This work analyzes the variation over time of the effect of geographic distance on knowledge flows. The flows are measured through the citations exchanged between scientific publications, including and excluding self-citations. To calculate geographic distances between citing and cited publication, each publication is associated with a “prevailing” territory, according to the authors’ affiliations. We then apply a gravity model to account for the research size of the territories, in terms of cognitive proximity of citing-cited publications. The field of observation is the 2010–2017 world publications citing the 2010–2012 Italian publications, as indexed in the Web of Science. The results show that in domestic knowledge flows, geographic proximity remains an influential factor through time, although with differences among disciplines and trends of attenuating effects. Finally, we replicate the analyses of knowledge flows but with the exclusion of self-citations: in this manner the effect of geographic proximity seems reduced, particularly at the national scale, but the differences (with vs without self-citations) lessen through time. As shown in previous works, the effect of distance on continental flows is modest (imperceptible for intercontinental flows), yet here too time has some influence, including concerning exclusion of self-citations.

这项工作分析了地理距离对知识流的影响随时间的变化。流量是通过科学出版物之间交换的引文来衡量的，包括但不包括自引。为了计算引用和引用的出版物之间的地理距离，根据作者的隶属关系，每个出版物都与一个“主要”地区相关联。然后，我们根据引文引用出版物的认知接近性，应用引力模型来说明这些地区的研究规模。观察领域是引用Web of Science的2010-2012年意大利出版物的2010-2017年世界出版物。结果表明，在国内知识流中，地理邻近度仍然是影响时间的因素，尽管各学科之间存在差异，但衰减效果的趋势不同。最后，我们复制了对知识流的分析，但排除了自我引文：以这种方式，地理邻近度的影响似乎有所降低，特别是在全国范围内，但是随着时间的推移，差异（带有或不带有自引文）减少了。如先前的研究所示，距离对大陆流动的影响是中等的（对于洲际流动是不可察觉的），但是在这里时间也有一些影响，包括关于排除自我引证的影响。