The biologic mechanism by which behavioral and psychological symptoms of dementia (BPSD) occur in some patients with Alzheimer's disease (AD) is not understood, but it may be related to the selected loss of different neuronal populations or specific neurotransmitters. In many patients, these neuronal and neurotransmitter losses are likely to be the end result of the inheritance of a number of genetic risk factors for AD. Thus BPSD may arise indirectly as a result of genetic risk factors for AD. However, an alternative, or possibly complementary, hypothesis is that genetic risk factors for neuropsychiatric phenomena may, in normal circumstances, be clinically silent until neurodegeneration occurs from whatever cause.