The best way to select a particular database for your research would be to think about what discipline best fits your subject. The best place to go for information on mental effects of family violence on children would be the psychological literature. While psychology is indeed a science and you might find some articles in this database, your best bet would probably be a database that is more specific. It is a great database because of its depth of coverage and full-text availability. You will probably find some information on the mental effects of family violence on children in Lexis-Nexis, but it will most likely be popular information.
Subject headings lend authority to a database.
When searching a book by subject heading in a library catalog, it must be searched only in the subject field only. If you link two terms together with AND, you are asking the database to look for records where both terms appear. However, you may not always retrieve the results you expect. Scholarly publications tend to cater to very focused, academic audiences. Articles are written by experts for fellow experts.