A not so big idea of Psychology

It is also a question I had in mind for very long time on a discovery of Psychology: That human eyesight is very apt at identify blurred images of human figure. Certainly that confer those with this trait an evolutionary advantage since human is, by and large. a social animal. But what is hanging in my mind is how could be the researcher certain about the sequence of learning. i.e. Are those subject learned how to figure out human face because they had enough exposure of human face when they are young, instead of being an innate ability? Can those experiments which led to this conclusion rule out this alternative chain of causality? Can we design experiments to differentiate whether human first learn the crucial features of human face then infer the human face in blurred image, or human is capable deduce the crucial features of human face from blurred image without learning?