Ingroup preference has been observed in infants before any inculturation could take place. Humans are extremely visual creatures; by the time you look at a stranger up and down, the reptilian part of your brain, its processes steeped in millions of years of evolution, has already constructed a low-res impression of that person. This primal circuitry has allowed us to make quick life-or-death judgments in order to survive, and they still regulate the higher brain functions in all modern humans. It's incredibly dangerous to pretend that the human brain is just a blank slate waiting to be molded.