The Economist has an interesting article on newborn babies' innate sense of numbers and ability to count.
People are born with an innate sense of how many items there are in small collections. Experiments in which older children and adults are shown randomly arranged dots and asked to say quickly how many there are show this sense is retained throughout life. Up to three or four items, and the number is immediately visible without counting. Within a limited range, humans are born arithmeticians, too.
Read more here.