The Scientist reports on a recent study (registration required) that suggests that open access journal articles will receive more citations than articles not available through open access.  Before you consign this to the dustbin of the obvious, not all studies have indicated such a correlation.  But the study at the focus of this article covered 26 million articles from 8 thousand journals over the course of seven years.  This comprehensive study noted an overall increase in citations of 8 percent, but significant variation across fields and countries.  Open access leads to much higher increases in citations in poorer countries, and fields with a culture of pre-print databases and personal archives did not show as high an increase in citations compared to fields like the biomedical sciences.  Expect this study to show up in future fights over the NIH open access requirements.