This longitudinal study of 50 families and their typically-developing children examined how parent input effects child vocabulary scores one year later. Controlling for factors like the child’s prior vocabulary skill, quantity of input, and SES, they found that:
At age 1 ½, quantity of parent input most predicted later vocabulary.
Note this doesn’t mean other things they didn’t measure couldn’t also impact it, like joint attention or parental responsivity
At age 2 ½, diversity of vocabulary in the input most predicted later vocabulary, even when controlling for input.
Also, other research on children this age has found that vocabulary grows best when directed to the child, not via ambient conversation.
At age 3 ½, language complexity matters most
e.g. decontextualized language like narratives, and explanations (such as answering “Why?” questions fully)
And for an Early Intervention SLP, this all seems pretty logical. But transforming it into a simplified version for coaching parents could also be quite useful, such as saying:
For babies and one-year-olds, talk to your child, and focus on amount.
For two-year-olds, talk to your child, and focus on words.
For three-year-olds, talk to your child, and focus on sentences and stories.
… and then coaching what this would look like, specifically. Then, of course, the question becomes—would this be adequate, and would it make a difference? We don’t know. The next review (actually, the next two!) show research that digs in deeper to what’s needed for success.