Previous research has shown strong evidence that there is a language bias in academic databases in the social sciences and humanities. However, evidence for the sciences is mixed: Some cannot find an US bias using the Science Citation Index (SCI), others actually identify also a strong language bias in the SCI.
Furthermore, you have to consider differences in publication cultures between disciplines: The humanities and social sciences still approve monographs and edited books as publications which are seldom listed in theses databases.
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