The explanation of this must lie in biology. In other words, some part of the capacity to learn langauges must be innate'. One issue dealt with by this chapter is an attempt to resolve this conflict. Innateness reflects deep, underlying similarities among all human languages. All of this implies biological basis for language learning.
Language learning is controlled, at least in part, by some underlying maturational timetable.
Second, simplicity of mother's language does not predict child's learning rate. Most deaf infants, though, are born into hearing families in which the parents know no sigh language. In many cases the parents make the decision not to allow the children access to sign language at all. This represents radical environmental variation, because children are deprived of linguistic input. While homemade system is ultimately much simpler than would be the case if full linguistic environment is present, its developmental sequence indicates underlying biology.