Failure to find evidence of stimulus generalization within pictorial categories in pigeons.
Generalization inside a category does not happen by itself—train at least two groups and check for transfer.
01Research in Context
What this study did
Carr et al. (2002) trained eight pigeons to peck at pictures of people.
The birds saw only one category during training.
Later the team showed new pictures from the same category plus pictures they had never seen.
They counted pecks to see if the birds treated the new same-category pictures like the old ones.
What they found
The pigeons pecked just as often at the brand-new category pictures as at the never-seen pictures.
There was no extra pecking for the same category.
In plain words, the birds did not automatically generalize within the picture group.
How this fits with other research
van Schrojenstein Lantman-de Valk et al. (2006) later asked the same kind of question.
They also used pigeons and also found no spontaneous class formation, backing up the 2002 result.
Rapport et al. (1996) looked at college students, not birds.
Those students did show equivalence classes and could even talk about them.
Together the three papers draw a clear line: birds need extra training to form a class, while humans often do it quickly and can describe it.
Alonso‐Álvarez et al. (2018) went further, showing that with the right cues people can sort same and opposite relations without inventing new mental rules.
Their work keeps the boundary set by Carr et al. (2002): generalization is not built in; it needs the right history or cues.
Why it matters
If you work on stimulus classes, do not assume your client will generalize for free.
Start with at least two clear categories and give direct practice across many examples.
Watch for the first signs of generalization, then add more examples to strengthen the class.
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02At a glance
03Original abstract
Pigeons' key pecks were reinforced in the presence of pictures from one of two categories, cats or cars. A single picture associated with reinforcement was used in Experiment 1, and 20 pictures from the same category were associated with reinforcement in Experiment 2. Pigeons then were presented with novel test pictures from the training category and from the other, previously unseen, category. During Session 1 of testing, pigeons pecked no more often at pictures from the reinforced category than at pictures from the previously unseen category. When pigeons were trained with pictures associated with reinforcement or its absence from different categories in Experiment 3, differential responding to novel pictures from different categories appeared during Session 1. These findings argue against a process of automatic stimulus generalization within natural categories and in favor of the position that category distinctions are not made until members of at least two categories are compared with one another.
Journal of the experimental analysis of behavior, 2002 · doi:10.1901/jeab.2002.78-333