Elucidating the Functional Relationship Between Working Memory Capacity and Psychometric Intelligence: A Fixed-Links Modeling Approach for Experimental Repeated-Measures Designs

Numerous studies reported a strong link between working memory capacity (WMC) andfluid intelligence (Gf), although views differ in respect tohow close these two constructs are related to each other. In the present study,we used a WMC task with five levels of task demands to assess the relationshipbetween WMC and Gf by means of a new methodological approachreferred to as fixed-links modeling. Fixed-links models belong to the family ofconfirmatory factor analysis (CFA) and are of particular interest forexperimental, repeated-measures designs. With this technique, processessystematically varying across task conditions can be disentangled from processesunaffected by the experimental manipulation. Proceeding from the assumption thatexperimental manipulation in a WMC task leads to increasing demands on WMC, theprocesses systematically varying across task conditions can be assumed to beWMC-specific. Processes not varying across task conditions, on the other hand,are probably independent of WMC. Fixed-links models allow for representing thesetwo kinds of processes by two independent latent variables. In contrast totraditional CFA where a common latent variable is derived from the differenttask conditions, fixed-links models facilitate a more precise or purifiedrepresentation of the WMC-related processes of interest. By using fixed-linksmodeling to analyze data of 200 participants, we identified a non-experimentallatent variable, representing processes that remained constant irrespective ofthe WMC task conditions, and an experimental latent variable which reflectedprocesses that varied as a function of experimental manipulation. This lattervariable represents the increasing demands on WMC and, hence, was considered apurified measure of WMC controlled for the constant processes. Fixed-linksmodeling showed that both the purified measure of WMC (β = .48) as well as theconstant processes involved in the task (β = .45) were related toGf. Taken together, these two latent variables explainedthe same portion of variance of Gf as a single latent variableobtained by traditional CFA (β = .65) indicating that traditional CFA causes anoverestimation of the effective relationship between WMC andGf. Thus, fixed-links modeling provides a feasible method for amore valid investigation of the functional relationship between specificconstructs.