A novel study: hypermnesia for books read years ago

Memory can increase across repeated tests without any further study, a finding known as hypermnesia (e.g., Erdelyi, M. H., & Kleinbard, J. (1978). Has Ebbinghaus decayed with time? The growth of recall (hypermnesia) over days. Journal of Experimental Psychology: Human Learning and Memory, 4(4), 275-289). This study is the first to examine hypermnesia in a recognition test over long delays between learning and test. The current experiment examined hypermnesia for popular novels across retention spans of up to 10 years. Participants took two tests separated by 24 hours on a novel they had previously read. The tests had identical questions presented in a different order. We found hypermnesia across the recognition tests, which was due to within-test memory improvements. Hypermnesia decreased as a function of retention time due to increased item losses at longer delays. We propose a guessing hypothesis to account for this result and suggest that increased item losses are in part due to greater instability of memory at longer intervals.