When looking for articles, try searching with the official terminology of the database. Look for a Thesaurus or Subject Terms link.
When looking for books in HoyaSearch or other libraries' online catalogs, look for relevant Library of Congress Subject Headings to get you started. Here is a short (non-comprehensive) list of examples:
Mental health or Child mental health
Mental illness -- Environmental aspects
Mental illness -- Genetic aspects
Behavior genetics
Personality disorders
Genetic markers
Genetic predisposition to disease
Neurosciences
Substance abuse