looking for investigations about the impact of agile methods to the motivation

Annika,

While I am not aware of the exact piece work you need, I would recommend that you had a look at the recent book written by Jeff Sutherland on Scrum. It's called "Scrum: The Art of Doing Twice The Work in Half the Time".

It's not about the Scrum framework, but much more about how Scrum can be used in several different ways, outside of the IT universe.

It's not the answer to what you seem to be looking for, but it can be helpful.

Cheers.

Thanks Demerson!

There are some positive things you have written, but stoicism can be another band aid if you have a mental illness, such as anxiety and depression. The willingness to endure pain is not always significant. Mental disease is a complicated disease consisting of multiple factors that are not well known by many. The solutions also mix multiple aspects through creativity and mistakes. 

As a product of OCD and depression, I have suffered a lot myself. I handle others as a piece of crap with compassion and empathy. I've been therapy for years, exercise, healthy food, medicine, meditation, every day ect. My brain still works against me, while I have made several advances. Usually, you must suffer from it yourself to understand mental wellbeing. The easiest way to cope with anxiety and depression is to seek clinical assistance.

There are some interesting blogs and videos on how people have used Scrum to help their families when it comes to some things like ADHD and Asperger's here.