7.9. In looking for the simple Keynesian multiplier, you regress the GNP on in- vestment and…

7.9. In looking for the simple Keynesian multiplier, you regress the GNP on in- vestment and find that there is some relationship. Now, thinking that it can not hurt much, you include the “irrelevant” variable “state and local taxes To your surprise, the investment variable loses its significance. How can an irrelevant variable do this?