Intelligence.

Back in the 60's, there were many studies done to try to predict the ideal qualities for programmers. There was a shortage and we were dipping into other fields for trainees. The most infamous of these was IBM's programmers' Aptitude Test (PAT). Strangely enough, despite the fact the IBM later repudiated this test, it continues to be (ab)used as a benchmark for predicting programmer aptitude. What IBM learned with follow-on research is that the single most important quality for programmers is raw intelligence-good programmers are really smart people-and so are good testers.