It is generally accepted that phobias arise from a combination of external events (i.e. traumatic events) and internal predispositions (i.e. heredity or genetics). Many specific phobias can be traced back to a specific triggering event, usually a traumatic experience at an early age. Social phobias and agoraphobia have more complex causes that are not entirely known at this time. It is believed that heredity, genetics, and brain chemistry combine with life-experiences to play a major role in the development of phobias.