Perhaps you should try some spelling variations:
"psychological theories" » "psychological theory", "psychological testing", "psychological tests"
"psychological studies" » "ecological studies", "psychological stress", "biological studies"
"morphological studies" » "ecological studies", "biological studies", "physiological studies"

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: (((("psychological studies") OR ("morphological studies"))) OR ("psychological theories")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"psychological studies\") OR (\"morphological studies\"))) or (\"psychological theories\"))".
• Removing quotes may allow a broader search: (((( psychological studies ) OR ( morphological studies ))) or ( psychological theories )).
• Adding a wildcard symbol may retrieve word variants: (((("psychological studies") OR ("morphological studies"))) or ("psychological theories"))*.