A unidirectional one-to-many association can only exist if the "many" is a stand alone entity, and that is fairly unusual (in my experience anyway). Since most one-to-many relationships tend to define parent-child relationships they would normally be bidirectional.
Thanks for replying Paul. I agree with you that most of the one-to-many association can be described as parent-child, although I cannot say I find the 'stand-alone' version an unasual scenario. Thanks again.