Some serious evidence for the oneness of speech and gesture is how closely the go together during children's language development. Here are some of the stages of child language development, as they relate to gesturing
6-8 months: babbling
-infants produce simple, non-communicative sounds
-same time frame as onset of simple hand clapping/banging gestures, which are also non-communicative
8-10 months: the beginnings of word comprehension
-infants start to produce deictic gestures, pointing at objects they see
-infants also start producing culture-specific gestures, like waving at people
12 months: naming (basically the ability to look at something and say what it is. Think "Mommy!" or "doggie!")
-children begin producing recognitory gestures, which are brief actions associated with specific objects, eg
-at this age, gestures and naming are positively correlated
-first words of infants are usually very similar in content
18-20 months: first word combinations
-this is basically stringing together more than one word, like "car go" or "want drink."
-the associated deictic gestures at this age are temporary, and go away as spoken language flourishes. Think of that as a precursor to the vocabulary spurt
24-30 months: "explosion" in grammar
-children are now producing sentences, rich in inflection and function words such as articles, conjunctions, and pronouns.
-studies have shown that the ability to remember and imitate arbitrary sequences of manual actions is correlated with the onset and growth of grammatical production from 24-30 months
Kidd and Holler (2009): Children’s use of gesture to resolve lexical ambiguity
Research question: do 3, 4, and 5 year-olds differ in the way they attempt to disambiguate homonyms?
Study: children read a picture book with an experimenter, then had to re-tell the story later to a second experimenter. The caveat here is that the picture book included two scenes, containing two homonyms for the word bat. In one scene it was a winged animal, in the other it was a baseball bat. The researches observed how age related to use of gestures and speech in resolving the ambiguous scenarios.
The results are quite clear. The older children excelled at completing their task, compared with the younger children. The 3 year-olds struggled, using lots of deictic (rather meaningless) gestures, and almost no iconic (meaningful) gestures. Additionally, the 5 year-olds were capable of using speech sophisticated enough that they did not need much help from their hands to get their point across. Two rather telling and interesting graphs follow:
As generalizable as this study may be, kids simply develop at different rates. Check out this wordy, advanced 3-year old explain the plot of Star Wars. I find it interesting how little she's using her hands to explain the plot of a science fiction movie...