“Doctor, as an expert on mental diseases, can you tell me what my chances are of being diagnosed with schizophrenia?” I am seeing a 23‐year‐old man, Albert, who came with his mother to my office. I learn that his monozygotic twin brother, Greg, who grew up with his father in Berlin, was recently diagnosed with schizophrenia. Albert has been living with his mother in a rural area near Hanover since his parents divorced, with each of them taking one of the twin babies along. There had obviously not been too much contact between them since.
Disease starts at a certain threshold, which is defined to some degree by societal and cultural standards.
What can I tell Albert? The concordance rate of schizophrenia in monozygotic twins is ~50%—indicating that, despite a 100% shared genome, there is a 50% chance that he will stay healthy, even though his twin brother is affected . It also means that there must be other factors at work, which significantly add to any genetic predisposition for behavioral abnormalities up to mental diseases.
When is a disease a disease?
What are these non‐genetic, environmental risk factors for behavioral abnormalities and mental disease? On the way to offering a satisfying answer based on state‐of‐the‐art research, a central, even philosophical question arises: How do behavioral abnormalities contribute to what we call a disease? And equally important, when does a disease start to be a disease?
Most of us can be depressed, lose our normal drive and pleasure at times, or be somewhat autistic when we intensely concentrate on a particularly interesting puzzle, or even become paranoid in certain situations. In other words, human (and animal) behavior can be regarded as consisting of quantifiable traits. Disease starts at a certain threshold, which is defined to some degree by societal and cultural standards [2 …
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