g attends to whether his or her performance
meets the standards he or she has learned 1 2 3 4 5
h enjoys grammar exercises 1 2 3 4 5
i begins learning in childhood 1 2 3 4 5
j has an above-average IQ 1 2 3 4 5
k has good academic skills 1 2 3 4 5
l has a good self-image and lots of confidence 1 2 3 4 5

All of the characteristics listed above can be classified into five main categories: motivation, aptitude, personality, intelligence, and learning style. However, many of the characteristics cannot be assigned exclusively to one category. For example, the characteristic `is willing to make mistakes' can be considered a personality and/or a motivational factor if the learner is willing to make mistakes in order to get the message across.

Research on learner characteristics
Perhaps the best way to begin our discussion is to describe how research on the influence of learner characteristics 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 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 investigates whether a learner with a high score on the proficiency test is also more likely to have a high score on the motivation questionnaire. If this is the case, the researcher usually concludes that high levels of motivation are correlated with success in language learning. A similar procedure can be used to measure the effects of intelligence on second language learning through the use of I Q 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 aptitude, 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.