Malcolm Knowles would be the only reference I could think of offhand. He's been around for a long time and has several principles of adult learning. Adults learn best when they have a need to know. Another one is something about adults learn better when they can direct their own learning. I'm sure if you search for his name you will find something.
The person who is done the most recent research is Ruth Clark. She has a number of books out and I have many of the but I have not seen anything in her writings about this that I can recall. You may be able to track her down and send her an email. I'm pretty sure she has a website.
