I did see one such study that looked at emotional expressions/body language rated on a Joy-Sadness Display Scale by 5 trained observers in addition to self-repo

I did see one such study that looked at emotional expressions/body language rated on a Joy-Sadness Display Scale by 5 trained observers in addition to self-reported measures: https://www.researchgate.net/publication/233265431_Experiencing_Flow_Is_Doing_it_Together_Better_Than_Doing_it_Alone https://www.researchgate.net/publication/233265431_Experienc... Of course, this wouldn't really work for a longitudinal study.