According the downloadable brochure published by the National Institute of Mental Health, question #3, "In any year, 2.4 million Americans have panic disorder."

from Gulli, Laith Farid and Bilal Nasser. "Panic Disorder." Gale Encyclopedia of Genetic Disorders. Ed. Brigham Narins. Vol. 2. 2nd ed. Detroit: Gale, 2005. 985-988.

Panic disorder definitely runs in families and twin studies suggest that about 20% of patients who have the criteria for diagnosis have first-degree relatives with the disorder. In families with no history of affected first-degree relatives the prevalence decreases to 4%. The ratio between monozygotic twins (identical) to dizygotic (non-identical) twins is 5:1 for PD. Recent evidence suggests that there is a genetic mutation in the SLC6A4 gene. This gene is related to a brain chemical called serotonin, a chemical in the brain, which is known to effect mood. If the transport of serotonin is imbalanced then certain parts of the brain may not receive the correct stimulus causing alterations in mood. Some studies have demonstrated that there is no positive family history in about 50% of patients diagnosed with PD. Other possible causes of PD include social learning and autonomic responsivity (the attack will affect the body and hypersensitizes nerve cells in the brain). PD usually begins during the affected persons late teens or in the twenties, and is uncommon after age 35 and unusual after age 45 years. Global studies suggest that the lifetime prevalence of PD is between 1.5% and 3.5%. In the United States approximately 3-5% of the population is affected with the disorder. In any given year approximately 1.7% of the U.S. population has PD. This represents about 2.4 million Americans. PD is twice as common in females compared to males (female:male ratio is 2:1).

Lesley Williams

Evanston Public Library

http://www.epl.org/library/reference-help.html