A child can have severe delay in language development yet understand the rules of social behaviour and show advanced ideas in the way he plays. One finds that the child who has slow language development is also immature in the development of attention control, the acquisition of social skills and the emergence of imaginative play. Many factors affect the rate of language development. The problem for the health professional is that between 10% and 20% of children are quite slow in language development and this figure may be substantially higher in areas of poverty or deprivation. The child who listens to and learns two languages from infancy has all their language activity in the same part of the brain; whereas if the child learns the second language later in childhood there will be two different areas of language activity. Children in bilingual families learn both languages almost as fast as monoglot children.