Although external events can affect a child's mood, a preexisting physiological vulnerability probably existed, causing the depressed reaction. Depression is thought to be the result of a chemical imbalance in the brain which is probably inherited. The chemicals involved in depression are neurotransmitters that regulate mood and endorphins that are responsible for producing positive moods. Research has consistently shown that depression runs in families. But however it originates, different life stresses can bring on depression or make it harder to manage.