  A recent analysis of scientific publication and patent citation networks by
Park et al. (Nature, 2023) suggests that publications and patents are becoming
less disruptive over time. Here we show that the reported decrease in
disruptiveness is an artifact of systematic shifts in the structure of citation
networks unrelated to innovation system capacity. Instead, the decline is
attributable to 'citation inflation', an unavoidable characteristic of real
citation networks that manifests as a systematic time-dependent bias and
renders cross-temporal analysis challenging. One driver of citation inflation
is the ever-increasing lengths of reference lists over time, which in turn
increases the density of links in citation networks, and causes the disruption
index to converge to 0. A second driver is attributable to shifts in the
construction of reference lists, which is increasingly impacted by
self-citations that increase in the rate of triadic closure in citation
networks, and thus confounds efforts to measure disruption, which is itself a
measure of triadic closure. Combined, these two systematic shifts render the
disruption index temporally biased, and unsuitable for cross-temporal analysis.
The impact of this systematic bias further stymies efforts to correlate
disruption to other measures that are also time-dependent, such as team size
and citation counts. In order to demonstrate this fundamental measurement
problem, we present three complementary lines of critique (deductive, empirical
and computational modeling), and also make available an ensemble of synthetic
citation networks that can be used to test alternative citation-based indices
for systematic bias.
