The human brain is the most complex object known to us. It contains billions of nerve cells, each of which may make thousands of connections, in immense networks of circuitry that control our sense of self and our appreciation of – and interaction with – the world around us. In the last half century we learned that we are born with raw circuitry that quickly tunes itself to the environment we encounter. Now we are learning that the properties that allow nerve cells to achieve this "plasticity" in response to the early environment are controlled by the very same genes that drive learning and memory in adults.