The reasons for learning a second language can be endless but the secret to success is rooted in the science of motivation

Second language professionals, after explaining what we do for a living, are inevitably asked "What's the fastest/best/most foolproof method for learning a language?" Some of us like to answer: language by partner, meaning, go to the country and fall in love with someone who speaks only the language you want to learn and not yours. In academic terms, what we are recommending is that people make the most of something that is crucial for language learning – motivation.

Most people find that learning another language after childhood is hard. Although not everyone who is motivated to learn manages to succeed, most people who do achieve a high level of success also turn out to be very strongly motivated.

Researchers who study the relationship between language learning and motivation explain that some learners are "intrinsically" motivated. These learners might want to learn a language to a higher level so that they can understand their grandparents who live in another country and are getting older. Or they might want to teach their children a second language because they want them to have the cognitive and social advantages of being bilingual, people like Guardian writer Louise Tickle, whose blog illustrates her efforts as an intrinsically motivated parent.

"Extrinsic" motivation, in turn, stems from outside sources, such as a student needing to pass a test in order to satisfy a language requirement. In practice, of course, people tend to have a variety of reasons, both intrinsic and extrinsic, for why they want to learn, and different aspects motivate them at different times.

Professors Robert Gardner and Wallace Lambert conducted pioneering work in studying motivation; they point out the importance of other qualities, such as aptitude. Based on their research in officially bilingual Canada, they noted that some learners are integratively motivated to learn a language, meaning they feel an affinity for it, they want to get to know people who speak that language, and they are often also interested in the culture associated with the language. This is similar to intrinsic motivation.

Instrumental motivation, on the other hand, kicks in where people primarily want to learn a language for practical reasons, such as a salary bonus or getting into university, similar to extrinsic motivation. It has been shown to be less powerful in language learning success.

Of course, people's motivation can change. Professor Zoltán Dörnyei from Nottingham University has pointed out that "every different psychological perspective on human behaviour is associated with a different theory of motivation". His own model is known as the "L2 motivational self system", and it focuses on learners' abilities to envision an ideal future for themselves as positive, competent and successful second language speakers.

Dörnyei's model also recognises that motivation is dynamic rather than static. So, motivation fluctuates as it is impacted by various factors in real time, moment to moment. A learner's motivation levels can go up or down depending on how motivated, or interested, or involved their conversational partners are (these might be native speakers, or fellow learners and their instructors in classrooms). In other words, motivation can be contagious.

It can also be influenced by the task that learners are undertaking and how interesting and engaging it is. Deborah Tannen, professor of linguistics at Georgetown University and author of several bestselling books, says that "each person's life is lived as a series of conversations". When asked how this applies to learning language, Tannen notes that "wanting to have certain kind of conversations, with people one knows or wants to know, can be a huge motivator in learning a new language". She follows this up with a personal example. Although her first husband, who was Greek, spoke English fluently, Tannen had to learn Greek in order to communicate with his mother, a woman she came to love deeply.

Dörnyei and his colleagues have recently carried out classroom research, finding that learners who can construct ideal self-images as successful second language speakers are better able to reach their goals. They need to have a clear roadmap of tasks and strategies to follow in order to do this, and Dörnyei makes the analogy between teaching a language and creating a training plan for an Olympic athlete. Like sports coaches, he explains, "instructors should take the perspective of trainers and cheerleaders, helping learners to imagine themselves in their ideal L2 personae and thus leverage their motivation to better their learning outcomes".

What about learners who are strongly motivated to learn a language but still don't succeed? Some of us just don't have strong aptitude for language learning. We might be good at other things, such as science, art, music or sports. Researchers traditionally look at the relationships among different kinds of aptitude and language learning outcomes. More recent approaches have started to study exceptionally successful second language learners – the rare few who end up sounding just like native speakers – to understand their particular cognitive skills.

It seems that at least three things are important for language learning success. These are your working memory, which can be thought of as how you hold a phone number in your head before you write it down, your associative memory, or how well you connect new and known information, and how strong your mechanisms are for implicit learning, which can be seen as the ability to figure out patterns in information. If you have these aptitude skills, along with high levels of various types of motivation, you're most likely to succeed, especially if you have access to instruction that has been tailored to match your abilities and interests.

However, what about those of us who aren't lucky enough to have this constellation of aptitude and motivation, or access to good instruction? As the evolutionary psychologist, linguist and bestselling science author Steven Pinker has pointed out in these pages "our behaviours are a result of neurophysiological activity in the brain. There is no reason to believe there is any magic going on." The idea of people being hard wired for second language learning is fascinating and, perhaps, appealing in that it might be used to get some of us off the hook if our language learning journey is less than successful.

Alison Mackey is professor of linguistics at Georgetown University and Lancaster University

• On 10 July, the Guardian and British Academy are holding a debate about the implications of medically enhanced language learning, asking the question of whether drugs are the answer to language learning