The “decline effect” refers to the observation that many effect sizes that scientists study seem to be declining over time. In ecology, Jennions & Møller (2002) found that decline effects were the rule in ecological meta-analyses. That is, if you plotted the magnitudes of the effect size estimates included in a meta-analysis vs. the years in which those effect size estimates were published, you typically got a negative correlation. The first published effect size estimates tended to be larger than more recent estimates. This is troubling because it suggests publication biases that, in the worst cases, might lead to wild goose chases. Everybody gets excited about some massive initial estimate of some effect, jumps on the bandwagon, and it’s only years later, after much additional research, that everybody realizes that those initial estimates were unusually large (whether because of sampling error, or for some other reason).
Fortunately, my own more recent compilation of data indicates that decline effects are no longer common in ecology. Further, they’re no more common than their opposite, which you might call “incline effects”: cases in which the first published effect size estimates are small in magnitude compared to subsequent estimates.
But still, I have found some cases that might represent real decline effects or incline effects. “Real” meaning “not just due to sampling error, I don’t think.” I’m curious whether you can pick them out of a lineup. After all, if someone tells you that studies of phenomenon X exhibit a decline effect, you’ll probably be able to come up with a post-hoc rationalization as to why. Everything is obvious once you know the answer. It’s much harder to predict which meta-analyses will exhibit a decline effect, or an incline effect.
Below is a list of seven ecological meta-analyses. It includes the clearest-cut decline effects I could find, the clearest-cut incline effects I could find, and a couple of cases that exhibit neither a decline effect or an incline effect. For each meta-analysis, I’ve provided a plot of the absolute values of effect size estimates vs. the years in which those estimates were published (with the oldest being coded as “year 0”). The plots are labeled A-G in their legends, and in case it’s not totally obviously I’ve labeled each plot as exhibiting a decline effect, an incline effect, or neither. But I haven’t told you which plot goes with which meta-analysis–it’s your job to guess! There’s an anonymous poll at the end for you to record your guesses. Good luck!
- Crawford et al. 2019 EcoLetts (plant-soil feedbacks)
- Winfree et al. 2009 Ecology (effect of anthropogenic disturbance on bee abundance)
- He et al. 2013 EcoLetts (stress gradient hypothesis)
- Gibson et al. 2011 Nature (effect of tropical forest disturbance/land conversion on biodiversity)
- Gange et al. 2019 New Phytol (effect of fungal “plant bodyguards” on insect herbivores)
- Koricheva et al. 2004 Am Nat (trade-offs among different anti-herbivore defenses in plants)
- Munguia-Rosas et al. 2011 EcoLetts (natural selection on plant flowering phenology)