What I have noticed very clearly is that some folks are better than others at spotting traffic. One colleague who flies quite often with me will spot other aircraft long before I do.
Yes, but I do wonder how relevant such data is. Some people are better at seeing movement against a cluttered background, which allows them to spot other aircraft in the vicinity. Unfortunately, those aren't the ones that are going to hit you in an enroute midair. The dangerous ones are the ones with no relative lateral movement, which are altogether harder to see.