The Naming Hypothesis (as detailed by Dugdale and Lowe, 1990; Horne and Lowe, 1996) states that in order to successfully demonstrate stimulus equivalence subjects must name the stimuli. Horne and Lowe quantify this by stating that if the subjects assign common names or name intraverbally
equivalence will emerge.
It was decided to evaluate the role of naming in stimulus equivalence by employing a population of subjects who are developmentally stable, of average or above average intelligence, but characterised by a deficiency in naming. For
this purpose developmentally dyslexic adults were employed. It was hypothesised that if naming is indeed necessary for the emergence of stimulus equivalence then dyslexic subjects should differ from control subjects on the equivalence test session. Specifically, the dyslexic performance should be more
prone to errors (Snowling, Wagendonk and Stafford, 1988) and longer response latencies should be observed on the test session trials, in line with the dyslexic tendency to be significantly slower in naming (Fawcett and Nicolson, 1994;
Watson and Brown, 1992).
Five matching-to-sample experiments were undertaken to compare the two groups' performances using a variety of stimuli and protocols which did or did not encourage the subjects to name. No statistically significant differences were
found between the groups on the tests of equivalence although more dyslexic subjects were found to persistently fail.
Looking across the studies it was found that subjects' performances on Experiments 1 and 2 (in comparison with Experiments 3, 4 and 5 which were explicitly verbal) were consistent with them using a nonverbal strategy which
presents the possibility that equivalence can be demonstrated without naming. The data from Experiments 3-5 also indicated that intraverbal naming is not always sufficient to bring about equivalence.
It was concluded that these data question the predictions made by Horne and Lowe (1996) and point to the need for more research regarding the role of naming in stimulus equivalence.