Purpose: Scientific manuscripts sometimes have two or more authors explicitly designated as having " contributed equally" to the study. The prevalence and characteristics of this practice are not known. The goal of this study was to identify longitudinal trends and characteristics of the practice of explicitly giving authors equal credit in publications found in major medical journals. Methods: We conducted electronic keyword searches looking for original research articles with equally credited authors (ECAs) published between Jan. 1, 2000 and Dec. 31, 2009 in the five general medicine journals with the highest impact factors (New England Journal of Medicine, Journal of the American Medical Association, Lancet, Annals of Internal Medicine, and British Medical Journal). The annual prevalence of original research articles with ECAs for each journal. Results: Original research articles with authors explicitly given equal credit were found in all five journals. Articles with ECAs formed a greater proportion of the total number of articles published in each journal in 2009 versus published in 2000 (NEJM: 8.6% vs. <1%; JAMA: 7.5% vs. 0%; Annals: 3.8% vs. 0%; Lancet: 3.6% vs. <1%; BMJ: 1.0% vs. 0%). There was a statistically significant increasing trend in yearly prevalence of ECA articles for all the journals (NEJM: p < 0.0001; JAMA: p < 0.001; Annals: p < 0.001; Lancet: p < 0.001, BMJ: p = 0.001]. The first two authors listed in the byline received equal credit the majority of the time, but the practice was also applied to authors in nearly every position in the byline. Finally, none of the journals provided specific guidance regarding this practice in their instructions to authors. Conclusions: The practice of explicitly giving authors equal credit is increasingly common in original research publications. Scientific journals should consider providing guidance for authors regarding this practice. Furthermore, the potential impact of this practice on evaluations for academic promotion should be assessed.
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