How much overlap should I expect between Medline and PsycINFO?
Unfortunately, I'm not sure there's a quick answer to the exact amount of overlap, but you may be able to get an estimate by comparing the list of journals included in Medline (http://www.nlm.nih.gov/archive/20130415/tsd/serials/lji.html.notice.html) to the list of journals covered by PsycINFO (http://www.apa.org/pubs/databases/psycinfo/coverage.aspx). Also, keep in mind that each database has it's own thesaurus and preferred search terms, and the terms you're using might pull up different results in different databases. You may be able to optimize your results by using search terms specific to each database.