This chapter discusses why variance might exist in cognitive decline, organizing the discussion around three principles. The first principle is that multiple, co-occurring causal mechanisms contribute to cognitive decline in aging. The second principle is that variability exists in the expression of causal mechanisms across individuals and in individuals' responses to them. The final principle is that causal mechanisms should be studied within integrative theories that span different levels of organization—from the genetic to the behavioral.

This chapter discusses why variance might exist in cognitive decline, organizing the discussion around three principles. The first principle is that multiple, co-occurring causal mechanisms contribute to cognitive decline in aging. The second principle is that variability exists in the expression of causal mechanisms across individuals and in individuals' responses to them. The final principle is that causal mechanisms should be studied within integrative theories that span different levels of organization—from the genetic to the behavioral.