When our modern selves contemplate immediate versus future rewards, the brain processes these two options very differently. The immediate reward triggers the older, more primitive reward system and its dopamine-induced desire. Future rewards don’t interest this reward system so much. Their value is encoded by the more recently evolved prefrontal cortex. To delay gratification, the prefrontal cortex has to cool off the promise of reward. It’s not an impossible feat—after all, that’s what the prefrontal cortex is there for. But it has to fight a feeling that’s been known to make rats run across
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