I sent an email to David Myers asking him about the umlaut article. Prof. Myers was kind enough to send me the following response:
This is a misreported story that took on a life of its own. I’ve never done research on German speech and personality (this was an utterly mistaken press report misattributed to me that has recycled for several years). But here is what I do report in my introductory psychology text:
Saying the phonemes e and ah, which activate smiling muscles, puts people—believe it or not—in a better mood than saying the German ü (rather like saying the English e and u together), which activates muscles associated with negative emotions (Zajonc & others, 1989).
Zajonc, R. B., Murphy, S. T., & Inglehart, M. (1989). Feeling and facial efference: Implications of the vascular theory of emotions. Psychological Review, 96, 395–416.
Let us set the record straight. The original press report was unreliable, Professor Myers does not stand for the proposition that frequent pronunciation of umlauts "causes German grumpiness." He has only cited, in passing, a study that appears to support a much more limited proposition. If I didn’t have a day job, I might actually want to track down the Inglehart & Murphy article.
The decision to wait and see whether I could find out more about this theory has paid off. I believe this is the first time anything approaching journalism has ever been committed on German Joys. I promise it won’t be the last!