Bilingual children, whether they’re sequential or simultaneous bilinguals have divided input. In their every day experiences. Because of this, they often know some words in their L1 and other words in their L2; some words (but not ALL) they know in both. There are a number of studies that shows this for children at different ages.
Pearson & colleagues tested Spanish-English bilingual and monolingual toddlers between 8 and 30 months. They compared expressive and receptive vocabulary in the two groups. When single-language scores were compared for bilinguals, they were lower than scores of monolingual norms in both languages. On average they were in about the 20th percentile in both. When they did conceptual scoring the average score was about the 35th percentile; and when they calculated a total vocabulary score, the average was about the 40th percentile– these are well within the normal range, and was right at the range for the monolingual children who scored at about the 40th percentile also. Comprehension showed similar patterns, in Spanish or English alone, average scores were in the 25th or 30th percentile. Total conceptual scores were at about the 45th percentile and total scores were at the 50th percentile, which is right at average. When they looked at the number of words produced by age, conceptual scores and total scores of the bilinguals showed the same means and growth as the monolinguals.
So, what does this look like?
- 16 months– monolinguals: 44 words bilinguals: 42 total words (28 conceptual)
- 20 months– monolinguals: 109 words bilinguals: 186 total words (118 conceptual)
- 26 months– monolinguals: 406 bilinguals: 414 total words (327 conceptual)
You can see a couple of things here– bilinguals and monolinguals show about the same growth rate and the same average numbers of words at each age. And the conceptual scores are a little lower than total words, but still show that they have a lot of words that are different in each language.
In a more recent study, Cynthia Core and colleagues compared total and conceptual scores in Spanish-English bilinguals from 22 to 30 months. They found that the total score was more comparable to monolingual norms (on the CDI) compared to conceptual scores. For this reason, it’s probably best clinically to look at total scores– at least in young children.
So, yes, if you look at only one language– bilingual children on average look a little delayed. But, it’s a matter of degree– a score 1SD below the mean would be about 225 words at age 26 (this is the 25th percentile). The 20th percentile would be about 180 words. Obviously, to get to the mean of 400 a child would need to know 220 in the other language. And you do see a lot of individual variation in how many words children know it each language. This knowledge is associated with how much they hear and use each language on a daily basis primarily, and to a lesser extent when they started learning a second language. It’s important to test children in both and put the two languages together and to know how delayed a normal bilingual “delay” might actually be.