  References are an essential component of research articles and therefore of
scientific communication. In this study we investigate referencing (citing)
behavior in five diverse fields (astronomy, mathematics, robotics, ecology and
economics) based on 213,756 core journal articles. At the macro level we find:
(a) a steady increase in the number of references per article over the period
studied (50 years), which in some fields is due to a higher rate of usage,
while in others reflects longer articles and (b) an increase in all fields in
the fraction of older, foundational references since the 1980s, with no obvious
change in citing patterns associated with the introduction of the Internet. At
the meso level we explore current (2006-2010) referencing behavior of different
categories of authors (21,562 total) within each field, based on their academic
age, productivity and collaborative practices. Contrary to some previous
findings and expectations we find that senior researchers use references at the
same rate as their junior colleagues, with similar rates of re-citation (use of
same references in multiple papers). High Modified Price Index (MPI, which
measures the speed of the research front more accurately than the traditional
Price Index) of senior authors indicates that their research has the similar
cutting-edge aspect as that of their younger colleagues. In all fields both the
productive researchers and especially those who collaborate more use a
significantly lower fraction of foundational references and have much higher
MPI and lower re-citation rates, i.e., they are the ones pushing the research
front regardless of researcher age. This paper introduces improved bibliometric
methods to measure the speed of the research front, disambiguate lead authors
in co-authored papers and decouple measures of productivity and collaboration.
