 A further challenge to modest modularity, not addressed by Prinz (2006), comes from evidence that susceptibility to the Müller-Lyer illusion varies by both culture and age. For example, it appears that adults in Western cultures are more susceptible to the illusion than their non-Western counterparts; that adults in some non-Western cultures, such as hunter-gatherers from the Kalahari Desert, are nearly immune to the illusion; and that within (but not always across) Western and non-Western cultures, pre-adolescent children are more susceptible to the illusion than adults are (Segall, Campbell, &amp; Herskovits, 1966). McCawley and Henrich (2006) take these findings as showing that the visual system is diachronically (as opposed to synchronically) penetrable, in that how one experiences the illusion-inducing stimulus changes as a result of one’s wider perceptual experience over an extended period of time. They also argue that the aforementioned evidence of cultural and developmental variability in perception militates against the idea that vision is an innate capacity, that is, the idea that vision is among the “endogenous features of the human cognitive system that are, if not largely fixed at birth, then, at least, genetically pre-programmed” and “triggered, rather than shaped, by the newborn’s subsequent experience” (p. 83). However, they also issue the following caveat: