Teens and babies may develop specific fears because of "genetic programming," according to two new studies from the University of Minnesota and the Virginia Commonwealth University.

Researchers at Virginia Commonwealth found that teens often develop fears of blood and injury, whereas the Minnesota team discovered that babies are "hard-wired" to be afraid of spiders. These studies used twins, so that scientists could determine whether heredity or environment caused fears to develop at certain ages. It is already known that separated identical twins develop similar phobias.

Dr. Kenneth Kendler believes that humans may be programmed for certain fears at certain ages because of our evolutionary history. For example, young adults often develop fears of strangers, which may be based upon long-ago risks of being attacked by members of a foreign tribe.

These studies appear in the Archives of General Psychiatry.