Understanding Formal Similarity in Verbal Behavior
Formal similarity exists when the response product physically resembles the stimulus that controlled it. In verbal behavior, this means the response "sounds like" or "looks like" the stimulus. The echoic is the clearest example—when someone says "ball" and you repeat "ball," your response sounds like what you heard. This physical resemblance between stimulus and response is what defines formal similarity.
Formal similarity is one of two key properties that distinguish duplic operants (echoics, copying text, imitation) from codic operants (textual, transcription). Codic operants have point-to-point correspondence but cross modalities, so the response doesn't physically resemble the stimulus (writing doesn't look like speaking). Understanding this distinction helps RBTs classify verbal behavior accurately for assessment and intervention planning.
Formal Similarity in Different Verbal Operants
In the echoic, formal similarity means the vocal response sounds like the vocal stimulus. In copying text, the written response looks like the written stimulus. In motor imitation (mimetic), the motor response looks like the motor stimulus. Each duplic operant maintains this physical resemblance between what was presented and how the learner responds.
The concept helps RBTs understand why some skills transfer and others don't. A child who can echo "ball" (formal similarity: vocal-vocal) may not automatically be able to write "ball" from dictation (no formal similarity: vocal-written). Each operant with different stimulus-response modality combinations must typically be taught directly.
Clinical Implications of Formal Similarity
Understanding formal similarity helps RBTs design interventions that build generalizable communication repertoires. When formal similarity exists, learners may show generalized imitation—the ability to reproduce novel stimuli after training with exemplars. This is why echoic training often produces generalized vocal imitation, allowing therapists to use modeling to teach new words efficiently.
Assessment of formal similarity helps identify learner strengths. Some individuals may have strong auditory-vocal formal similarity (excellent echoing) but weak visual-motor formal similarity (poor copying). Understanding these modality-specific strengths guides treatment planning and communication modality selection.
Technical Classification
Formal similarity combined with point-to-point correspondence defines the duplic class of operants. Understanding this classification system helps RBTs communicate precisely about verbal behavior with supervisors and in professional documentation. Precise terminology supports effective treatment planning and implementation fidelity across team members.
Frequently Asked Questions About Formal Similarity
What is formal similarity in ABA?
Formal similarity exists when a response physically resembles the stimulus that controlled it—the response sounds like, looks like, or feels like the stimulus. It is a defining property of duplic operants like echoics and motor imitation.
Which verbal operants have formal similarity?
Duplic operants have formal similarity: echoic (vocal-vocal), copying text (written-written), and mimetic/motor imitation (motor-motor). Codic operants like textual (written-vocal) and transcription (vocal-written) do not have formal similarity.
Why is formal similarity important for teaching?
Understanding formal similarity helps RBTs predict skill transfer. Skills with formal similarity often show generalization (echoing new sounds after training). Skills without formal similarity typically need separate instruction.
How does formal similarity relate to generalized imitation?
Generalized imitation emerges when learners can imitate novel responses without direct training. This is facilitated by formal similarity—the learner recognizes that their response should match the presented model across new examples.
Can formal similarity vary in degree?
Yes, formal similarity can range from perfect (exact match) to approximate. A child who echoes "ball" as "baw" shows partial formal similarity. Training often begins by reinforcing closer approximations toward accurate matching.