Snack
Snack
[Ponytail, holding a cookie up in one hand and an apple up in the other, addresses an alarmed Cueball. His alarm is shown by seven lines radiating away from his head, and he also holds his arms stretched out.] Ponytail: Hey, do you want a cookie? Or an apple? Cueball: Who are you!? Did the IRB approve this!? Is everyone here an actor!?
[Caption beneath the panel:] The best prank you can play on psych majors is just to offer them a snack.

Many psychological studies involve participants being asked to make decisions under varying conditions, to determine how those conditions influence decision making. A common example is to give subjects a choice between eating a healthy snack (such as an apple) or a tasty snack (such as a cookie), which may be used as a simple proxy for whether they're prioritizing long-term health or short-term gratification. In most cases they are not made aware of the nature of the experiment, as knowing the premise of the study is liable to influence their behavior and alter the results. Instead subjects may deliberately be given a false impression of the purpose of the study, or they may be offered a choice under conditions where they're not aware that they're part of an experiment at all.
Examples of experiments like this are the Stanford marshmallow experiment and this study .
This sort of psychological study is most commonly done by universities, which means that using university students as subjects is generally the most convenient option. This means both that psychological studies tend to be heavily skewed towards the demographics of college students, and that university students have a pretty good chance of being invited to participate in a study at some point.
The joke in this strip is based on the premise that psychology majors are sufficiently aware of such studies that it would make them suspicious of any circumstances which could be part of a study.  If they've studied (or even conducted) such experiments, anything that reminded them of such a study could cause them to become suspicious. In Cueball 's case this is exaggerated into outright paranoia, and Ponytail is apparently playing on that to prank him, offering options that could easily be part of such an experiment just to spook him into suspicion.
Studies done on humans are subject to important ethical controls, particularly if the subjects are not fully informed of the study's purpose. "IRB" stands for Institutional Review Board , which is a committee (for example, at a university) which must approve such research to ensure that there's no significant risk of doing harm to the subjects of the study.
The title text jokes that graduate students have so much work to do that they are liable to forget to eat entirely and stereotypically too impoverished to afford adequate amounts of food; when presented with an offer of a snack, they don't ponder the implications or potential ulterior motives; they just eat it quickly and get back to work.
[Ponytail, holding a cookie up in one hand and an apple up in the other, addresses an alarmed Cueball. His alarm is shown by seven lines radiating away from his head, and he also holds his arms stretched out.] Ponytail: Hey, do you want a cookie? Or an apple? Cueball: Who are you!? Did the IRB approve this!? Is everyone here an actor!?
[Caption beneath the panel:] The best prank you can play on psych majors is just to offer them a snack.
