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Traditionally, Switzerland avoids alliances that might entail military, political, or direct economic action and has been neutral since the end of its expansion in 1515. Its policy of neutrality was internationally recognised at the Congress of Vienna in 1815. Only in 2002 did Switzerland become a full member of the United Nations and it was the first state to join it by referendum. Switzerland maintains diplomatic relations with almost all countries and historically has served as an intermediary between other states. Switzerland is not a member of the European Union; the Swiss people have consistently rejected membership since the early 1990s. However, Switzerland does participate in the Schengen Area.

What have the Swiss people continually rejected membership in since the 1990's?
European Union