Initial research into bilingualism and cognitive functioning and into bilingualism and educational attainment often found bilinguals to be inferior to monolinguals.This connects with a naive theory of bilingualism which represents the two languages as existing together in a balance.The picture is of weighing scales，with a second language increasing at the expense of the first language.An alternative naive picture-theory attached to the early research is of two language balloons inside the head.The picture portrays the monolingual as having one well filled balloon.The bilingual is pictured as having two less filled or half filled balloons.As the second language balloon is pumped higher（e.g.English in the US），so the first language balloon（e.g.Spanish）diminishes in size.As one language balloon increases，the other decreases.

The balance and balloon picture theories of bilingualism and cognition appear to be held intuitively by many people.Many parents and teachers，politicians and large sections of the public appear to latently，subconsciously take the balloon picture as the one that best represents bilingual functioning.Cummins（1980a）refers to this as the Separate Underlying Proficiency Model of Bilingualism.This model conceives of the two languages operating separately without transfering and with a restricted amount of‘room’for languages.

What appears logical is not always psychologically valid.While both the balance or balloon ideas are plausible，neither fits the evidence.Earlier evidence in this chapter has shown when children become balanced bilinguals，there are cognitive advantages rather than disadvantages for being bilingual.Similarly，certain types of bilingual education（e.g.early total immersion and heritage language bilingual education）appear to result in performance advantages（e.g.in two languages and in general curriculum performance）compared with submersion or monolingual education.

Research has also indicated that it is wrong to assume that the brain has only a limited amount of room for language skills，such that monolingualism is preferable.Evidence suggests that there is enough cerebral living quarters not only for two languages，but for other languages as well.The picture of the weighing scales，of one language increasing at the expense the second language，does not fit the data.

Figure 7.10　Separate Underlying Proficiency Model of Bilingualism.Source：Amado，M.，Padilla，H.H.，Valadez.M.（1990）

There is another fallacy with the balance or balloon theory.The assumption of the theory is that the first and second language are kept apart in two‘balloons’inside the head.The evidence suggests just to the contrary that language attributes are not apart in the cognitive system，but transfer readily and are interactive.For example，when school lessons are through the medium of Japanese，they do not solely feed a Japanese part of the brain.Or when other lessons are in English，they do not only feed the English part of the brain.Rather lessons learnt in one language can readily transfer into the other language.Teaching a child to multiply numbers in Japanese or use a dictionary in English easily transfers to multiplication or dictionary use in the other language.A child does not have to be re-taught to multiply numbers in English.A mathematical concept can be easily and immediately used in English or Spanish if those languages are sufficiently well developed.Such easy exchange leads to an alternative idea called Common Underlying Proficiency（Cummins，1980a）.