Use the default Everything for a Keyword Search option to begin.
The dropdown menu under Everything helps you limit your search results to:
...and more
The dropdown menu under All Fields helps you search by:
...and more
--Instead, use the basic keywords related to your topic, for example, population growth.
--So, search for exact phrases by enclosing them in quotation marks. Words enclosed in double quotes will appear together in all results exactly as typed. Example: "child abuse"