  Measuring the rate of innovation in academia and industry is fundamental to
monitoring the efficiency and competitiveness of the knowledge economy. To this
end, a disruption index (CD) was recently developed and applied to publication
and patent citation networks (Wu et al., Nature 2019; Park et al., Nature
2023). Here we show that CD systematically decreases over time due to secular
growth in research and patent production, following two distinct mechanisms
unrelated to innovation -- one behavioral and the other structural. Whereas the
behavioral explanation reflects shifts associated with techno-social factors
(e.g. self-citation practices), the structural explanation follows from
`citation inflation' (CI), an inextricable feature of real citation networks
attributable to increasing reference list lengths, which causes CD to
systematically decrease. We demonstrate this causal link by way of mathematical
deduction, computational simulation, multi-variate regression, and
quasi-experimental comparison of the disruptiveness of PNAS versus PNAS Plus
articles, which differ only in their lengths. Accordingly, we analyze CD data
available in the SciSciNet database and find that disruptiveness incrementally
increased from 2005-2015, and that the negative relationship between disruption
and team-size is remarkably small in overall magnitude effect size, and shifts
from negative to positive for team size $\geq$ 8 coauthors.
