Cheerful people regroup faster and are still going strong on the 3rd/4th attempt to solve a problem.

The higher-IQ person is more likely to get it right the first time. But there won't be much conviction on try No. 2, and after that, his/her position will be: "It doesn't work. Can't be done. Dumb idea."

In a lot of work that I see, the IQ adjustment might be more like 10-15 points. For gnarly problems, we might still get better results with the really smart guy who's not 100% invested. And even grouchy geniuses can get enthused once they see that their ideas might work in a problem that has most other people stumped.

Still, for problems that just require a lot of different attempts until something clicks, well-aimed enthusiasm can work wonders.

IQ is, at best, a measure of raw capacity. But what matters in practice is how you apply that capacity. If somebody is enthusiastic then they are alert to details, excited to try things, and resilient in iterating. That means they're going to end up with a smarter end product than somebody who is smart but not really into it.