SQL: Select (null = null);

This question is out of a databases exam i was looking at, so it might be nothing one can ever use in a real situation...

What output does the following valid SQL statement produce? Explain your answer!

SELECT (NULL = NULL);

I can easily produce the output of the statement, which is

school=> select (null = null);
 ?column?
----------

(1 row)

in psql 8.4.11, but how and why this is the answer i don't know... I would have guessed it tries to evaluate the expression inside the brackets and comes up with true/false but apparently it doesn't.

Any hints on why it behaves like it does?

Thanks