I worked out for 7 years, without skipping a single day; motivation: External
I gotten thru 4 years of University; motivation: External;
I've been with my Company for almost 5 years now; motivation: External;
In fact the only thing I stick to that is of internal cause is my hobbies.
I.e: Playing games, watching Animes, reading comics, eating & etc...
I'm pretty sure this is an example that the research doesn't reflects the real life...
It would be better to do an population analysis as compared to conducting participated studies.