In the acclaimed BBC documentary Life of Birds (1998), David Attenborough
narrated male house sparrows displaying a black patch of feathers on their chest
or bib as a ‘badge of status’ to signal their fighting ability. Ten years later, a synthesis of published studies confirmed that the bigger the patch, the higher the social status. However, another ten years later an updated synthesis included unpublished studies, and questioned whether the bib signals their status at all.
This sparrow example may seem benign but consider the following example.
An early synthesis of clinical trial evidence led governments to spend billions of
US dollars stockpiling the anti-flu drug Tamiflu. It took another ten years to access the unpublished data that showed Tamiflu was probably not as effective, or as safe, as was first believed. There are an increasing number of examples of evidence weakening or disappearing over time, not only in the field of ecology and evolution, but also in other fields. This phenomenon is called a ‘decline effect’. Such a phenomenon suggests that we should reconsider the way we synthesize evidence.