[Zope] 'Delete objects' permission

Are permissions handled differently on folders than other objects? I have a community site where members should be able to add objects anywhere, but only delete their own objects. How do I achieve this? I have a folder and in it an object. If I add 'Delete objects' and 'Modify portal content' for Owner on the object I still can't delete it. If I add 'Delete objects' for Owner on the folder I still can't delete the object, but if I add 'Delete objects' for Member, then I can delete the object. How should I set the permissions to get this community policy? I've searched The Zope Book, Zope mailing lists and google without finding an answer. Maybe there isn't one ;-) Thanks, Tim