Welcome to the forums-
Don't worry, your question won't be on your checkride...
You may find a document somewhere that states, "pilots abilities decrease by x with x amount of time out of the cockpit or something to that effect". I doubt you will be able to find a logical link from scientific-based rational research to FAA or *gasp USAF regulations. If you do, I would venture to guess that the date of the document was well after the regulation was written (someone trying to justify a reason).
Many moons ago, the regulations were written by groups of pilots, based on years of experience/experimenting/accidents/deaths/etc... and then someone that controls the money to pay for currency got involved, (Enter your own humorous reasoning/rationale (or lack of) for USAF/FAA guidance). I could imagine the rational is something like this:
A new plane built by my company leaves somewhere traveling at 351 knots. The pilot spins the VV knob instead of the IAS knob. The plane crashes and burns with everyone trapped inside. Now, should we initiate a new training currency requirement? Take the number of planes in the field, A, multiply by the probable rate of another pilot screwing up, B, multiply by the average out-of-court settlement, C. A times B times C equals X. If X is less than the cost of a new training currency requirement, we don't do one.
... Of course, I think that gives too much credit, it is probably more like:
Experienced Pilot says, "Pilots need to fly x things in y days"
Leadership dude climbing the ranks with no concern for anyone else's bottom line but his own, and whoever's he needs to take care of to get further up the ranks; and who could not tell you how many engines a glider has, says, "Well, don't they have an autopilot or something?", etc, etc...
Is this research for something? Give us a little more insight and we might be able to point you in a better direction...