“There is a significant hereditary contribution to ADD but I do not believe any genetic factor is decisive in the emergence of ADD traits in any child. Genes are codes for the synthesis of the proteins that give a particular cell its characteristic structure and function. They are, as it were, alive and dynamic architectural and mechanical plans. Whether the plan becomes realized depends on far more than the gene itself. It is determined, for the most part, by the environment.

To put it differently, genes carry potentials inherent in the cells of a given organism. Which of multiple potentials become expressed biologically is a question of life circumstances. Were we to adopt the medical model — only temporarily, for the sake of argument — a genetic explanation by itself would still be unsuitable. Medical conditions for which genetic inheritance are fully or even mostly responsible, such as muscular dystrophy, are rare.

“Few diseases are purely genetic,” says Michael Hayden, a geneticist at the University of British Columbia and a world-renowned researcher into Huntington’s disease. “The most we can say is that some diseases are strongly genetic.” Huntington’s is a fatal degeneration of the nervous system based on a single gene that, if inherited, will almost invariably cause the disease. But not always. Dr. Hayden mentions cases of persons with the gene who live into ripe old age without any signs of the disease itself. “Even in Huntington’s, there must be some protective factor in the environment,” Dr. Hayden says.”