While [depression] is typically defined in terms of its emotional symptoms - this led a generation of researchers to search for the chemicals, like serotonin, that might trigger such distorted moods - researchers are now focusing on more systematic changes in the depressed brain, such as reduced neurogenesis and increased cell death. What causes this neurodegeneration? You guessed it: chronic stress. Those same hormones that make you alert and escalate your pulse can also damage the brain.