Abstract

In today’s increasingly bilingual world—where one in twenty adults is diagnosed with attention-deficit/hyperactivity disorder (ADHD)—surprisingly little is known about how bilingualism and ADHD interact. This dissertation investigated how vocabulary size and inhibitory control are influenced by bilingualism, ADHD, and their intersection in young adults.

Manuscript 1 examined vocabulary size in 391 English and/or French-speaking young adults. Study 1 grouped participants into two categories: monolinguals and bilinguals, with and without ADHD. Study 2 analyzed bilingualism and ADHD symptomatology as continuous variables. Contrary to expectations, individuals with ADHD had larger vocabularies than their non-ADHD peers—in both monolinguals and bilinguals and across languages. Furthermore, ADHD symptomatology was positively associated with vocabulary size, especially in monolinguals and less-balanced bilinguals, challenging deficit-based views of ADHD.

Manuscript 2 examined response inhibition and inhibition of interference in 274 young adults using the stop-signal and flanker tasks. Bilinguals with ADHD showed enhanced response inhibition, particularly under high task demands. No group differences were found for inhibition of interference, where performance was high across groups. These results support the idea that bilingual advantages emerge under cognitive challenge—whether through aging, as previously documented —or in the presence of ADHD, as demonstrated here.

Together, these findings refute the notion of a ‘double disadvantage’ for bilingual individuals with ADHD. Instead, both bilingualism and ADHD served as sources of cognitive strength. Their combination did not impose an additional burden and enhanced performance when cognitive demands were high. These findings highlight how linguistic and neurodevelopmental diversity can foster increased adaptability.