Adult hippocampal neurogenesis promotes the formation of new memories, but little is known about its effect on existing memories. In this new study, Akers and colleagues demonstrate that neurogenesis — in infancy or adulthood — induces forgetting of established memories.
They show that increasing neurogenesis after the formation of a memory was sufficient to induce forgetting in adult mice. By contrast, during infancy, when hippocampal neurogenesis levels are high and freshly generated memories tend to be rapidly forgotten (infantile amnesia), decreasing neurogenesis after memory formation mitigated forgetting.
In precocial species, including guinea pigs and degus, most granule cells are generated prenatally. Consistent with reduced levels of postnatal hippocampal neurogenesis, infant guinea pigs and degus did not exhibit forgetting. However, increasing neurogenesis after memory formation induced infantile amnesia in these species.

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Akers KG, Martinez-Canabal A, Restivo L, Yiu AP, De Cristofaro A, Hsiang HL, Wheeler AL, Guskjolen A, Niibori Y, Shoji H, Ohira K, Richards BA, Miyakawa T, Josselyn SA, Frankland PW: Hippocampal neurogenesis regulates forgetting during adulthood and infancy. Science 344(6184):598-602 (2014).
http://www.ncbi.nlm.nih.gov/pubmed/24812394