In order to cover your bases, you'll want to have (and document) a thorough search strategy that includes:
Reference lists (of existing systematic reviews & studies chosen for inclusion)
hand-searching key journals
contacting authors, organizations, conferences
Best practice for systematic searching recommends searching at least 3 databases with one of those databases being interdisciplinary. For reviews in the social sciences, I'd recommend that, regardless of your topic, you include both of the below databases.
Using Medical Subject Headings in PubMed
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