  Articles in high-impact journals are, on average, more frequently cited. But
are they cited more often because those articles are somehow more "citable"? Or
are they cited more often simply because they are published in a high-impact
journal? Although some evidence suggests the latter, the causal relationship is
not clear. We here compare citations of preprints to citations of the published
version to uncover the causal mechanism. We build on an earlier model of
citation dynamics to infer the causal effect of journals on citations. We find
that high-impact journals select articles that tend to attract more citations.
At the same time, we find that high-impact journals augment the citation rate
of published articles. Our results yield a deeper understanding of the role of
journals in the research system. The use of journal metrics in research
evaluation has been increasingly criticized in recent years and article-level
citations are sometimes suggested as an alternative. Our results show that
removing impact factors from evaluation does not negate the influence of
journals. This insight has important implications for changing practices of
research evaluation.
