Child development refers to biological, psychological and emotional changes that happen in humans between birth and the end of adolescence, at which time the individual goes from dependence to independence. The development is strongly influenced by genetic, mental, physical and social factors that may happen during the progression. Children develop at different levels. In particular, children having autism spectrum troubles or Down syndrome may have different development than usual or substandard motor development. Ideas about how children develop psychologically have changed over time. There are several important theories on how children develop.