I kept careful records of what they ate and how they behaved since results could take several days to happen. We found that eating sugar made no difference at all, but the other things made a huge difference. When on the diet the boys (who also had other problems) went from several tantrums a day to one or less a week. However eating even a tiny bit of the problem chemicals would set them off.
A couple of times the boys went wild when I was sure there hadn't been any exception to the diet, but we later found out the ingredients of certain foods had been changed but the old labels were still used.
Obviously that reaction couldn't have been a psychosomatic. However the Feingold diet was considered quackery by most of the medical community and there was a lot of negative publicity about it.
But a few years ago a study showed that preservatives can cause behavior problems. You can see an article about that here:
And this week it hit the news that scientists have discovered artificial colors in foods may be causing the increase in the number of hyperactive kids and the FDA is doing a study about it. Here's a link to an article about that: