Infants, they contend, can tell if they're looking at one, two or three objects and can base their judgment on the number of items, not on the size, shape or mass of the objects. For higher numbers, infants can see that one group of objects has fewer discrete members than another group. But over the past several years another cadre of researchers has risen to challenge the assertions that infants know their numbers. It may be that, instead of measuring infants' ability to notice the number of objects they see, studies have been measuring their ability to register a change in the general amount of object they're seeing, say researchers, including Indiana University's Kelly Mix, PhD. Others land in the middle. No single study proves that infants can tell the difference between discrete numbers of items.
All researchers agree that studying infants' understanding of simple numbers is anything but easy.
The standard approach uses a habituation paradigm, a technique that takes advantage of the fact that babies tend to look longer at new or surprising visual stimuli. The trick for number researchers is to design stimuli so that the only thing different between the habituation stimuli and the test stimuli is the number of objects the babies see. Over the years, hundreds of studies have found that after infants habituate to, say, two objects, they look longer if they then see three objects than if they again see two objects. Many researchers interpret the findings to mean that infants understand discrete numbers, at least up to three or four. It's been done in too many different ways in too many different labs.