English Language Learner versus Language Learner
I am a bilingual and I think it is a worthwhile endeavor to become one. Becoming proficient and conversant with two languages is truly a rewarding accomplishment. Becoming a bilingual, unfortunately, is not just a matter of wanting to become one. Cognitive abilities in one language are expected to influence progress in a second language. Not taking into account the quality of instruction and exposure can easily impede a child's progress in both languages.
A recent paper published in the Journal of Educational Psychology looks at how the language used at home affects a child's proficiency in two languages. Kijoo Cha and Claude Goldenberg of Stanford University report that there is indeed a relationship between a child's proficiency in two languages. A child who is proficient in Spanish is more likely to be proficient in English as well, but the relationship is moderated by what happens inside a child's home. The following figure, for instance, shows how complex these relationships really are.
An opposite trend is seen when oral proficiency in English (instead of Spanish) is plotted against the language spoken at home and students are grouped as separate lines, this time, according to their proficiency in Spanish. The line that turns out to have the least slope corresponds to students who are well above average in Spanish. The English proficiency of children who are orally fluent in Spanish seems less correlated with the amount of Spanish/English spoken at home.
This study shows that it is important to consider where children are in their language learning. In order for children to grow in their mother tongue, one must take into account not just use of the language, but how the language is used. How cognitive abilities are emerging in the child with the use of the mother tongue needs consideration as deficiency in either language points to a higher likelihood of deficiency in both.