People with ADD are hypersensitive. That is not a fault or a weakness of theirs, it is how they were born. It is their inborn temperament. That, primarily, is what is hereditary about ADD. Genetic inheritance by itself cannot account for the presence of ADD features in people, but heredity can make it far more likely that these features will emerge in a given individual, depending on circumstances. It is sensitivity, not a disorder, that is transmitted through heredity. In most cases, ADD is caused by the impact of the environment on particularly sensitive infants. Sensitivity is the reason why allergies are more common among ADD children than in the rest of the population. It is well known, and borne out again and again in clinical practice, that children with ADD are more likely than their non-ADD counterparts to have a history of frequent colds, upper respiratory infections, ear infections, asthma, eczema and allergies, a fact interpreted by some as evidence that ADD is due to allergies. Although the flare-up of allergies can certainly aggravate ADD symptoms, the one does not cause the other. They both are expressions of the same underlying inborn trait: sensitivity.