Whilst chapters 2-4 spelt out in some detail what is known about the structure of individual differences, they said nothing about how or why they emerge. In order to have a proper scientifi c understanding of cognitive abilities, it is crucial to understand what causes people to differ in g and other abilities. Merely establishing the structure of abilities is not enough, in the same way that just being able to identify the symptoms of various diseases does not mean that we understand them enough to develop cures. A proper scientifi c understanding of any topic requires that we should be able to establish the basic structure (lists of diseases, lists of cognitive abilities), understand their interrelationships (discovering classes of diseases, such as infections or cancers; establishing the hierarchical structure of abilities), and fi nally developing models (to explain what causes some people to develop various diseases, or to develop a particular level of some cognitive ability). Models are usually developed piecemeal, by identifying weaknesses in the previous models and successively refi ning them. But even a partial understanding of how bacteria cause infections or how nervous system activity can affect intelligence can be of considerable practical use.