Recently I spoke with a missionary department staff person (female) who told me the following:
The most missionaries serving was 62,000 about 5-7 years ago. Currently there are about 52,000 out and about.
Of these 2,800 are "older missionaries" and 13,000 are sister missionaries.
About 5% of missionaries are on some form of psychiatric medications for depression, moods swings or anxiety. On average more sisters than elders are on psych medications.
About 20 elders a year have a psychiatric break down (either become psychotic or suffer dibilitating depression) and most do not return to their missions.
The missionary department works much harder at stabilizing and maintaining the elders than the sisters with psychiatric problems in order to keep them out in the field. On average more sisters return home early with psych/emotional problems than elders. (no percentage given)
An elder may be returned home for psych treatment for up to six months and then sent back out the mission field.
On average missionaries in South America demonstrate more psychiatric problems.
The reason for decline in overall missionary numbers is due to baby boom demographics, etc. They saw the decline coming and tried to prepare for it.
Note: This was the best info I could get from this person without them becoming suspicious. She was not in a position to cite actual reports or studies. Rather, these were her impressions from working in the department the past few years.
She seemed to think that prospective missionaries with severe emotional problems are being screened out better thus preventing them from entering the mission in the first place.
So there you have my "return and report from my labors in the Church Office Building day."