How do I find out *why* a user has certain effective permissions? I know that I can query effective permissions by using sys.fn_my_permissions:

USE myDatabase;
SELECT * FROM fn_my_permissions('dbo.myTable', 'OBJECT') 

 entity_name | subentity_name | permission_name 
------------------------------------------------
 dbo.myTable |                | SELECT          
 dbo.myTable |                | UPDATE          
 ...


This tells me whether the current user has SELECT, INSERT, UPDATE, etc. permissions on myTable in database myDatabase.

Is it possible to easily find out why the user has these permissions? For example, I'd love to have a function fn_my_permissions_ex which outputs an additional reason column:

USE myDatabase;
SELECT * FROM fn_my_permissions_ex('dbo.myTable', 'OBJECT') 

 entity_name | subentity_name | permission_name | reason
------------------------------------------------------------------------------------------------------------------------------------
 dbo.myTable |                | SELECT          | granted to database role public
 dbo.myTable |                | UPDATE          | member of group MYDOMAIN\Superusers, which belongs to database role db_datawriter
 ...


Unfortunately, I could not find such a function in the SQL Server documentation. Is there a tool or script that provides this functionality?
You can find some good information regarding security from the article below.  

Reviewing SQL Server Permissions | TechRepublic http://tek.io/KfzEyp

Except: 


  The following query uses the sys.database_permissions system view to
  indicate which users had specific permissions inside the current
  database.


SELECT
        dp.class_desc
       ,dp.permission_name
       ,dp.state_desc
       ,ObjectName = OBJECT_NAME(major_id)
       ,GranteeName = grantee.name
       ,GrantorName = grantor.name
    FROM
        sys.database_permissions dp
        JOIN sys.database_principals grantee
        ON dp.grantee_principal_id = grantee.principal_id
        JOIN sys.database_principals grantor
        ON dp.grantor_principal_id = grantor.principal_id