Guy Kawasaki said, paraphrasing the point in the video linked above or in [1, 2, 3, 4, 5] (it was also in the biography, "Steve Jobs", by Walter Isaacson): “Steve Jobs has a saying that A players hire A players; B players hire C players; and C players hire D players. It doesn't take long to get to Z players. This trickle-down effect causes bozo explosions in companies.”