  Changes in citation distributions over 100 years can reveal much about the
evolution of the scientific communities or disciplines. The prevalence of
uncited papers or of highly-cited papers, with respect to the bulk of
publications, provides important clues as to the dynamics of scientific
research. Using 25 million papers and 600 million references from the Web of
Science over the 1900-2006 period, this paper proposes a simple model based on
a random selection process to explain the "uncitedness" phenomenon and its
decline in recent years. We show that the proportion of uncited papers is a
function of 1) the number of articles published in a given year (the competing
papers) and 2) the number of articles subsequently published (the citing
papers) and the number of references they contain. Using uncitedness as a
departure point, we demonstrate the utility of the stretched-exponential
function and a form of the Tsallis function to fit complete citation
distributions over the 20th century. As opposed to simple power-law fits, for
instance, both these approaches are shown to be empirically well-grounded and
robust enough to better understand citation dynamics at the aggregate level.
Based on an expansion of these models, on our new understanding of uncitedness
and on our large dataset, we are able provide clear quantitative evidence and
provisional explanations for an important shift in citation practices around
1960, unmatched in the 20th century. We also propose a revision of the
"citation classic" category as a set of articles which is clearly
distinguishable from the rest of the field.
