1. How is it advantageous?

2. Assuming it really is: Why would this make it unlikely to be encoded on a single gene? As a layman, the only condition I'm aware of with this "downside but also upside" quality is sickle cell anaemia (downside is anaemia, upside is increased resistance to malaria), which is decided by a single nucleotide change on chromosome 11.

Am biologist and keeping genes that are "advantageous" is the most common trap that people fall into about evolution. Evolution isint actually about keep advantageous traits the real idea is about differences in reproductive success, not about evolution systematically preserving only traits that are beneficial to the individual.

In fact Darwin's theory of evolution is kind of outdated as well. We now understand genetic drift and things like the neutral theory of molecular evolution from moto kimura and the nearly neutral theory of evolution from tomoko ohta may give some sort of explanation to polygenic nature of adhd.

Sometimes the upside is just being different. I heard about a group of cattle, and one of them was ill, while the others feasted. The feed was tainted, and so only the sick one survived.

Though, with ADHD, I think it's definitely more than that. It doesn't just make it hard to focus, it makes it hard to focus on what you need to, until the moment that it is obviously now or never. Focus, though - there's tons of that, flitting about, and sometimes grabbing you up and running away with you - sometimes for months. I would imagine that there are a lot of novel discoveries attributable to it.

> As a layman, the only condition I'm aware of with this "downside but also upside" quality is sickle cell anaemia

Essentially all traits that you might think of as "traits" and not "features of the species" have downsides and also upsides. Where there's any significant imbalance, the genes for that trait will be purged from the gene pool (if the trait is basically all negatives) or sweep to become universal (if all positives).

By seeking out novelty, someone could be a more effective forager or better able to spot predators/prey hiding. ADHD often comes with a high attention to detail, so there's lots of advantages that could come with that (e.g. that twig wasn't broken before, something has passed through here).

Also, having different sleeping patterns can mean that the people with ADHD are likely better equipped to keep a fire burning overnight and possibly be alert for predators.