I first became acquainted with the phenomenon during my year-and-a-half stint in a child development lab: hearing parents would sign while speaking to their hearing children, and their children would sign back. I originally attributed the curious habit to the presence of disabled family members in the home. But as I have since come to understand, these people were engaging in baby sign.

Infants can produce recognizable gestures earlier than they can produce recognizable speech. And indeed, from a young age, gestures are a critical component of the communication arsenal, complete with their own developmental trajectory, one that begins with pointing and ends with references to things unseen. So why not put that natural affinity for gesture to good use?

Such is the logic behind courses in baby sign, where parents learn gestures—sometimes borrowed from sign language, sometimes not—for simple words like milk and more. The gestures are then taught to infants, who, in lieu of throwing tantrums to acquire more milk, can go about the process with some dignity, making for happier babies, happier parents, and stronger baby-parent bonds.

Baby sign also, it’s been argued, makes for smarter babies, or at least more linguistically advanced ones. There’s something about gesture, some believe, that prepares children for language learning. One telling piece of evidence: bilingual children learning both a spoken language and a sign language have been shown to hit language milestones in their spoken language earlier than monolingual children learning only that spoken language.

This is all the more striking because bilingual babies have it tough. They have to learn two labels for milk, two labels for more, and two ways of structuring milk and more before they can master the art of more milk. Whatever advantages these bilingual babies may experience later in life—and they may well be legion—bilingual babies often fall ever-so-slightly behind monolingual babies when it comes to reaching milestones for a given language, in part because the bilinguals, on average, hear less of it.

Given that bilinguals who both sign and speak seem to have an advantage, however, many parents consider a course in baby sign a prudent investment. But is it? In the latest issue of Child Development, researchers Elizabeth Kirk, Neil Howlett, Karen Pine, and Ben Fletcher report results from the largest, best-controlled study of baby sign yet.

The researchers recruited 80 mothers with eight-month-old infants, whom they promptly (and randomly) split into four groups of 20. The first group was instructed to use a set of 20 words as often as possible, accompanying each word with its equivalent sign in British Sign Language. The second group was instructed similarly, only instead of borrowing from British Sign Language, they paired words with symbolic gestures—things like extending your arms (for airplane) or pulling on an imaginary shoe (for shoe). In the third group, parents were simply asked to speak the words often. The final group served as a control and was not given the word set.

Periodically, over the course of the next year, researchers visited the families. They discovered that children in the two gesture conditions had indeed learned several of the gestures. But importantly there were no group differences in how often children produced the related spoken words. Indeed, there were no group differences in any of the measures of language development the researchers examined: whatever satisfaction baby sign brought mother and child in the short term, it appeared to do absolutely nothing for the child’s long-term language development. (Researchers did however note upticks in a few measures of maternal responsiveness, suggesting a modest benefit to the mother, if not to her child.)

Why the null result? The researchers admit that their sample—high socioeconomic status (SES) infants—may have already been “beyond the threshold of improvement.” In other words, children who are read to, engaged with, simply spoken to often and at length just don’t need whatever benefits gesture training might provide.

The researchers did not set out to recruit high-SES families. But as they point out, the highly educated, motivated parents likely to enroll in academic research studies are the same parents likely to enroll in baby sign classes. Alas, for the families who might actually benefit from baby sign, the classes likely aren’t on their radar.