Lessons from a Paradoxical Hypothesis : A Methodological Critique of the Threshold Hypothesis

Up to the 1960s, bilingualism was regarded as having a harmful effect on cognitive development. Early studies (e.g., Laurie, 1890; Saer, 1923; Smith, 1923) showed that bilingual children scored lower on measures of verbal intelligence than monolingual children, concluding that bilingualism hampers children’s development of intelligence and leads children to psychological confusion. More recent studies on bilingualism (e.g., Ianco-Worrall, 1972; Peal and Lambert, 1962) supported the opposing view that bilingualism has a positive effect on children’s cognitive development. For example, by studying 10-year-olds from French schools in Montreal, Canada, Peal and Lambert (1962) found that the bilingual group scored higher than the monolingual group on the measures of both verbal and nonverbal intelligence. This finding contradicted the results of the previous research that bilingual children were considered to be cognitively inferior to monolingual children. Cummins (1976) attempts to provide an adequate explanation for the inconsistent findings from studies on the relationship between bilingualism and cognitive development. The hypothesis proposed by Cummins to account for the data available at that time is called the threshold hypothesis. The threshold hypothesis proposes that there may be a threshold level of bilingual proficiency that children must attain in order to gain advantages and avoid disadvantages in their cognitive development. Cummins (1976) identified two thresholds, the higher and lower threshold levels of bilingual proficiency: This raises the possibility that there may be not one but two thresholds. The attainment of the first threshold would be sufficient to avoid cognitive retardation but the attainment of a second, higher, level of bilingual competence might be necessary to lead to accelerated cognitive growth. (p. 24) In other words, bilingual children must attain the lower threshold level in either language to avoid negative effects on their cognitive development. However, the attainment of the lower threshold level does not guarantee bilingual children cognitive advantages. Bilingual children must attain the higher threshold level in both languages to obtain positive effects on their cognitive development. The threshold hypothesis with two threshold levels is shown in Figure 1. This hypothesis seems useful in accounting for the findings that some children benefit from bilingualism whereas others do not. According to the hypothesis, if bilingual children have not attained the lower threshold level in both languages, they are regarded as limited bilinguals and their language proficiency is at a low level in both languages. This situation is what Cummins (1979) calls semilingualism. If bilingual children attain the lower threshold level in either language, they are regarded as partial bilinguals and their language proficiency is at an age-appropriate level in only one language. Partial bilingualism has neither positive nor negative effects on children’s cognitive development. This type of bilingualism is called dominant bilingualism. If bilingual children attain the higher threshold level in both languages, they are regarded as balanced bilinguals and their language proficiency is at an age-appropriate level in both languages. This type of bilingualism is called additive bilingualism. Thus, the findings of the early studies on bilingualism are associated with semilingualism whereas the results of more recent research are associated with additive bilingualism. However, with some knowledge of methodology and common sense, it is easy to conclude that the threshold hypothesis is paradoxical and it is virtually impossible to support in a meaningful manner. If policymaking should be driven by tested theory, as Cummins (1999) himself asserts, theories in the research on bilingualism are integral to bilingual education policy. This means that incorrect conclusions based on a theory may affect policymaking in a way that students are put at a disadvantage. Although the hypothesis has already been criticized by other researchers (e.g., MacSwan, 2000; Martin-Jones & Romaine, 1986; Valadez, MacSwan, & Martinez, 2000), some researchers still claim that the hypothesis is supported by research (e.g., Cummins, 2000; Lasagabaster, 1998; Lee & Schallert, 1997; Ricciardelli, 1993; Roller, 1988; Schoonen, Hulstijn, & Bossers, 1998). Thus, additional discussion on the threshold