It would be impossible to publish research in any respectable peer-reviewed psychology journal that was not up to this high standard with concomitant rigorous statistical analyses. These are the standards in psychology. When someone comes up with a new theory in psychology or any other science, for that theory to be of any use, it has to be able to make predictions and those predictions have to be verifiable. If there is no way to disprove a theory, it can explain everything that happens, yet it doesn't predict anything specific that can be tested, it is considered a poor theory.
We tend to think of gratitude as a spontaneous emotion, something that just happens to us in moments of triumph or success. In reality, though, gratitude is something we develop.And just like all the other not-so-secret secrets on this list, it is something we choose, something we make a wide-eyed, premeditated, self-determined decision to experience.