A strong case for “baby number sense”Were the babies really responding to the change in numerosity? It certainly seems so.
Babies reacted similarly when presented with pairs of small numbers (2 vs 3) and distant, large numbers (4 vs 12).
And researchers controlled for several, non-numerical variables that could have influenced babies—like the total surface area of the objects, the average area devoted to each object, the total luminance of each image, and the total area occupied by each group of objects.
So babies weren’t simply attending to the continuous extent of “stuff” in each image.
In fact, other experiments suggest that babies pay more attention to change in number than they do to changes in continuous extent or surface area (Cordes and Brannon 2008).
Nor is it all “in the eye.” If babies’ feats of numerical discrimination were confined to visible objects, we might suspect that the ability is domain specific—a specialization of the visual system rather than a more general conceptual ability. But babies aren’t one-trick ponies. In addition to distinguishing visual arrays, babies can also detect differences in the number of sounds and in the number of actions (Lipton and Spelke 2003; Wood and Spelke 2005).
Babies can even understand ordinality—the idea that numbers can be arranged in order of magnitude. When 11-month old babies were presented with sequences of numerosities, they could discriminate between sequences that increased and sequences that decreased (Brannon 2002).
Of course, this doesn’t mean that babies can distinguish any two numbers. There are limits. For instance:
- Six-month old babies have a hard time distinguishing two quantities if the ratio between them is less than 2:1. In other words, they can handle 8 vs. 16, but fail to distinguish 8 vs. 12 (Xu and Arriaga 2007; Lipton and Spelke 2003)
- Nine- and 10-month olds can make finer distinctions (8 vs. 12). But even these older babies don’t seem to discriminate between 8 and 10 (Xu and Arriaga 2004; Lipton and Spelke 2003)
- Babies don’t distinguish between increasing and decreasing sequences of numbers until the approach the end of their first year. When Elizabeth Brannon tested 9-month old babies, they failed the test (Brannon 2002)