I don't know of any medical evidence, but my 9yr old son has been on a daily regimen multivitamin / fish oil / magnesium & zinc.  I do believe that it has helped him.  One of the things I noticed is that he has a strong immune system, everyone in the house may come down with something, but he never catches it.  The most he gets is a few sniffles.  I have also noticed better temperment, not 24/7 but more often.  I have seen other positive changes so we have continued the regimen.  With these being natural supplements I don't have a problem with him being on them.  As I said, there isn't any concrete proof that I know of, but I was willing to try it as I knew it would be safe for him.