http://www.psychologytoday.com/articles/200306/our-brains-negative-bias
"The brain, Cacioppo demonstrated, reacts more strongly to stimuli it deems negative. There is a greater surge in electrical activity. Thus, our attitudes are more heavily influenced by downbeat news than good news."
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http://en.wikipedia.org/wiki/Negativity_bias
"Negativity bias is the psychological phenomenon by which humans pay more attention to and give more weight to negative rather than positive experiences or other kinds of information.
"In the brain, there are two different systems for negative and positive stimuli. The left hemisphere, which is known for articulate language, is specialized for positive experiences; whereas, the right hemisphere focuses on negative experiences. Another area of the brain used for the negativity bias is the amygdala. This specific area of the brain uses about two-thirds of its neurons searching for negative experiences. Once the amygdala starts looking for the bad news, it is stored into long-term memory.
"Positive experiences have to be held in awareness for more than twelve seconds in order for the transfer from short-term memory to long-term memory to take place. We remember more after we hear disapproving or disappointing news than before; this shows how the brain processes criticism.
"As a result, Cliff Nass, professor of communication from Stanford University has suggested managers offer praise after criticism, not before, so that the praise actually makes an impression on the receiver. Implicit memory registers and responds to negative events almost immediately. It takes five to twenty seconds for positive experiences to even register in the brain.
"Emotional information revolves within the limbic system. Therefore, the limbic system ties perfectly into the negativity bias."