The third child syndrome is when a child enters a household with parents who are relaxed and not worried about simple things like diaper rashes or each sniffle from their experience in raising the other older children, making the third born a calmer, more easy-going child. For example, if your first child eats dirt, you'll call the doctor, then if your second one does the same, you'll clean their mouths, but if the third child eats dirt, you'll simply start to wonder if they are hungry.