Thank you for your post.
If a record doesn't exist, then any related fields will be empty. Therefore, first evaluate if any records exist in the portal. For example: Count ( <Related Field> ) > 0
Or, in your case, since the relationship is based on parent::parentID:
If ( IsEmpty ( parent::parentID ) or parent::Nationality = "USA" ; 0 ; 1 )
If you want an empty value if parent::parentID is empty, then use:
Case ( IsEmpty ( parent::parentID ) ; "" ; parent::Nationality = "USA" ; 0 ; 1 )