Invariant measures concentrated on countable structures

Let L be a countable language. We say that a countable infinite L-structure M admits an invariant measure when there is a probability measure on the space of L-structures with the same underlying set as M that is invariant under permutations of that set, and that assigns measure one to the isomorphism class of M. We show that M admits an invariant measure if and only if it has trivial definable closure, i.e., the pointwise stabilizer in Aut(M) of an arbitrary finite tuple of M fixes no additional points. When M is a Fraisse limit in a relational language, this amounts to requiring that the age of M have strong amalgamation. Our results give rise to new instances of structures that admit invariant measures and structures that do not.Comment: 46 pages, 2 figures. Small changes following referee suggestion

Universal graphs with a forbidden subtree

We show that the problem of the existence of universal graphs with specified forbidden subgraphs can be systematically reduced to certain critical cases by a simple pruning technique which simplifies the underlying structure of the forbidden graphs, viewed as trees of blocks. As an application, we characterize the trees T for which a universal countable T-free graph exists

Family Structure, Early Sexual Behavior, and Premarital Births

In this paper, we argue that entry into first sexual intercourse is a key process mediating the effects of family structure on premarital childbearing. We explicate three ways in which onset of sexual activity can mediate effects of family structure on premarital first births. First, the gross association between family structure and premarital birth risks may be due entirely to the effect of family structure on age at first intercourse. Second, the earlier the age at first intercourse, the longer the duration of exposure to the risk of a premarital first birth. Third, an early age at first intercourse may proxy unmeasured individual characteristics correlated with age at onset but uncorrelated with other variables in the model. We develop methods to assess such mediating effects and analyze data from two sources, the 1979-93 National Longitudinal Survey of Youth and the 1988 National Survey of Family Growth. We find that age at first intercourse partially mediates the effect on premarital birth risks of both snapshot measures of family structure at age 14 and a time-varying measure of the number of family transitions, but that significant effects of these variables remain net of age at first intercourse. Delaying age at intercourse by one year reduces the cumulative relative risk of a premarital first birth by a similar amount for both white and black women. For black women, the magnitude of this effect is roughly the same as that of residing in a mother-only family at age 14.

Living Together Apart in France and the United States

Union formation involves a number of stages, as does union dissolution, and new couples often spend an initial period in a noncohabiting intimate relationship. Yet while certain couples never share the same dwelling, "living apart together"(1) has not developed widely as a long-term lifestyle option. Claude MARTIN in France, and Andrew CHERLIN and Caitlin CROSS-BARNET in the United States have studied a symmetrical phenomenon, that of couples who continue to live together while considering themselves to be separated. In this article, they draw together their analyses to describe an arrangement which, while marginal, reveals situations where residential separation is not possible, either because of the need to keep up appearances, often for the children's sake, or because total separation is too frightening or living in separate homes is unaffordable. Beyond the differences between the two countries and the two survey fields, the authors analyse the ways in which persons who "live together apart" describe their loveless relationship that has led to explicit conjugal separation within a shared home.La formation des couples est un processus qui connaît des étapes, et il en va de même pour leur dissolution. Le début de la vie en couple passe souvent par une période de " conjugalité non cohabitante ". Mais, bien que certains couples ne partagent jamais le même logement, " vivre en couple chacun chez soi "(1) ne s'est guère développé comme mode de vie durable. Claude Martin en France, Andrew Cherlin et Caitlin Cross-Barnet aux États-Unis, se sont intéressés à un phénomène symétrique des couples non cohabitants, celui des couples qui continuent à vivre ensemble bien qu'ils se considèrent comme séparés. Ils rassemblent ici leurs analyses pour décrire un phénomène qui, s'il reste également marginal, révèle des situations dans lesquelles la séparation résidentielle n'est pas possible, soit pour maintenir la fiction du couple, notamment pour les enfants, soit par crainte des difficultés qu'entraînerait une rupture complète ou pour des raisons matérielles liées aux difficultés de logement dues à la " rupture résidentielle ". Au-delà des différences entre les deux pays et les terrains d'enquête, ils analysent la manière dont ces personnes qui " vivent ensemble séparés " décrivent leur situation dans laquelle la disparition du sentiment amoureux conduit à une séparation conjugale explicite dans une résidence commun

Are You Applying to More Than One Specialty?

Although the majority of U.S. medical students predominantly apply to only one specialty, some apply to more than one. When it comes to emergency medicine (EM), applicants may apply to additional specialties due to several reasons: being international medical graduates as well as their inability to make a decision regarding the choice of specialty, fear from the growing competitiveness of EM, or the desire to stay in a specific geographic area. Accordingly, in this article we aim to guide medical students through the process of applying to more than one specialty, including using the Electronic Residency Application Service application, writing a personal statement, getting letters of recommendation, and an Early Match. Moreover, we elaborate on the effect of applying to more than one specialty on a student's application to a residency in EM

The social geography of unmarried cohabitation in the USA, 2007-2011

US studies of marriage and cohabitation have mainly highlighted the social and racial differentials as they were observed in cross-sections, and have as a result essentially focused on the "pattern of disadvantage". The evolution of such social differentials over time and space reveals that this pattern of disadvantage has clearly persisted, but that it is far from covering the whole story. Historically, there has been a major contribution to the rise of cohabitation by white college students, and later on young white adults with higher education continued to start unions via cohabitation to ever increasing degrees. Only, they seem to move into marriage to a greater extent later on in life than other population segments. Also, the religious affiliation matters greatly: Mormons and evangelical Christians have resisted the current trends. Furthermore this effect is not only operating at the individual but at the contextual level as well. Conversely, even after controls for competing socio-economic explanations, residence in areas (either counties or PUMA-areas) with a Democratic voting pattern is related to higher cohabitation probabilities. And, finally, different legal contexts at the level of States also significantly contributed to the emergence of strong spatial contrasts. Hence, there is a concurrence of several factors shaping the present differentiations, and the rise of secular and liberal attitudes, i.e. the "ethics revolution", is equally a part of the explanation