Many of us worry about memory loss, but it's surprisingly important to forget. Scott Small, MD, director of the Alzheimer's Disease Research Center at Columbia, says pruning our memories is good for us. We all know "forgive and forget" is key to emotional health, but forgetting is also critical to cognitive health. Find out why a healthy dose of forgetting is not a pathology, but a way of clearing away extraneous information and improving our more important memories.   Plus... why their memories keep chimps in a state of rage and fear, while forgetting makes bonobos so happy.