As described in Chapter 1, personality development is contributed to by genetic factors and environmental factors. In our present state of knowledge there is little we can do about the more subtle forms of abnormal inheritance, so understandably most work has been concentrated on environmental factors. Many professional workers in the last hundred years have been examining possible theories of personality development, with especial emphasis on stages of intellectual, social or emotional development, according to the particular interest of the professional worker concerned. Perhaps it is unfortunate that theories have developed in isolation and that those interested in emotional aspects of personality development have given little consideration to cognitive functioning and vice versa. In a text of this nature it is not possible to consider any theory of personality development in great depth and so the reader is referred to the appropriate literature, but brief mention will be made of some relevant thebries.