Rationalists hold that the mind contains ideas innate (built-in) from birth. The Empiricists hold that the mind is a tabula rasa or “blank slate” since birth, and all ideas come from our experience of the world. Does the life of Kaspar Hauser provide evidence for the rationalist perspective on the mind, or that of the empiricists?

The competing ideas of inbred intelligence and "Tabula rasa" polarize the debate about human intelligence and learning. If we are to believe the rationalist perspective, individuals are born with some innate knowledge that manifests itself throughout the life cycle, essentially saying that there is archaic human experience in our genetics that holds over knowledge from previous generations. However, the empiricist perspective makes the argument that humanity is born with nothing, a blank slate on which to make our memories and create knowledge.
While both of these concepts have their merits, there is some scientific evidence for instinctual knowledge—essentially knowledge that, like the rationalist believe, is held over from prior generations—genetic tendencies that allow creatures to survive and grow. For instance, the grasping and rooting reflexes are certainly innate and instinctual; however, these are much simpler than adult knowledge.
The story of Kaspar Hauser is an interesting one because it presumably presents itself as an experiment to validate these two opposing concepts. Hauser was, purportedly, kept in a small cage-like cell in pitch-black darkness for his entire youth, only learning to speak and write a few words. While the account is dubious and seemingly a plot for financial gain, the underlying precepts are very interesting in light of the debate.
Hauser essentially claimed to have no knowledge or conception of the world, but he did, however, intrinsically know to eat and drink and play. His development shows the rationalist concept, that there were some genetic tendencies or innate knowledge that helped guide and groom him, even though he lacked the intellectualism and learning of most people of his day.
It is interesting because studies have shown that infants respond best to the native language of their mother, presumably because prior to stepping into the world, that is what they hear most frequently. In this same way, it seems that some modicum of knowledge slips in prior to Hauser's birth that would predispose him to normal human life and intellectual capacity.