Penile Sufficiency: How Hard Is Hard Enough?

Yes folks, somebody paid somebody to answer the question.

In an effort to obtain a more precise definition of erectile sufficiency for vaginal penetration 19 normal men were exposed to an erotic videotape and recordings of penile tumescence were obtained. The subjects were instructed to signal with a push button switch when they had attained an erection they judged sufficient for penetration. They were asked to signal a second time when a full erection had been reached.

from NCBI ROFL via Science Roll