Novelty at second glance: a critical appraisal of the novel object paradigm based on meta-analysis

The study of consistent individual differences has become an important focus in research on animal behaviour. These behavioural differences are typically measured through standardized testing procedures. One frequently used paradigm is the novel object test, in which animals are exposed to unfamiliar objects and their reaction is quantified. We used meta-analysis to evaluate how reliably novel object trials quantify individual differences. Overall, we found repeatability of responses to novel objects was strong and significant and was larger in short-term than in long-term studies. Average sample size and long-term estimates have both increased over the past three decades. Most short-term studies used different novel objects in repeated presentations, while long-term studies used either the same or different novel objects almost equally often. Novelty, the time interval between trials and their interaction together explained little of the total heterogeneity, while between-study heterogeneity remained large. Overall, novel object trials reliably estimate individual differences in behaviour, but results were very heterogeneous even within the same study species, suggesting susceptibility to unknown details in test conditions. Most studies that use novel object trials in a foraging context label the trait as neophobia, while novel object trials in a neutral context are labelled variously as shyness–boldness, exploration–avoidance or neophilia. To avoid ambiguity, we argue for the use of object–neophobia for trials near resources and object–neophilia for trials in a neutral context as the most specific labels for novel object responses.