I'm reminded of a set of experiments in which rats were placed in cages and provided with levers that would dispense food under varying circumstances.
1) The lever never dispenses food; the rat learns not to press the lever.
2) The lever dispenses food every time it is pressed; the rat learns to press the lever when it is hungry.
3) The lever randomly dispenses food when it is pressed; the rat goes crazy and pushes the lever like mad because it never knows when it will get food and when it won't.
Don't be rat 3.
Are you ok with being evil scientist 3?
I heard about this experiment in relation to gambling. Simple creatures like rats show signs of addiction to gambling.
You might be interested: they have made rats addicted to various drugs and arranged their cage so that they have to run across an electrified runway to get the next fix. By finding the maximum voltage the rat will endure, they can measure (one dimension of) the addictiveness of a drug.