  The bibliometric measure impact factor is a leading indicator of journal
influence, and impact factors are routinely used in making decisions ranging
from selecting journal subscriptions to allocating research funding to deciding
tenure cases. Yet journal impact factors have increased gradually over time,
and moreover impact factors vary widely across academic disciplines. Here we
quantify inflation over time and differences across fields in impact factor
scores and determine the sources of these differences. We find that the average
number of citations in reference lists has increased gradually, and this is the
predominant factor responsible for the inflation of impact factor scores over
time. Field-specific variation in the fraction of citations to literature
indexed by Thomson Scientific's Journal Citation Reports is the single greatest
contributor to differences among the impact factors of journals in different
fields. The growth rate of the scientific literature as a whole, and
cross-field differences in net size and growth rate of individual fields, have
had very little influence on impact factor inflation or on cross-field
differences in impact factor.
