Phenotypic plasticity is the ability of a given genotype to express different phenotypes in response to environmental variation. This seemingly simple definition encompasses several challenges for the study of plasticity - how do we define different types of plasticity with less or more long-lasting phenotypic effects? How do we measure phenotypic plasticity when we cannot fully account for genetic or environmental variation? How do we address the ethical issues of the study of phenotypic plasticity in humans?