These interactions were videotaped, and the ten-minute tapes were later broken down second by second. Every time the baby looked to the mother, or babbled, or reached for a toy was noted. The children did this, on average, about 65 times in ten minutes, but some kids were very quiet that day and others very active. Every time the mother responded, immediately, was also noted. The moms might say, “Good job,” or “That’s a spoon,” or “Look here.” The moms responded about 60 percent of the time. Responses that were late, or off-timed (outside a five-second window), were categorized separately.
The researchers then telephoned the mothers every week, for the next year, to track what new words the child was using that week—guided by a checklist of the 680 words and phrases a toddler might know. This created a very accurate record of each child’s progression. (They also repeated the in-home videotape session when the infant was thirteen months old, to get a second scoring of maternal responsiveness.)
On average, the children in Tamis-LeMonda’s study said their first words just before they were thirteen months old. By eighteen months, the average toddler had 50 words in her vocabulary, was combining words together, and was even using language to talk about the recent past. But there was great variability within this sample, with some tots hitting those milestones far earlier, others far later.
The variable that best explained these gaps was how often a mom rapidly responded to her child’s vocalizations and explorations. The toddlers of high-responders were a whopping six months ahead of the toddlers of low-responders. They were saying their first word at ten months, and reaching the other milestones by fourteen months.
Remember, the families in this sample were all well-off, so all the children were exposed to robust parent vocabularies. All the infants heard lots of language. How often a mother initiated a conversation with her child was not predictive of the language outcomes—what mattered was, if the infant initiated, whether the mom responded.
“I couldn’t believe there was that much of a shift in developmental timing,” Tamis-LeMonda recalled. “The shifts were hugely dramatic.” She points to two probable mechanisms to explain it. First, through this call-and-response pattern, the baby’s brain learns that the sounds coming out of his mouth affect his parents and get their attention—that voicing is important, not meaningless. Second, a child needs to associate an object with a word, so the word has to be heard just as an infant is looking at or grabbing it.
In one paper, Tamis-LeMonda compares two little girls in her study, Hannah and Alyssa. At nine months old, both girls could understand about seven words, but weren’t saying any yet. Hannah was vocalizing and exploring only half as often as Alyssa—who did so 100 times during the ten minutes recorded. But Hannah’s mom was significantly more responsive. She missed very few opportunities to respond to Hannah, and described whatever Hannah was looking at twice as often as Alyssa’s mother did with Alyssa. At thirteen months, this gap was confirmed: Hannah’s mom responded 85% of the time, while Alyssa’s mom did so about 55% of the time.
Meanwhile, Hannah was turning into a chatterbox. Alyssa progressed slowly. And the gap only increased month by month. During their eighteenth month, Alyssa added 8 new words to her productive vocabulary, while in that same single-month period, Hannah added a phenomenal 150 words, 50 of which were verbs and adjectives.
At twenty-one months, Alyssa’s most complicated usages were “I pee” and “Mama bye-bye,” while Hannah was using prepositions and gerunds regularly, saying sentences like: “Yoni was eating an onion bagel.” By her second birthday, it was almost impossible to keep track of Hannah’s language, since she could say just about anything.
This variable, how a parent responds to a child’s vocalizations—right in the moment—seems to be the most powerful mechanism pulling a child from babble to fluent speech.
Now, if we take a second look at the famous Hart and Risley study, in light of Tamis-LeMonda’s findings, this same mechanism is apparent. In Hart and Risley’s data, the poor parents initiated conversations just as often with their tots as affluent parents (about once every two minutes). Those initiations were even slightly richer in language than those of the affluent parents. But the real gap was in how parents responded to their children’s actions and speech.
The affluent parents responded to what their child babbled, said, or did over 200 times per hour—a vocal response or a touch of the hand was enough to count. Each time the child spoke or did something, the parent quickly echoed back. The parents on welfare responded to their children’s words and behavior less than half as often, occupied with the burden of chores and larger families. (Subsequent analysis by Dr. Gary Evans showed that parent responsiveness was also dampened by living in crowded homes; crowding leads people to psychologically withdraw, making them less responsive to one another.)
Tamis-LeMonda’s scholarship relies on correlations—on its own, it’s not actually proof that parent-responsiveness causes infants to speed up their language production. To really be convinced that one triggers the other, we’d need controlled experiments where parents increase their response rate, and track if this leads to real-time boosts in infant vocalization.
Luckily, those experiments have been done—by Dr. Michael Goldstein at Cornell University. He gets infants to change how they babble, in just ten minutes flat