In the 1980s, Dr. Vincent Felitti, now director of the California Institute of Preventive Medicine in San Diego, discovered something potentially revolutionary: childhood abuse and neglect could affect adult health. In the 1990s, Felitti got together with an epidemiologist named Dr. Rob Anda, who at the time was on staff at the Centers for Disease Control and Prevention. They came up with a set of questions to trace, in a larger group, how tough childhood experiences might affect adult health, [and] called their work the study of Adverse Childhood Experiences, or ACE. [Upon] getting a rough measure of the severity of the patients' experiences, when Anda's team at the CDC crunched the numbers, he was shocked. One in 10 of the patients surveyed had grown up with domestic violence. Two in 10 had been sexually abused. Three in 10 had been physically abused. Now, 15 years after the ACE study came out, some scientists are trying to connect the dots - to get a clearer picture of what exactly adverse childhood experiences do to the body and why the study results came out the way they did. "Well, you've reshaped the biology of the child," says Megan Gunnar, a developmental psychologist at the University of Minnesota who, for more than 30 years, has been studying the ways children respond to stressful experiences. "This is how nature protects us," Gunnar adds. We all become adapted to living in "the kinds of environments we're born into." And if you have scary, traumatic experiences when you're small, Gunnar says, your stress response system may, in some cases, be programmed to overreact, influencing the way your mind and body work together.