Dr. Rick Hanson, Senior Fellow of the Greater Good Science Center at UC Berkeley, in an article on his website says, “To keep our ancestors alive, Mother Nature evolved a brain that routinely tricked them into making three mistakes: overestimating threats, underestimating opportunities, and underestimating resources (for dealing with threats and fulfilling opportunities).” Thus evolved the “negativity bias,” our tendency to react to negative stimuli more intensely than positive.
