When Timmy drinks orange juice he has no problem. But Timmy is just one of close to a dozen personalities who alternate control over [his body]. If those other personalities drink orange juice, the result is a case of hives. If Timmy comes back while the allergic reaction is present, the itching of the hives will cease immediately, and the water-filled blisters will begin to subside. For more than a century clinicians have ... reported isolated cases of dramatic biological changes in people with multiple personalities. These include the abrupt appearance and disappearance of rashes, welts, scars and other tissue wounds; switches in handwriting and handedness; epilepsy, allergies and color blindness that strike only when a given personality is in control of the body. One patient ... had a blood pressure of 150/110 when one personality was in control, and a pressure of only 90/60 when another personality took over. The effects found in these patients ... are graphic examples of the power of states of mind to regulate the body's biology. [They] are leading scientists to study the physiology of patients with multiple personalities to assess how much psychological states can affect the body's biology. If the mechanisms through which these differences occur can be discovered, it may be possible to teach people some similar degree of control over these problems. Multiple personalities typically develop in people who were severely and repeatedly abused as children. Often only one or two of the sub-personalities will be conscious of the abuse, while others will have no memory or experience of the pain.