the formation of star clusters. Using an N-body simulation of the Antennae galaxies, we derive the positions of these compressive regions and the statistics of their duration. Excellent agreement between the spatial distribution of tides and observed young clusters is found, while the characteristic e-folding times of 10 to 30 Myrs derived for the tidal field compare well with cluster formation time-scales.Comment: 3 pages, 5 figures, to be published in the proceedings of the "Galactic and Stellar Dynamics 2008" conference, Ed. C.M. Boil

Let me sleep on it: Delay reduces rejection rates in Ultimatum Games

We show that delaying acceptance decisions in the Ultimatum Game drastically increases acceptance rates of low offers. While in standard treatments without delay less than 20% of low offers are accepted, these numbers increase to around 65-75% as we delay the acceptance decisions by around 10 minutes. Our findings provide precise evidence for familiar notions such as ''sleeping on it'' and show that there may be a good reason why public administrations often communicate bad news on Friday afternoons. They shed new light on recent evidence in Neuroscience on brain activation after receiving bad news and raise questions about the extent to which decisions reveal the preferences of a decision-maker.microeconomics ;

GROUP SELECTION WITH IMPERFECT SEPARATION - AN EXPERIMENT

We experimentally investigate the effect of imperfect separation of groups on group selection and cooperation in a standard prisoner¿s dilemma environment. Subjects can repeatedly choose between two groups, where in one of them an institutionalized norm fosters cooperation. The degree of separation of the two groups is varied between treatments. We find that both, the share of participants that choose into the group where the norm is implemented and the share of participants that cooperate, rise monotonously with the degree of group separation. Furthermore with higher group separation significantly more subjects support the enforcement of the norm.Experiments, Cooperation, Group Selection, Social Norms, Population Viscosity.