Undetermined. The to& fro on food additives first peaked in the early 70's. By eliminating dyes & additives parents often saw improvement in behaviors although most testing could not verify effects. Later studies show minor effects of food additives as well as various foods, exclusion of which improves performance. A subset of the population is probably sensative but identification of the subgroup is difficult.
Yes, in some. And the connection is very powerful in some kids.