From Scientific American:
By measuring response time, the researchers got a sense of how quickly people learned which one of the abstract pictures indicated money would follow. They noticed an inverse correlation between how much money a person had (assets and income) and the swiftness with which they were conditioned. The poorer people tended to figure out which card signaled money ahead within about 12 trials, says neurobiologist Philippe Tobler, the study's lead author, whereas the richer people took about 35 trials.
The team next repeated the experiment while the subject's brains were scanned by an fMRI (functional magnetic resonance imaging) machine. Researchers focused their scans on the midbrain (which contains neurons or nerve cells that produce dopamine, a neurotransmitter central to reward-based learning), and the striatum, another reward-based center located under the cerebral cortex. This time, however, the participants did not have to physically respond. “We didn't want them to do that because there are neurons in the striatum that are responding to initiate an action of responding to reward,” Tobler says. It was this response preparation that the researchers timed.
Once again, an inverse association between wealth and learning appeared, with poor people displaying more increased activity in the midbrain and striatum when compared with the more affluent subjects.
It seems like every experiment I see confirms the idea that effort is not free. Take a unit of anything valuable — money, happiness, whatever — and there will some point at which the modicum of effort to acquire it isn't worth it.