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Perhaps the best way to begin our discussion is to describe how research on the influence of learner chararcteristics on second language learning has been carried out. When researchers are interested in finding out whether an individual factor such as motivation affects second language learning, they usually select a group of learners and give them a questionnaire to measure the type and degree of their motivation. The learners are then given a test to measure their second language proficiency. The test and the questionnaire are both scored and the researcher performs a correlation on the two measures, to see whether learners with high scores on the proficiency test are also more likely to have high scores on the motivation questionnaire. If this is the case, the researcher concludes that high levels of motivation are correlated with success in language learning. A similar procedure can be used to assess the relationship between intelligence and second language acquisition through the use of IQ tests.

Although this procedure seems straightforward, there are several difficulties with it. The first problem is that it is not possible to directly observe and measure qualities such as motivation, extroversion, or even intelligence. These are just labels for an entire range of behaviours and characteristics. Furthermore, because characteristics such as these are not independent, it will come as no surprise that different researchers have often used the same labels to describe different sets of behavioural traits.

(from Lightbown P.M. and Spada, N. How languages are learned. OUP. 1999/2008. p 51).