This paper studies evidence from Thomson Scientific about the citation process of 3.7 million articles published in the period 1998-2002 in 219 Web of Science categories, or sub-fields. Reference and citation distributions have very different characteristics across sub-fields. However, when analyzed with the Characteristic Scores and Scales technique, which is replication and scale invariant, the shape of these distributions over three broad categories of articles appears strikingly similar. Reference distributions are mildly skewed, but citation distributions with a five-year citation window are highly skewed: the mean is twenty points above the median, while 9-10% of all articles in the upper tail account for about 44% of all citations. The aggregation of sub-fields into disciplines and fields according to several aggregation schemes preserve this feature of citation distributions. It should be noted that when we look into subsets of articles within the lower and upper tails of citation distributions the universality partially breaks down. On the other hand, for 140 of the 219 sub-fields the existence of a power law cannot be rejected. However, contrary to what is generally believed, at the sub-field level the scaling parameter is above 3.5 most of the time, and power laws are relatively small: on average, they represent 2% of all articles and account for 13.5% of all citations. The results of the aggregation into disciplines and fields reveal that power law algebra is a subtle phenomenon.

This article studies massive evidence about references made and citations received after a 5-year citation window by 3.7 million articles published in 1998 to 2002 in 22 scientific fields. We find that the distributions of references made and citations received share a number of basic features across sciences. Reference distributions are rather skewed to the right while citation distributions are even more highly skewed:The mean is about 20 percentage points to the right of the median, and articles with a remarkable or an outstanding number of citations represent about 9% of the total. Moreover, the existence of a power law representing the upper tail of citation distributions cannot be rejected in 17 fields whose articles represent 74.7% of the total. Contrary to the evidence in other contexts, the value of the scale parameter is above 3.5 in 13 of the 17 cases. Finally, power laws are typically small, but capture a considerable proportion of the total citations received.

This paper studies some empirical implications of models with limited risk sharing due to the imperfect enforceability of contracts. We test whether the amount by which public transfers reduce private transfers is affected by features of the economy, such as the variance of income and its persistence. These implications are unique to models with imperfect enforceability. We use data from Mexico collected to evaluate a public transfer programme. It included a randomised component that we exploit as a source of exogenous variation. Our results support the theoretical model in that the crowding out of private transfers is larger in villages where the variance of income is smaller.Since the pioneering work of Townsend (1994) it has become increasingly clear that the ideal paradigm of perfect insurance markets that allow individuals to smooth idiosyncratic income shocks is not a good description of reality, even in small village economies. Several empirical papers, such as Udry (1994), have documented large deviations from the empirical implications of such a model.From a theoretical point of view, several authors have now modelled explicitly imperfections that prevent full risk-sharing. In particular, various authors have looked at models where first best allocations are not achieved either because of asymmetric information, or because of the impossibility of enforcing contracts. In the latter, because of the lack (or the imperfection) of an enforceability mechanism, individuals only enter contracts that are self-enforceable.Models with imperfect enforceability seem to be particularly apt at describing small village economies characterised by repeated interactions and good information flows within the village. They can generate very complex dynamics and allocations of resources that differ substantially from those implied by complete markets. More importantly, they can replicate features of inter-households agreements that are reportedly prevalent in many village economies. Platteau and Abraham (1987), for instance, claim that the risk sharing agreements observed in fishing communities are half way between credit and insurance, while Udry (1994) reports that interest rates and maturities on loans in Northern Nigeria seem to be state contingent and vary not only with shocks affecting the borrower but also with those affecting the lender.The empirical evidence on models with imperfect enforceability is limited. The papers by Foster and Rosenzweig (2001), Ligon et al. (2002) and Krueger and Perri (2001) consider different implications of imperfect enforceability and test them on data from Bangladesh, India and the US. In Albarran and Attanasio (2001b), we propose two tests that are somewhat in the spirit of the original Townsend (1994) paper. Such an approach can afford to be silent on the instruments individuals use to smooth idiosyncratic shocks.