1) We still don't know yet. If we are not observing some statistical noise, then it's possible that it's either bimodal (some have it, some don't) or it has a very weak effect.
2) Visualizing only the positive outcome, as far as I know, doesn't work. There's an interesting book about it: Rethinking positive thinking, by G. Oettingen. I've only skimmed it though, and I don't know how sound are the citations.
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