The Rolf Furuli "study" is highly flawed for many reasons which I will explore in a bit when I have more time.
His pro-JW bias is blatantly evident. Also, he relied on the opinions of JW elders in determining if people suffered from mental illness or not. Yes he did. I kid you not!
It's really a joke.
As LisaRose previously commented about this "study":
Being a lecturer in Semitic studies does not qualify you to do a study on mental health, it's frankly laughable. He may mean well but what are his credentials in mental health? What are his credentials in statistical analysis? Was this even peer reviewed?
You are talking about a religion where it is not acceptable to be unhappy, much less admit to mental illness, so how accurate could any casual study actually be? And in a religion that disfellowships people for bad conduct, how meaningful is any one time study anyway? Anyone seriously mentally ill will soon get disfellowshipped, which is going to skew the results. You would have to do a long term study from birth to determine how many actually became mentally ill, which I doubt was done here. I don't even know how he got this published.