I've tried in earnest to find studies to back up my following hypothesis, but I don't think I'm using the correct search terms.
I believe that teaching your child more vocabulary now is much better than trying to teach them all that same vocabulary and their age-appropriate vocabulary 4 years years from now. There's a soft limit of how many new words per day can be learned by any individual, given the methods used, intelligence level, and amount of effort expended. Whichever way you slice up that time, I don't see making up 4 years of opportunity to be a worthwhile process. The time and effort you expend doing that at a later age, no matter how trivial, could be used for any other educational experience. (This is my hypothesis.)
Unfortunately, most studies done on lexical development and language acquisition that I have access to are done to study second language acquisition, and not native language acquisition.
So, the only authoritative answer I can give on the subject must come from personal observation and experience. I am the authority when it comes to language acquisition, lexical development, and verbal comprehension of my son, who is now 2.25 years of age.
We teach my son numbers, colors, shapes, sizes, and any number of adjectives (and adverbs!) in addition to nouns (objects, animals, etc.)
It has had an amazing effect on our ability to communicate with him, and his ability to communicate with us. It has had an immediate functional value.
Here are examples of the things we can say to my son, where he has full comprehension and can perform the related tasks:
- Get your green shoes
- Get your socks. You need two of them.
- Which color shirt do you want to wear today?
- Can you get a blue car to play with?
- You can't throw hard balls inside. Please go get a soft ball.
- Do you want to eat more of the yellow chicken or the red chicken? (Actually Sesame vs General Tso's)
- Can you get Papa's blue shoes, with the white bottoms?
- What color melon do you want?
- That is hot, don't touch it!
- We can play outside if it's not raining.
- Can you pick up the toy with the letter A on it?
- Which shape do you want to use? (This is used a lot when using Play-Doh tools and cookie cutters!)
- Do you want the blue hat or the red hat?
- Your toy is on your top shelf, on the right
- Your car is behind you
- Don't go under the chair
- Stay next to Papa in the parking lot
And here are things he'll say to us without any prompting:
- "Want big green ball."
- "Want red berries."
- "Want white milk."
- "Want yellow frisbee."
- "Too loud."
- "Want big lion."
- "Too hot." (Food-wise.)
- "Me two crayons." (I have two crayons)
- "My red cup."
"My draw 'H'" (I drew an 'H')
These are examples that have happened within the last week, some of them today.
Our efforts towards increasing our son's vocabulary at 2 years of age have enabled us to:
- Communicate to him our desires and requests
- Elicit detailed responses from him
- Enabled him to more accurately and quickly express his needs and desires to us
I would never trade the ability I have to communicate meaningfully with my son just to remove a few moments of teaching time from our lives each day. We don't see these learning opportunities as a bother, and actually enjoy them.
I suspect that if my son could not communicate so well with us, we would spend considerably more time failing to communicate than we would ever spend teaching him these simple concepts. The nature of this assertion, however, makes it nearly impossible to study scientifically.