Perhaps you should try some spelling variations:
"psychological theories" » "psychological theory", "psychological testing", "psychological tests"
"morphological studies" » "ecological studies", "biological studies", "immunological studies"
"physiological studies" » "biological studies", "epidemiological studies", "pharmacological 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: (((("morphological studies") OR ("physiological studies"))) OR ("psychological theories")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"morphological studies\") OR (\"physiological studies\"))) or (\"psychological theories\"))".
• Removing quotes may allow a broader search: (((( morphological studies ) OR ( physiological studies ))) or ( psychological theories )).
• Adding a wildcard symbol may retrieve word variants: (((("morphological studies") OR ("physiological studies"))) or ("psychological theories"))*.