Long ago, people believed that physical diseases were caused by sins or demons, but now we know that most are triggered by environmental factors such as microorganisms and chemicals. And recently, we’ve started to understand the important role in illness of defective genes inherited from our ancestors.

There is increasingly evidence that mental illnesses have similar causes. As with some physical ailments, mental illness may be caused by the convergence of a number of factors, both genetic and environmental.

It may require clusters of defective genes and environmental factors such as drugs or upbringing to trigger the illnesses, not just one bad gene or one outside cause.

That makes it difficult to predict potential outcomes and cures, for some people with similar patterns of genes get sick while others don’t.

The same is true for physical illness; at a chemical and biological level, scientists know that there’s no difference between the mind and body.

At the Staglin Family Music Festival for Mental Health in September, Daniel Weinberger, M.D., of the National Institute of Mental Health, addressed this controversial subject in a talk, “Genes and Mental Illness, What is the connection?”

Weinberger is director of the “Genes, Cognition & Psychosis Program,” an intramural research program of the NIMH. A specialist in neurology and psychiatry, he has focused on brain mechanisms involved in the origins and treatment of neuropsychiatric disorders, especially schizophrenia.

He started with what has become obvious: Genes matter. They affect conventional illness and mental health as well as physical well-being. “Something about people seems to be inborn,” and not just their color of eyes and build. “Many human behavior characteristics are determined by their genes to a considerable degree.”

This behavior includes not only how smart or aggressive people are, but even complex social behavior.

Mental changes can be due to one small change in a gene, the replacement of one tiny segment or “letter” of its DNA.

Weinberger noted that mapping the human genome sequence was completed in 2003, and he said that no field of medicine has been more affected by this research than the field of mental illness.

Even so, experts are still not sure what mental illness is, at the basic level. It’s a very complex phenomenon, but understanding its genetic causes is a first step.

He added that he believes, as do many other experts, that the greatest risks for psychiatric illness are related to genetic inheritance, not environment.

He noted that understanding genetic makeup can assist understanding and overcoming mental problems because genes are more objective than diagnoses based on observation of behavior, they can help identify individuals at risk for mental illness and they can suggest the biological paths to take for developing new treatment.

Weinberger focused much of his talk on schizophrenia, which he called “the cancer of mental illness.” “It’s a serious, chronic disorder of cognition, perception and comportment,” he said, one that usually starts in early adulthood. He added that it’s one of the 10 most devastating diseases of man, and costs the United States more than $50 billion a year.

We don’t know yet what causes schizophrenia, but have identified many risk factors, including some aspects of a person’s environment — and their genetic inheritance.

Genetic research includes comparing the genes of those affected and others who are not, and so far about 15 genes seem most relevant for studying schizophrenia.

Weinberger explained, however, that having some or all of those genes appears to indicate risk, not inevitability, as would be true for cystic fibrosis, which is caused by specific genes.

Instead, psychiatric disorders are determined by many genes and in many different patterns. “It’s not one single collection of genes that causes the problem,” he stated.

Likewise, sometimes something compensates for the pattern of genes, so the individual with them doesn’t get the disorder. “Genes don’t encode a psychiatric personality,” he said, but instead could make one possible or even likely.

As far as the impact of genetics in psychiatry, Weinberger doesn’t anticipate genetics diagnosing mental illness, but he does believe it might suggest steps to prevent it. He doesn’t believe scanning genes should be used to make decisions about marriage or conception, for example.

Studying a person’s genes may help predict the outcome of treatment. Better understanding genetics could also suggest mechanisms to fight the disease and suggest new targets of therapy.

This knowledge can also help develop drugs to target mental illness; he pointed out that every drug now used to treat psychiatric problems was discovered serendipitously, not by targeting specific causes.

He doesn’t believe we will modify genes, however, as predicted by some proponents of genetic engineering, and noted that “defective” genes are found in people with no psychiatric problems.

Weinberger also dealt with a few other concerns, such as the likelihood of multiple problems in a family. He says that while identical twins have a 50/50 chance of sharing the problem, it’s only one in 15 or 20 among siblings and even less for further relations. “The odds that others in the family will have mental illness if one does is less than one in 10 — and mental illness runs in a family due to genes, not environment.”

And he emphasizes that the environment represents a small risk factor compared to a person’s genes in all common mental illnesses, but even genes are only part of the story. “Everyone has genes that carry the risk of psychiatric problems. The environment can impact them, as can the number and relationship of the genes.”

A continuing undercurrent of Weinberger’s talk was how an understanding of the role of genetics in mental illness helps establish the physical reality of the disease and identify it, bringing mental illness into equity with other illnesses as part of the field of medicine.