A Minority pulls the sample mean: on the individual prevalence of robust group-level cognitive phenomena – the instance of the SNARC effect

The aim of cognitive psychology is to obtain insights into human cognition in general. For this purpose, group-studies are typically conducted on representative samples, such that the results can be generalized to the population. Within this approach, individual differences are typically neglected or considered to be error variance. Little is known about the prevalence of group-level phenomena at the individual level. Such information is nevertheless important for claims about the universality of phenomena, as, in theory, significant effects at the group-level can be driven by a minority of participants. Here we used a uniform analysis of 18 existing data sets revealing a well-replicated phenomenon in numerical cognition at the group level, the so-called SNARC (Spatial-Numerical Association of Response Codes) effect, to investigate its prevalence at the individual level. Three methods of analyzing the presence of this effect at the individual level were utilized: one psychometric and two bootstrapping methods. The results suggest that the group-level SNARC effect is driven by a minority of individuals (≤ 45%). Our findings are pertinent to an important theoretical question: whether group-level effects are reflecting general principles of cognition. We discuss advantages and drawbacks of the present methods and their utility in investigating the prevalence of other cognitive phenomena. We posit that testing the presence of robust group-level cognitive effects at the individual level, as well as ensuring their reliable measurement, is an important step towards integrating two traditionally separate experimental and correlational approaches of scientific psychology, as previously proposed by Cronbach in the 1950s.