Format the field with check boxes. This allows the user to select both checkboxes. If they do, both values are entered into the field, separated by a return. Used as a match field in a relationship, it will match to records with a match value of 1 OR 0.
You can also get the same list of values with a drop down list or pop up menu if you hold down shift or ctrl while clicking in order to select both values.
The same technique could be used with your negative 1 value if you set up fields with lists of values such as
List ( 0 ; -1 ) for female and
List ( 1 ; -1 ) for male
that way, -1 would link to both records.
Big tx Phil it really helped me out.
Just linked my parent field - set as a check box - to my child calc field
and it rocks !!