a b s t r a c tResearch networks play a crucial role in the production of new knowledge since collaboration contributes to determine the cognitive and social structure of scientific fields and has a positive influence on research. This paper analyses the structure of co-authorship networks in three different fields (Nanoscience, Pharmacology and Statistics) in Spain over a three-year period (2006)(2007)(2008) and explores the relationship between the research performance of scientists and their position in co-authorship networks. A denser co-authorship network is found in the two experimental fields than in Statistics, where the network is of a less connected and more fragmented nature. Using the g-index as a proxy for individual research performance, a Poisson regression model is used to explore how performance is related to different co-authorship network measures and to disclose interfield differences. The number of co-authors (degree centrality) and the strength of links show a positive relationship with the g-index in the three fields. Local cohesion presents a negative relationship with the g-index in the two experimental fields, where open networks and the diversity of co-authors seem to be beneficial. No clear advantages from intermediary positions (high betweenness) or from being linked to well-connected authors (high eigenvector) can be inferred from this analysis. In terms of g-index, the benefits derived by authors from their position in co-authorship networks are larger in the two experimental fields than in the theoretical one.

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A positive influence of international collaboration on the impact of research has been extensively described. This paper delves further into this issue and studies to what extent the type of collaborating country-high, medium or low R&D intensive countryand which country is the leader in the research may influence the impact of the final scientific output. Among 9,961 papers co-authored by scientists from Spain and from another country (bilateral collaboration) during 2008-2009, papers with high R&D intensive countries predominated (60 %) and received the highest number of citations. This holds true in eight out of nine fields, being Social Sciences the one which benefited the most from partnerships with high R&D intensive countries. Mathematics emerges as a special case where other factors such as the partner's specialisation in the field may have a greater influence on research impact than the level of investment in R&D of the collaborating country. No significant influence of the type of country leading the research on the impact of the final papers is observed in most fields. Research policy implications are finally discussed.

This paper analyses existing trends in the collaborative structure of the Pharmacology and Pharmacy field in Spain and explores its relationship with research impact. The evolution in terms of size of the research community, the typology of collaborative links (national, international) and the scope of the collaboration (size of links, type of partners) are studied by means of different measures based on co-authorship. Growing heterogeneity of collaboration and impact of research are observed over the years. Average journal impact (MNJS) and citation score (MNCS) normalised to world average tend to grow with the number of authors, the number of institutions and collaboration type. Both national and international collaboration show MNJS values above the country's average, but only internationally co-authored publications attain citation rates above the world's average. This holds at country and institutional sector levels, although not all institutional sectors obtain the same benefit from collaboration. Multilateral collaboration with high-level R&D countries yields the highest values of research impact, although the impact of collaboration with low-level R&D countries has been optimised over the years. Although scientific collaboration is frequently based on individual initiative, policy actions are required to promote the more heterogeneous types of collaboration.