When the classical reaction norm is corrected by body size

• We propose a theoretical framework to correct the classical reaction norm.
• We apply this framework to quantify the apparent and true plasticity.
• Apparent plasticity significantly contributes to nearly all traits.
• The role of true plasticity varies throughout ontogeny.
• No plasticity in some traits is compensated for by true plasticity in other traits.

For nearly half a century, plasticity has been a controversial issue in ecology and evolution. Observed plasticity was classically quantified as the slope of a reaction norm or quantified in time series, which concealed its relationship with body size. With a theoretical framework and an experiment, here we demonstrated that: (1) body size significantly contributes to nearly all traits’ variability, which produces apparent, but not true plasticity; (2) the classical reaction norm seriously misestimates the origins and levels of plasticity, and a size-correction to the reaction norm can eliminate its size-dependency and leave only the environmental-induced plasticity; and (3) the absence of true plasticity in beneficial traits can be compensated for by true plasticity in compensating traits. This study emphasizes that the role of true plasticity varies throughout ontogeny. It also suggests that the classical reaction norm or function-valued traits needs to be integrated with body size (i.e., size serial analysis) when we evaluate environmental effects on phenotypes.