According to Spitz, it is incorrect to look upon the first few years of the child's development as a simple linear development. Instead, Spitz believes, there are multiple lines of development which come together to make a new form of behavior possible for the child. This new behavior is so different from what the child was able to do before, that Spitz regards such new behavior as being indicative of a higher level of psychic structure.

Borrowing the language of biology, Spitz calls this new behavior an indicator of a new structure. The moment when such new behavior appears he calls a critical period. Since these critical periods have as their function the synthesizing of several developments into a new form of behavior that is in turn the beginning of a new period of psychological development, Spitz