What the rat enrichment experiments suggest is that one contributor to addiction can be boredom. Or more specifically, a lack of awareness, or obfuscation, of opportunity cost. Boredom is effectively synonymous with the “opportunity cost” having gone to zero: if you believe there is nothing else enriching that you could spend your time on, then the calculation of whether or not to engage with a particular substance or action is skewed, particularly if that substance or action results in a feeling of enrichment, even a false one.