Once thought to be caused by personality defects, bad parenting or demons, scientists now know that schizophrenia has a biological basis: It has to do with faulty brain wiring. Brain abnormalities that occur during development are not evident until late adolescence or early adulthood. The result is a dramatic disturbance in the way a person thinks.

But getting at the underlying biological and genetic mechanisms is a tough problem.

For one thing, scientists believe that multiple genes are involved, with each contributing a small effect. In fact, while schizophrenia runs in families, it’s not really simply a matter of genetics. Studies of identical twins show that one twin has only a 48 percent risk of developing schizophrenia if the other twin has the disease. Both twins would be expected to have the illness if it were solely a genetic disease because they share the same genes. What is inherited seems to be a genetic vulnerability to the disease, not the disease itself.

Apparently, environmental factors — perhaps a virus, poor prenatal nutrition or complications during birth — must interact with several genes on different chromosomes for illness to occur. Scientists believe that this interaction of genes and environment somehow disrupts the cell migration and reproduction that occurs during normal brain development.

“There are lots of different reasons someone may have schizophrenia. The brain pathology seems to be inherited, probably the result of some neuro-developmental impairment,” Miller said. “But this is where we get unsure whether it’s genetic — a problem with genes — or whether it is due to some insult early on during a critical time in the brain development.”

To complicate matters, the very medications that keep the hallucinations, delusions and other symptoms at bay make it difficult to study the link between deficits in cognition and how the brain works.