Brain and cognitive reserve: mediator(s) and construct validity, a critique.

The concept of "reserve" has traditionally been defined on the basis of a single indicator (e.g., education or intracranial volume) that purports to moderate or buffer the effects of brain damage on different clinical outcomes. While studies have shown modest effects for some indicators, it has left the concept of "reserve" wanting as an explanatory construct. More recently efforts have been made to identify groups of indicators hypothesized to represent a construct for brain or cognitive reserve. These efforts have also proved wanting because of the lack of evidence to justify such a priori groupings of variables into a brain or cognitive reserve construct. This theoretical paper addresses the issue of construct validity (convergent and discriminant) for both brain and cognitive reserve as single or multiple reserve factors. Conceptual models are proposed that are (a) derived from the current extant reserve literature and (b) empirically testable in order to facilitate establishment of construct validity for the commonly used, and perhaps misused, brain and cognitive reserve concepts.