It was theorized long before this study that ADD/ADHD is a legacy from our hunter-gatherer ancestors. I had not heard of the data that American Indians have the highest rate of ADD, but it certainly fits the model: Native Americans were among the penultimate hunter-gatherers.
The flip side of this is to imagine what it must be like, if you have a novelty-seeking disposition and are easily distracted, to have to get up every morning in the same place, and spend dawn to dusk tilling the soil, sowing seeds, pulling weeds, and harvesting crops. People with this gene were never suited for that kind of existence.
Vocabulary alert: Penultimate doesn't mean the best, it means "next to the ultimate". But the word certainly sounds like it's the max.