Ron, thanks for your reply & comments. Our contractor-built new deck is Southern yellow pine, pressure-treated. I went online & found that it IS correct to place those boards together, so I'm relieved about that. My concern that led me to this great forum is about the balusters that surround the deck - 11 of them (at last count!) have split at the ends where the nails were nail-gunned in. I gather from all the previous comments that I should certainly bring up the issue with the contractor, that this kind of problem can happen, and that a good contractor will want to protect his name & reputation, and replace the balusters. (All the stuff about the older deck that my husband built really isn't relevant to the new deck - just that he had spaced the deck boards apart, so we didn't know that the contractor's method was to put them close together.)
Sorry if my messages have been confusing, and for the delay in answering questions. Had to be away from the computer for a while.