I think not= Yes, she does OR No, she doesn't?

[joham]

--She doesn't want a real investigation, does she?
--I think not.

This dialogue is taken from the entry of 'I THINK SO' in the COLLINS COBUILD ENGLISH USAGE. Could I ask if 'I think not' in this context means 'Yes, she wants a real investigation' or 'No, she doesn't want a real investigation'?

And how about if the answer is 'I think so'?

 

[probus]

"I think not": the speaker's opinion is that "she does not want a real investigation."

"I think so" could be ambiguous here, and therefore should be avoided in this situation.