  Few studies that examine the neurogenesis--behaviour relationship formally
establish covariation between neurogenesis and behaviour or rule out competing
explanations. The behavioural relevance of neurogenesis might therefore be
overestimated if other mechanisms account for some, or even all, of the
experimental effects. A systematic review of the literature was conducted and
the data reanalysed using causal mediation analysis, which can estimate the
behavioural contribution of new hippocampal neurons separately from other
mechanisms that might be operating. Results from eleven eligible individual
studies were then combined in a meta-analysis to increase precision
(representing data from 215 animals) and showed that neurogenesis made a
negligible contribution to behaviour (standarised effect = 0.15; 95% CI = -0.04
to 0.34; p = 0.128); other mechanisms accounted for the majority of
experimental effects (standardised effect = 1.06; 95% CI = 0.74 to 1.38; p =
1.7 $\times 10^{-11}$).
