Perhaps you should try some spelling variations:
depressive disorders » depressive disorder
2.1 etiology » 1.1 etiology, 4.1 etiology, 8.1 etiology
2.2 etiology » 12.2 etiology, 1.2 etiology, 3.2 etiology
21 etiology » 11 etiology, 28 etiology, 1.1 etiology
22 etiology » 12 etiology, 28 etiology, 52 etiology

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %depressive disorders %((((2.1 etiology) OR (21 etiology))) OR (((22 etiology) OR (2.2 etiology))))%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%depressive disorders %((((2.1 etiology) OR (21 etiology))) or (((22 etiology) or (2.2 etiology))))%22".
• Adding a wildcard symbol may retrieve word variants: %depressive disorders %((((2.1 etiology) OR (21 etiology))) or (((22 etiology) or (2.2 etiology))))%22*.