Stewart, I., Barnes-Holmes, D., Roche, B., & Smeets, P. M. (2002). Stimulus equivalence and non-arbitrary relations. The Psychological Record, 52, 77-88.
The authors, researchers at the National University of Ireland and Leiden University, Investigated the effect of conflicting nonarbitrary (color) relations on equivalence responding. Three groups of 8 Ss (aged 18-25 yrs) were trained and tested for the formation of three 3-member equivalence classes using nonsense syllables as stimuli: (1) the No-Color group, for which all stimuli in both training and testing phases were in black lettering; (2) the All-Color group, for which all stimuli were in color (thus, training effectively involved learning to ignore color); and (3) the Color-Test group, for which training and testing stimuli were in black and color lettering respectively. During testing, color had little or no effect upon performance, in that there was no significant difference in levels of equivalence responding between the No-Color and All-Color groups. For the Color-Test group, levels of equivalence responding were significantly lower than for the other 2 groups. Levels of responding in accordance with color matching were significantly higher for the Color-Test group than for the All-Color group. These data are consistent with Relational Frame Theory.