ScienceDirect lets you easily narrow to peer-reviewed research articles.
After you enter some search terms and click "search," then select "research articles" from the "Article type" menu on the left to narrow to original research studies.
Web of Science lets you easily narrow to peer-reviewed research articles.
After you enter some search terms and click "search," then select "Article" from the "Document Types" menu on the left to narrow to original research studies.
Google Scholar is a freely accessible web resource that can help you find a wide range of academic journals, books, conference proceedings, case law, patents, content from university repositories, and more.
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To access Advanced Google Scholar Search, click on the menu in the top left corner when conducting a search (but note that its features are not very robust).