For example, in motivation questionnaires, learners are often asked whether they willingly seek out opportunities to use their second language with native speakers and if so, how often they do this. The assumption behind such a question is that learners who report that they often seek out opportunities to interact with speakers of the second language are highly motivated to learn. Although this assumption seems reasonable, it is problematic because if a learner responds by saying `yes' to this question, it not only suggests that the learner is highly motivated, but also that the learner has more opportunities for language practice in informal contexts. Because it is usually impossible to separate these two factors (i.e. willingness to interact and opportunities to interact), many researchers have been seriously criticized for attempting to present these characteristics as independent.

Another factor which makes it difficult to reach conclusions about relationships between individual learner characteristics and second language learning is how language proficiency is defined and measured. To illustrate this point let us refer once again to the personal characteristic of motivation. In the second language learning literature, it is not uncommon to find that while some studies report that learners with a higher level of motivation are more successful language learners than those with lower motivation, other studies report that highly motivated learners do not perform any better on a proficiency test than learners with much less motivation to learn the second language. One explanation which has been offered for these conflicting findings is that the language proficiency tests used in these studies do not measure the same kind of knowledge. That is, highly motivated learners are found to be more successful in some studies because the proficiency tests measure oral communication skills. In other studies, however, highly motivated learners are not found to be more successful because the tests are primarily measures of grammatical knowledge. Results such as these imply that motivation to learn a second language may be more related to particular aspects of language proficiency than others.

Finally, there is the problem of interpreting the correlation of two factors as being due to a causal relationship between them. That is, the fact that two
things tend to occur together does not necessarily mean that one caused the other. Research on motivation is perhaps the best context in which to illustrate this. Learners who are successful may indeed be highly motivated. But can we conclude that they became successful because of their motivation? Is it not also plausible that their success heightened their motivation? In fact, it is very difficult to show with certainty that the correlations that are found between learner characteristics and success in second language acquisition are indicative of a one-way causal relationship. It seems more likely that, at least for some of these individual differences, the characteristic may contribute to success, but success also contributes to the enhancement of characteristics such as motivation, risk-taking behaviour, or even performance on tests which measure aptitude for language learning. As we will see, research on individual differences often permits multiple interpretations. In the sections below, we will look at `what people say' about individual differences and `what the research shows' to support or refute these opinions.