The various stages of the psychological development of children have a profound impact on a childs ability to learn. Over the years, there are many different theories of the cognitive and psychological stages a child goes through as he or she matures into young adulthood. The psychology of children has been studied from various perspectives, including the classic dichotomy of issues of nature versus nurture. Essentially, the proponents of nature believe that the child is a product of genes and heredity. On the other hand, the proponents of the nurture theory of psychological development believe that the child is primarily the product of society and the environment. Other, more contemporary theories of the psychological development of children focus on the different stages of sensory discrimination, perception, emotional expression and learning through language and cognitive development, as well as the socialization process.