Mental contrasting is one of the more tested useful interventions in psychology, including in field studies intended to help actual people with their actual goals. (Though in field studies it's often paired with implementation intentions, an even-more-tested intervention, to give people a double dose of help, which makes those studies less informative about the benefits of either intervention.)
The papers cited in the op-ed, and many others, are available on Oettingen's website. One of the papers is a 2012 review article summarizing the research; it's rather long but you can get a decent picture by reading all the section headings (and then diving into whatever seems interesting):
Oettingen, G. (2012). Future thought and behavior change.%20In%20W.%20Stroebe%20&%20M.%20Hewstone.pdf). In W. Stroebe & M. Hewstone (Eds.), European Review of Social Psychology, 23, 1-63.
"Positive thinking" is not a very precise phrase. In this case it seems like it's used to mean "daydreaming".
I'm more convinced that the kind of positive thinking described in this article is useful: http://tynan.com/how-to-be-happy-always (If anyone has a convincing rebuttal to it, I'm interested.)
I like that article. For people capable of thinking about what methods make humans happy, it seems unlikely that simply performing any feel-good method will overcome barriers as difficult as what happiness means or what use is happiness anyway. They might improve one's outlook in the short term, or provide an easier platform to help answer those questions, but to me the notion that therapy works because of therapists (a sort of research supported idea if I recall correctly) corresponds well to the intuition that humans are just too wrapped up for overly easy feel-good solutions to work. (This is as opposed to psychiatric solutions to psychiatric issues, for which you should be following this algorithm if you're depressed).
I've had trouble with the notion that happiness is even a goal to be strived for at all, because of the self-referential reality that a really good way to become happy is to become less self-focused, but that thinking about being happy is sort of self-focused. In that sense, I'd much rather seek out "fulfillment" or "goodness" than "happiness," but I now think that my issue here is just an artifact of the language of people using the word "happy." That word is just too wrapped up in ideas that make it out to be something like wireheading, which as we know is something that nobody actually wants. And so while I do think people looking for X often stop short with not-very-desirable things, it's good to separate this from people who actually want to be the most good kind of happy, the kind that one would always want, and maybe still even call "happy."
One paper by the the authors (the directly referenced paper is paywalled):
Sad Mood Promotes Self-Initiated Mental Contrasting of Future and Reality.%20Emotion.pdf)
Self-regulation by mentally contrasting a positive future with negative reality leads people to differentiate in their goal commitments: They commit to goals when expectations of success are high and let go when expectations of success are low. On the contrary, when indulging in the positive future or dwelling on negative reality, people fail to consider expectations of success and do not form selective goal commitments (Oettingen, Pak, & Schnetter, 2001). Whereas prior research has examined the effects of experimentally induced mental contrasting, we address sad mood as a contextual influence promoting self-initiated mental contrasting. Across various mood inductions, sad moods--which are associated with problem solving strategies--facilitated self-initiated mental contrasting more than neutral moods (Studies 1, 5) or happy moods (Studies 2, 3, 4, 6). Importantly, mood did not affect the relation between mental contrasting and selective formation of goal commitment (Studies 5, 6). The results suggest that sad moods aid in self-regulation by making people self-initiate goal commitments that are sensitive to their expectations of success.
Psychology researchers discuss their findings in a New York Times op-ed piece.
The take-home advice:
Positive thinking fools our minds into perceiving that we’ve already attained our goal, slackening our readiness to pursue it.
...
What does work better is a hybrid approach that combines positive thinking with “realism.” Here’s how it works. Think of a wish. For a few minutes, imagine the wish coming true, letting your mind wander and drift where it will. Then shift gears. Spend a few more minutes imagining the obstacles that stand in the way of realizing your wish.
This simple process, which my colleagues and I call “mental contrasting,” has produced powerful results in laboratory experiments. When participants have performed mental contrasting with reasonable, potentially attainable wishes, they have come away more energized and achieved better results compared with participants who either positively fantasized or dwelt on the obstacles.
When participants have performed mental contrasting with wishes that are not reasonable or attainable, they have disengaged more from these wishes. Mental contrasting spurs us on when it makes sense to pursue a wish, and lets us abandon wishes more readily when it doesn’t, so that we can go after other, more reasonable ambitions.