This is because the child assumes that because you didn’t say ‘car’ or ‘ball’, things they already know, then it must be the things they do not know. The whole-object assumption is another reason a child might map the novel label to the unfamiliar object. This is because children assume words are referring to the entire object instead of parts of the object. So the novel label cannot be the color of the ball or a part of the car, but the new object (Hoff 155, 156).
One reason most developmental scientists would be hesitant to agree with Joan is that baby Erin is only 6.5 months, and is most likely in the reduplicated babbling stage. Erin is repeating “mamama” and this is probably not to communicate with Joan (Hoff 117). One necessary feature of Erin’s utterance in order for it to be her first real word is that it has to be an approximation of a word in the target language, instead of a sting of syllables like Erin’s (Hoff