In the Dimensional Change Card sorting (DCCS) task, 3-year-olds can usually sort cards successfully by a first rule - whether by shape, color, size, etc. When asked to switch then to another rule, most 3-year-olds will perseverate by continuing to sort cards according to the first and now-irrelevant rule. This occurs even when the current rule is repeated every single time they're asked to sort a card! Children will even correctly repeat the name of the rule they should be using, and then proceed to actually sort the card by the old rule.
By age 4, however, most kids are able to successfully switch to a second rule (though many will still have trouble when asked to switch again). What then changes between 3 and 4 to allow this shift away from a remarkably strange behavior?
A new study by Wolfgang Mack begins to answer this question.
Mack concludes that the verbal information conveyed by the experimenters at the end of the first sorting rule—at the beginning of the second is merely insufficient for many 3 year olds to understand what they should be doing. Accordingly, providing pre-training may help through this route.
Oddly, Mack showed that switching performance can be improved simply by asking an irrelevant question, but he provides no explanation for this fascinating finding. One idea, based on Kirkham et al's attentional inertia account of perseveration, is that children are merely fixated on the first rule and need some sort of strong external prompt to think of something else.