Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative disorders. Why some individuals develop one disease rather than the other is not clear. Association studies with a case-control design are the time-honored approach to identifying risk factors. Extensive association studies have been carried out in both diseases creating a large knowledge database, however, reproducible risk factors remain rare. This general lack of knowledge of pathogenesis prevents us from reducing the worldwide burden of these diseases. Case-control studies are reductionist paradigms that assume, for maximum power, that the two populations being compared are exclusive and homogenous. The common occurrence of incidental AD and PD-type pathology combined with 'intermediate phenotypes' such as dementia with Lewy bodies suggest that aging itself, AD, and PD are part of a complex continuum characterized by variable amounts of amyloid-β, tau, and α-synuclein pathology. This heterogeneity may be a contributor to the lack of reproducibility in association studies to date. Here, we speculate on alternative experimental approaches to the case-control paradigm and consider how the association-study literature for AD and PD might be re-interpreted in terms of a disease spectrum.