"The hypothesis was more or less buried during the '60s. Arrow used it in his paper on learning-by-doing in the '60s. Prescott and I used it in that paper of ours on investment. People were aware of it, I didn't understand then how fundamental a difference it mad econometrically. I didn't realize that if you took it seriously you had to rethink the whole question of testing and estimation. I guess no one else did either, except for Muth."