Bilingualism, a subset of multilingualism, means having proficiency in two languages. A bilingual individual is traditionally defined as someone who understands and produces two languages on a regular basis. A bilingual individual's initial exposure to both languages may start in early childhood, e.g. before age 3, but exposure may also begin later in life, in monolingual or bilingual education. Equal proficiency in a bilingual individuals' languages is rarely seen as it typically varies by domain. For example, a bilingual individual may have greater proficiency for work-related terms in one language, and family-related terms in another language.

Being bilingual has been linked to a number of cognitive benefits. Research on how a bilingual individual's first language (L1) and second language (L2) interact shows that both languages have an influence on the function of one another and on cognitive function outside of language. Research on executive functions like working memory, perception, and attentional and inhibitory control, suggests that bilinguals can benefit from significant cognitive advantages over monolingual peers in various settings. There are also age-related benefits which seem to protect against cognitive decline in older adults.

Throughout the history of research into the cognitive advantages of bilingualism, views have shifted from a subtractive to an additive perspective: it is now believed that being bilingual adds to an individual's abilities rather than subtracting from it.

There is, however, strong disagreement over how findings on this subject should be interpreted. Systematic reviews and meta-analyses of executive functioning studies have failed to find compelling evidence for cognitive advantages in healthy adults or in participants across a broader age range. Moreover, the distribution of effect sizes in meta-analyses suggest publication bias, or that the reporting of bilingualism effects on executive functioning give a distorted view of the evidence.

According to the Singapore Management University (SMU) School of Social Sciences, research before the 1960s on bilingual individuals varied but commonly supported the idea that there were disadvantages to bilingualism. Researchers believed that bilinguals would have smaller vocabularies and stunted cognitive abilities. They also thought that children learning two languages at a young age would struggle differentiating and building proficiency in two languages to become competent in either. The idea that being bilingual was harmful to a child's linguistic and cognitive development, persisted. According to a historical review in "The Journal of Genetic Psychology," various researchers held these beliefs, noting a "problem of bilingualism" or the "handicapping influence of bilingualism." Following studies reported that bilinguals performed worse in IQ tests and suffered in most aspects of language development. These perspectives on bilingualism may have come from studies that did not control for socioeconomic status (SES) and gave IQ tests to non-proficient speakers of a second language in that second language. Many of these studies also used unstandardized and subjective definitions of bilingualism and of a bilingual individual, labeling someone as bilingual or monolingual through assumptions based on parent national origin, or based on family name.

Researchers began to change tone in the late 1950s/early 1960s, when Lenneberg, Chomsky, and Halle co-founded the field of biolinguistics and explored the role of biology in language. Their ideas led others to consider the role of human development more. In 1962, a turning point came about from a study that emphasized the importance of controlling for factors like age, sex, and SES, as well as of having a standardized measure for bilingualism when selecting a sample of bilinguals to be studied. Researchers carefully matched bilingual to monolingual participants and found that the bilinguals appeared to have significant advantages to that of their monolingual peers, outperforming in both verbal and non-verbal tests, more specifically in the non-verbal tests. In continuation of this study, research after this point began to shift focus, investigating areas of cognitive development and aptitude like perception and executive functioning. In 1967, publication of Lenneberg's seminal book, Biological Foundations of Language, first introduced the idea of a critical period of language acquisition, now better known as a sensitive period and further influenced bilingualism ideas. In 1977 the American Institutes for Research published an influential study which discussed bilingualism as it relates to education - how it affects a child's performance compared to peers. This study played a large role in our understanding of multilingualism and the effects that it has on the brain.

Since the late 1970s, researchers have found more cognitive benefits of bilingualism, including better attention, task-switching, and protection against aging declines. Over time, the prevalence of bilinguals in the United States has also increased. While the United States Census Bureau does not directly poll for bilingualism, they do poll for household languages. Non-English-speaking households are also surveyed for English proficiency. A 2012 interpretation of Census Bureau results observed that 11% of the population was bilingual in 1980, 14% in 1990, and 20% in 2012.

Executive function is the group of high-level cognitive processes that assists in goal-oriented tasks, such as problem solving, mental flexibility, attentional control, inhibitory control, and task switching. Much of the current research on cognitive effects of bilingualism studies the potential relationship between bilingualism and executive function; monolinguals and bilinguals may have executive function differences. Studies that match age and background factors like SES have found that bilinguals, who experience the mental demands of learning multiple languages, may have greater executive control and experience other cognitive benefits than their monolingual peers. Executive function may also have effects for older adults. Adults who learned a second language at a young age show better control and inhibitory processing than monolingual adults, as well as greater protection from cognitive decline such as dementia. Notably, some scientists continue to debate whether a bilingual advantage in executive functioning exists, with some studies and meta-analyses coming to opposite conclusions.