The Organisation for Economic Cooperation and Development�s (OECD) Frascati Manual is the internationally accepted methodology for collecting and reporting data on R&D. This study interviewed countries in the OECD that do � and those that do not � permit Social Sciences and Humanities (SSH) R&D expenditure within their R&D tax credit programmes. It finds that how countries choose to adopt the Frascati Manual�s definition of R&D is a policy choice led by its domestic technical, financial, and political objectives. The UK, like many OECD countries, claims to use the Frascati Manual to define R&D within its R&D tax credit programme, but despite these claims, it excludes SSH R&D from its definition of R&D. By excluding AHSS R&D from its R&D definition for the purpose of tax relief, the UK government risks ignoring the full value of R&D in the UK economy and risks missing out on incentivising investment in AHSS-related innovation. To the best of our knowledge, this is the first comparative study of R&D definitions for tax relief.