Are internationally co-authored journal articles better quality? The UK case 2014-2020

International collaboration is sometimes encouraged in the belief that it generates higher quality research or is more capable of addressing societal problems. In support of this, there is evidence that the journal articles of international teams tend to be more cited than average. Reasons other than the benefits of international collaboration could explain this, however, such as increased national audiences from researcher networks. This article investigates research quality using 148,977 UK-based journal articles with post publication peer review scores from the 2021 Research Excellence Framework (REF). Based on an ordinal regression model controlling for collaboration, international partners increased the odds of higher quality scores in 27 out of 34 Units of Assessment (UoAs) and all four Main Panels. At the country level, the results suggests that UK collaboration with other advanced economies generates higher quality research, even if the countries produce lower citation impact journal articles than the UK. Conversely, collaborations with weaker economies tend to produce lower quality research, as judged by REF assessors. Overall, the results give the first large scale evidence of when international co-authorship for journal articles is beneficial, at least from a UK perspective, and support the continuation of research policies that promote it.