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Vaughn, K. A., & Hernandez, A. E. (2018). Becoming a balanced, proficient bilingual: Predictions from age of acquisition & genetic background. Journal of Neurolinguistics, 46, 69–77. doi:10.1016/j.jneuroling.2017.12.012

Summary by Kelly A. Vaughn


In this study, Vaughn and Hernandez (2018) examine how age of second-language acquisition interacts with genetic variants from dopamine-related genes to predict bilingual proficiency in young adulthood. These genetic variants have been previously found to predict cognitive flexibility, which is necessary for switching between tasks, goals, or even languages. For example, one might use cognitive flexibility to take a different route to work upon seeing a lot of traffic on the normal commute.

The researchers genotyped Spanish-English bilingual young adults for genetic variants associated with cortical (frontal lobe) and subcortical (deep brain) dopamine. Participants reported their age of English acquisition (AoA) and completed measures of proficiency in English and Spanish. The proficiency scores were combined to create a “bilingual proficiency” score, which considered the level of proficiency in each language along with the similarity of proficiency levels. Bilingual proficiency was predicted by an interaction between AoA and the genotype of both dopamine-related genetic variants. Overall, results indicated that becoming a proficient bilingual from an early AoA is related to genetic markers of subcortical dopamine and that becoming a proficient bilingual at a later AoA is additionally related to markers of cortical dopamine.


Current neurobiological theories of language learning highlight the importance of connections between the cortical and subcortical regions of the brain for language learning and flexibility in the use of multiple languages, depending on the context. Dopamine may be important for these connections, as cortical and subcortical dopamine have associations with learning, motivation, and cognitive flexibility. The Sensorimotor Hypothesis proposed by Hernandez and Li (2007) suggests that earlier language learning is associated with the development of subcortical structures and that later language learning is associated with the development of cortical structures. Therefore, it may be important to consider AoA to understand how cortical and subcortical dopamine relate to language learning.

The role of dopamine in language learning may have to do with flexibility. To develop high and balanced proficiencies in two languages, a bilingual often needs to flexibly switch between the two languages in different contexts. For example, when a bilingual child is at home working on homework, he or she needs to complete the assignment in English but may need to switch to Spanish to communicate with other family members in the room. A bilingual child who has trouble switching between the two languages may have difficulty completing his or her homework or switching out of English to respond to Spanish-speaking relatives. Therefore, the genotypes that predict higher levels of flexibility may be beneficial to achieving a high level of bilingual proficiency. However, being too flexible may make it difficult to stay in a single language during a conversation or task. A very flexible child may not notice that he or she completed some of his or her homework in Spanish or may use English words with a Spanish-speaking parent because they came to mind more quickly. It may be the case that having a balance between flexibility and stability is better for achieving bilingual proficiency. This balance between flexibility and stability would allow the child to focus on English while completing homework but still comfortably switch to Spanish to speak to a family member. This balance can be achieved by having a more flexible genotype in the subcortical variant and a more stable genotype in the cortical variant or vice versa.

Finally, it is possible that the amount of stability and flexibility that is beneficial for bilingual proficiency is different when the language is learned early in life compared to when the language is learned later in life. Based on the Sensorimotor Hypothesis, subcortical dopamine should matter more early in life while subcortical (deep) brain structures are still developing. Cortical dopamine may be more important for a language learned later in life, as cortical regions such as the frontal lobe are the last brain regions to develop. This study examined whether the genotypes interacted with AoA to predict bilingual proficiency.

Study Purpose and Methodology

The goal of the study was to understand how genetic variants associated with cortical and subcortical dopamine interacted with AoA for Spanish-English bilinguals to predict adult bilingual proficiency. Participants were young adult Spanish-English bilinguals who reported their AoA to be between 0 and 17 years. They provided saliva as a DNA sample, which was used to determine their genotypes for the genetic variants associated with cortical and subcortical dopamine. The researchers measured bilingual proficiency using the picture vocabulary and passage comprehension subtests of the Woodcock-Muñoz Language Survey – Revised. Bilingual proficiency was calculated as a sum of the scores for each language (English and Spanish) multiplied by a measure of how similar the scores were between the two languages. This gave participants who had mid-level proficiency in two languages a higher score of bilingual proficiency than participants who had high proficiency in one language and lower proficiency in the other language because they are more balanced in their proficiency. A multiple regression model was conducted to predict bilingual proficiency from the two genetic predictors, AoA, and the two-way and three-way interactions among these variables, while controlling for participants’ socioeconomic status.

Key Findings

There was a significant three-way interaction between the two genetic variants and AoA. The significant interaction predicted that when participants learned a language before age 5, having the more flexible variant of subcortical dopamine was associated with the highest levels of bilingual proficiency. Around age 5, the genotypes did not matter; all participants with an AoA around age 5 achieved about the same level of bilingual proficiency. At later AoAs, there was the most variability based on genotype, with both cortical and subcortical dopamine-related variants predicting bilingual proficiency.

Conclusions and Recommendations

These findings support the Sensorimotor Hypothesis (Hernandez & Li, 2007), which states that subcortical development is associated with early language learning and that cortical development is associated with later language learning. Early in life, having higher levels of subcortical dopamine predicts higher levels of bilingual proficiency, which is probably the case because the subcortical regions of the brain are still developing and are therefore responsive to the two languages in the environment. Later in life, cortical and subcortical dopamine both predict bilingual proficiency, with the best outcomes for individuals who have more subcortical dopamine and balanced levels of cortical dopamine. That finding suggests that bilinguals who achieve high proficiency later in life can be flexible when needed and stable when appropriate, reflected in balanced levels of cortical dopamine. Finally, bilinguals who acquire their second language around age 5, as they are entering school, seem to achieve similar levels of bilingual proficiency regardless of their genetic background. That may be the case because the school environment, which involves immersion in the second language, provides a foundation that allows everyone to achieve a similar level of bilingual proficiency. These results suggest that children with various genetic backgrounds can achieve bilingual proficiency if they are immersed in elementary school and that balancing when you should be flexible with your two languages or stick to one language may aid in achieving bilingual proficiency at a later age.

For Further Reading

Hernandez, A. E., Greene, M. R., Vaughn, K. A., Francis, D. J., & Grigorenko, E. L. (2015). Beyond the bilingual advantage: The potential role of genes and environment on the development of cognitive control. Journal of Neurolinguistics, 35, 109–119. doi:10.1016/j.jneuroling.2015.04.002

Hernandez, A. E., & Li, P. (2007). Age of acquisition: Its neural and computational mechanisms. Psychological Bulletin, 133, 638–650. doi:10.1037/0033-2909.133.4.638

Vaughn, K. A., Ramos-Nuñez, A. I., Greene, M. R., Munson, B. A., Grigorenko, E. L., & Hernandez, A. E. (2016). Individual differences in the bilingual brain: The role of language background and DRD2 genotype in verbal and non-verbal cognitive control. Journal of Neurolinguistics, 40, 112–127. doi:10.1016/j.jneuroling.2016.06.008

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