Does bumping your head to wall lower your iq?

I suppose: If there is brain damage with lower cognitive functioning as a result, a loss of IQ could, I suppose, occur.

Depends: There is all degrees of "bumping" . You could bump your head in a minor fashion passing though a tight hallway or be blown against it in an explosion. If you fell off the bed of a truck & sustained brain damage you would expect loss of some function. However, we know that many kids head bang as a self soothing practice & grow up to be fine. So in many instances there is no effect.