  We study how a single paper affects the Impact Factor (IF) by analyzing data
from 3,088,511 papers published in 11639 journals in the 2017 Journal Citation
Reports of Clarivate Analytics. We find that IFs are highly volatile. For
example, the top-cited paper of 381 journals caused their IF to increase by
more than 0.5 points, while for 818 journals the relative increase exceeded
25%. And one in 10 journals had their IF boosted by more than 50% by their top
three cited papers. Because the single-paper effect on the IF is inversely
proportional to journal size, small journals are rewarded much more strongly
than large journals for a highly-cited paper, while they are penalized more for
a low-cited paper, especially if their IF is high. This skewed reward mechanism
incentivizes high-IF journals to stay small, to remain competitive in rankings.
We discuss the implications for breakthrough papers appearing in prestigious
journals. We question the reliability of IF rankings given the high IF
sensitivity to a few papers for thousands of journals.
