Plasticity, stereotypy, intra-individual variability and personality: handle with care.

Only recently, variability within individuals has become of importance to evolutionary thinking. The boom in the literature on behavioural variability has led to the emergence of concepts such as behavioural plasticity, stereotypy, imprecision, and intra-individual variability (IIV). The proliferation of new terms has resulted in overlapping concepts, spreading confusion in understanding the origins of variability. Here we provide a critical overview of the concepts related to behavioural variability within the individual. We conclude that although there is no overlapping between behavioural plasticity and IIV, these concepts do overlap with stereotypy; they also face problems with ideas of abnormality and absence of function in stereotyped behaviour. We further provide a critical overview of the sometimes confusing relationship between (1) within individual variability, and (2) consistent variability across individuals (personality). We point out that personality is logically independent of both activational plasticity and IIV, because personality emerges at the population level, whereas plasticity and IIV emerge at the individual level. We conclude that, in personality studies, the failure to acknowledge the existence of either internal variability or consistent between-individual differences in internal variability will result in mixing different phenomena, and inhibit building unified accounts from heterogeneous databases.