You can't just throw your thesis into a database-they cannot parse whole sentences properly.
If you want to know if family income affects children's school attendance in the southwestern United States, you need to break that up.
Look at your results, what terms are popping up? You might not have phrased it like that, but maybe that is what it is called in the literature.
If there's a thesaurus tool available, use it! PsycINFO's thesaurus tool is invaluable! Also be sure to look at an article's subject terms or author supplied keywords.