A post on Pixel Poppers looks at the psychological underpinnings of the types of challenges offered by different game genres, and the effect those challenges have on determining which players find the games entertaining. Quoting: "To progress in an action game, the player has to improve, which is by no means guaranteed — but to progress in an RPG, the characters have to improve, which is inevitable. ... It turns out there are two different ways people respond to challenges. Some people see them as opportunities to perform — to demonstrate their talent or intellect. Others see them as opportunities to master — to improve their skill or knowledge. Say you take a person with a performance orientation ('Paul') and a person with a mastery orientation ('Matt'). Give them each an easy puzzle, and they will both do well. Paul will complete it quickly and smile proudly at how well he performed. Matt will complete it quickly and be satisfied that he has mastered the skill involved. Now give them each a difficult puzzle. Paul will jump in gamely, but it will soon become clear he cannot overcome it as impressively as he did the last one. The opportunity to show off has disappeared, and Paul will lose interest and give up. Matt, on the other hand, when stymied, will push harder. His early failure means there's still something to be learned here, and he will persevere until he does so and solves the puzzle."
Researchers from the School of Medicine at the University of California have shown that the more germs a child is exposed to, the better their immune system in later life. Their study found that keeping a child's skin too clean impaired the skin's ability to heal itself. From the article: "'These germs are actually good for us,' said Professor Richard Gallo, who led the research. Common bacterial species, known as staphylococci, which can cause inflammation when under the skin, are 'good bacteria' when on the surface, where they can reduce inflammation."