This study examined the hypothesis that age-related increases in reasoning ability are associated with the ability to represent relations of increasing complexity, defined as the number of entities related. The study's purpose was to determine the extent to which this ability to process relations with three entities increased between ages 4 and 8 years, and whether this ability is domain-general. A total of 136 children, ages 3 to 8 years, performed relational tasks in 6 content domains: (1) transitivity; (2) hierarchical classification; (3) cardinality; (4) comprehension of relative clause sentences; (5) hypothesis testing; and (6) class inclusion. Relational complexity was manipulated within each domain. Tests of working memory and fluid intelligence were also given. Results showed significant correlations among tasks from different domains. The age by which 50 percent of the children processed relations with three entities was between 5 and 6 years. In a factor analysis, all tasks loaded on a single factor labeled Relational Complexity. Relational Complexity factor scores were estimated and interpreted as a content-independent measure of the ability to process relations. Relational Complexity was significantly correlated with age, fluid intelligence, and working memory. (Contains 12 references.) (Author/KDFB)