@harvon said "live with my parents for many years (though the house is under their names)".
The support value of the home, provided by the parents, is the fair market rental value of the home divided by the number of occupants. So, if they live in their own home, that has to be taken into account in determining if you provide more than half their support.
The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
