1. How Can Researchers Tell Whether a Baby Understands Words?
Babies rarely speak until about age 1, but this does not mean they lack understanding. Since researchers cannot ask them directly, they observe what the baby looks at [5]. In these experiments, two pictures appear on a screen. The researcher says the name of one picture, such as "look at the apple," and measures which image the baby gazes at longer. If the baby looks longer at the named picture than when it is not named, it suggests a link between the word and the object [5].
However, there is a risk that babies simply prefer certain pictures regardless of words. To fix this, a 2012 University of Pennsylvania experiment measured how long each picture was viewed when not named, then subtracted that baseline [5]. Parents hold the baby but wear a cover so they cannot see the screen and use headphones to hear instructions. This prevents parents from accidentally signaling the answer with their expressions [5].
2. Do 6-Month-Olds Look at the Right Person When They Hear "Mama"?
First, babies recognize their own names. A 1994 report from the University at Buffalo found that 24 infants aged 4.5 months listened longer to their own name than to other names with similar rhythm and intonation [1]. This shows they are learning sound patterns, but it is not yet proof of meaning.
The first clue about meaning came in 1999. Tincoff and Jusczyk showed 6-month-olds videos of their two parents side-by-side while saying "mama" or "papa." The babies looked at the named parent more often than chance [2]. This study involved two groups of 24 infants each [2].
A second experiment used videos of other babies’ parents. In this case, no preference appeared [3]. This suggests that for 6-month-olds, "mama" and "papa" are linked specifically to their own parents, not just any woman or man [3]. Jusczyk stated that by 6 months, these words stop being generic terms for anyone [2].
3. Do Babies Know the Names of Food and Body Parts?
In 2012, Bergelson and Swingley tested 33 infants aged 6 to 9 months at the University of Pennsylvania [4]. They heard names for common items like apples or nose and looked at two pictures. The babies looked longer at the named picture [4]. There was no difference in scores between 6–7 months and 8–9 months, meaning performance did not improve within this age range [4]. For comparison, 50 children aged 10 to 20 months took the same test [4].
Researchers concluded that 6-month-olds seem to understand many nouns they encounter frequently [4]. Reports also indicate that 6-month-olds understand words for body parts like hands and feet [8]. Generally, understanding is thought to develop before speaking [8].
However, "understanding" here is inferred from eye gaze. It was not confirmed by speech or pointing. The precision of this understanding is explored in other experiments.
4. Can Babies Tell Similar Things Apart, or Is Their Understanding Too Broad?
In 2017, Bergelson and Aslin tested 51 infants aged 6 months [6]. When shown two unrelated items, like a bottle and a nose, babies looked longer at the named item. But when shown related items, like a bottle and a spoon, or juice and milk, they looked at both equally [6].
Researchers interpreted this to mean that 6-month-olds’ word meanings are still broad. Hearing "bottle" might trigger a general category like "eating tool," making it hard to distinguish from a spoon [6]. This is how they performed in this experiment; it does not prove all 6-month-olds cannot distinguish similar objects [6].
So, when and how does this detail change? The next section looks at daily exposure to words.
5. Does Hearing More Object Names at Home Help Babies Learn Words?
The 2017 study also recorded infants’ homes using audio and small cameras [7]. They measured how often parents named objects that were physically present in the room [7].
Infants from homes with a higher rate of naming present objects scored better on language comprehension tests [7]. Hearing names for things right in front of them may provide clues [7].
However, this study only shows a correlation. It does not prove that naming objects more causes better scores. Researchers stated it is too early to give advice to parents based on this [7].
6. Why Do Babies' Word Test Scores Stay Flat from 6 to 14 Months?
In the 2012 experiment, scores remained nearly flat from 6 to about 14 months, then jumped up [5]. Swingley noted that the reason for this plateau is unknown [5]. Possible factors include understanding the task as a game, distinguishing sounds, or grammar [5]. If babies do not realize it is a game, they may not look at the pictures even if they understand [5].
We cannot say understanding stops growing until 14 months. The experiment only measures what appears in eye gaze during testing [5]. Another question remains: this study focused on concrete objects like food and body parts. Bergelson noted that words with changing meanings, like "oops" or "gone," are harder for babies to learn [4].
Generally, understanding precedes speaking. A rough estimate is about 50 words understood by age 12 months, though individual differences are large [8].
7. Where Do Your Eyes Go When You Hear a Name in a Picture Book?
Try this at home: open a picture book with two or more images. Ask someone to say, "Which one is the apple?" Notice where your eyes move immediately after hearing the word [5]. In infant experiments, eye movement after hearing a name was recorded as the "answer" [5].
Thinking about your own eye movements helps imagine what the infant’s "answer" might be. However, this is not a formal test. Eye gaze alone cannot determine understanding, as it can change based on mood or preference [4]. Do not use this to judge babies or compare children.
This article uses university press releases and abstracts. Exact statistics require checking the original papers [5].