Sex is innately rewarding. The behavior is associated with the activation of opioid receptors in “reward centers” of the brain. Activation of these centers is also a key component to the development of addiction. The process of addiction involves learning to associate some sort of external cue, like a particular environment, with a rewarding outcome, like a high. But this reward-driven process is not just involved in the development of addictive behaviors – it’s also important for the development of both vocalizations and movements. So, strangely, opioids (the reward that makes us feel good) released during sex (the behavioral cue) are therefore well-suited to be involved in behavioral learning.

To examine this process, researchers observed how good male rats were at sex – they counted both how long it took to initiate a sexual interaction and how long it took them to ejaculate. In this case, quick interactions and ejaculations were considered better, since the goal of male rats is to reproduce as much as possible. The scientists watched rats with a lot of sexual experience, rats with no prior sexual experience, and rats that had recently ejaculated. They then examined the densities and distributions of opioid receptors in the rats’ brains and compared the three groups.