For example, positing that the impulsivity in ADHD is caused by an impaired executive function - this doesn't really tell us anything useful when we define the executive function as something which governs self-control and the ability to resist impulsive urges.
This is a great example! In class, however, I have a really hard time getting my students to understand the problem. Alas, no matter how many examples I provide from psychology, about half the class still thinks the tautological claims seem reasonable. However, I have come up with an example that seems to work for all the students. I live in a somewhat religious area, but I suspect this would work just about anywhere.
My common intervention is to have them pretend we are "Christian Scientists" with a grant from the Vatican to investigate the hypothesis:
Committing sin causes you to go to hell.
They all roll their eyes, but I assure them we are going to take this seriously and determine a method of investigation. I even mention the tension in traditional Christianity about this point. I even contrast the Calvinist doctrine that pretty much everyone is going to hell and that is unlikely anyone in the classroom has ever met anyone who isn't, with the Universalist doctrine that God would never really condemn anyone to hell and that somehow God's grace redeems everyone. I point out that most christian denominations fall somewhere in the middle, so this is really an up-for-grabs issue in religious circles. At this point they usually seem to be satisfied that I have some knowledge of religious issues and am not just being a jerk (an easy assumption given that at least a few will know of my role in the campus skeptics group).
I write the hypothesis on the board, in big letters. Then I tell them that, for the sake of the exercise, we assume that we can take advantage of the full resources of the Vatican, including papal knowledge regarding which dead people are or are not in hell. As we, being Vatican scientists, assume the pope is infallible, we need not doubt the accuracy of this data. The task, then, centers not in determining who is in hell, but in determining who has sinned.
We then write on the board a series of potential definitions of "sin". The students come up with the definitions, and I allow pretty much anything. Common answers include "things the bible says not to do", "evil things", and "things that make God cry" (the latter almost always comes up as a joke, but I write it on the board just the same). I keep pushing them until there are at least 5 potential definitions, and usually I can get as many as 8 reasonably distinct definitions. Then I point out that we can, in principle, investigate any of those things. We can, in principle, determine who has acted in a manner the bible says not to act, and then determine if they have gone to hell. We can, in principle, determine who has made God cry, and then see if they go to hell. I also suggest we might want to skip the observational phase and go straight to randomized control trials... but usually the students don't think it will get past the IRB.
At this point most students are pretty engaged, and seem to think the idea of such a study is interesting, even if it is entirely hypothetical. Then, if the students have not already said it, I set up the tautology: We could define sin as "things that make you go to hell."
At that point, the problem is immediately obvious to most of them... if you define sin in this way, then you don't have a hypothesis anymore, there is nothing to investigate. Just to be safe, though, I spell it out for them. If we substitute this definition for the word "sin", then we get:
Doing 'things that make you go to hell' causes you go to hell.
I ask them how many people think the Vatican will continue our funding if we propose that definition, and everyone agrees it is pretty stupid. As the rest of the definitions are still on the board, the contrast is clear. Then I go back to a psychological example...
This has been surprisingly effective. I'm pretty sure I have a 100% success rate in getting students to understand what a tautology is and why it is problematic. I certainly don't have perfect success with getting them to apply the principle back to examples in psychology, but I have a much higher success rate than with any other examples I have tried.
Admittedly, it can be difficult to detect tautology when there is a lot of jargon, and when explanations are unnecessarily long winded. And in psychology, we use a lot of jargon, and are often unnecessarily long winded. Thus, I am at least a little sympathetic when my students initially struggle. It takes a lot of intellectual fortitude to determine if there is a tautology in a dense paragraph. Mike's example about 'impulsivity in ADHD' and 'executive function' heads in the jargon-laden direction, but is not nearly as hard to unpack as some examples I have seen (as demonstrated by how quickly he unpacks it).
If you also have this problem getting students to understand this problem, and you try my intervention, please let me know how it goes.