Bilingualism depends upon the quality, not quantity, of exposure to languages from birth.
Children grow to be bilingual when they learn any two languages from the time they were born and as they grow up. Of course, people can also become bilingual or multilingual when they learn other languages when they're older. In the case of languages learned from birth, however, researchers have some interesting findings.
Each language develops in the child independently from each other, but their development reflects the exposure of the children to the languages. There's a strong relationship between vocabulary and grammatical complexity as the language develops in monolingual children. According to this study, bilingualism develops in the same way that language develops in monolingual children.
A team of researchers conducted a longitudinal study on children, aged two and a half to four years old, who had been exposed to English and Spanish since birth. These children spoke both languages fluently. The researchers analyzed the vocabulary and grammatical development in these children every six months.
The researchers wanted to know if grammar and vocabulary have a relationship specific to particular language, or if the relationship is language general. They theorized that perhaps vocabulary and grammar develop at the same rate because of certain qualities in a child. They also theorized that either vocabulary is the foundation of learning grammar, or grammar is the foundation of learning vocabulary. A third theory is that the development of vocabulary and grammar relies on something external to the child.
According to the data the researchers collected, the third theory makes the most sense. If an internal quality in the child is what drives language development, then there should be a better relationship between the development of English and Spanish in the child. If learning vocabulary and learning grammar depend on each other, then the relationship between the two should be language-specific. The findings ruled these two possibilities out.
Researchers also found that English can overtake Spanish, though Spanish doesn't overtake English. When a child learns both these languages simultaneously, strengthening English skills can lead to weakening Spanish skills. However, growing Spanish skills don't affect English skills. This shows that learning one language doesn't necessarily affect learning the other.
Thus, if a child is learning English at a better rate than another child is, this is because of the quality of exposure to the English language. If a child is better at Spanish, it may be because its parents have a richer quality of Spanish than English. Exposure to external influences is what drives the growth and development of a certain language in a child.
Bilingualism doesn't develop as easily or as well due to simple exposure, however. The researchers stress that the exposure to the language has to be rich and high-quality. This way, language develops at a better rate.
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