Anxiety Disorders result from a combination of family and biological influences. Studies suggest that young children who are temperamentally (at birth) shy or tentative in unfamiliar situations may be more prone to anxiety. Some research suggests that anxiety may be caused by a chemical imbalance involving norepinephrine and serotonin. Other research implicates specific brain mechanisms, involving hormones and respiratory functions, as potential pathways to anxiety. Anxiety Disorders tend to run in families, but the complex relationship between genes, biological systems, and anxiety is not yet well understood. Moreover, evidence suggests that anxiety and phobic reactions can be learned, either through direct experience or observations of others.