Genes are essential for getting the various cell lines started in the embryo, and genes guide brain development toward a “first draft” in utero. But experience matters, too, even while the baby is in the uterus; and after birth, it matters enormously. Experience is so essential for wiring a large brain that the “first draft” of the brain includes a strong motivation to practice behaviors that will give the brain the right kind of feedback to optimize itself for success in the environment that happens to surround it. That’s why young mammals are so keen to play, despite the risks.
