The United States is the main partner for Japan when it comes to internationally co-authored research in the Nature Index, as measured by bilateral collaboration score (BCS), followed by China, Germany, the United Kingdom and France.
In 2015, Japan had a very similar BCS with China (316.0) as it did with Germany (306.1). Since then, the partnership with China has grown more rapidly, reaching a BCS of 558.7 in 2021.
The key international research partnerships involving Japan fluctuate across the four subject areas covered by the Nature Index. In chemistry, China was the leading partner in 2021 by BCS, but collaboration was strongest with the United States across other areas, and especially in life sciences.