This study shows a new approach concerning the concept of family in developing countries. This new approach notably takes into account numerous family related constraints placed upon members of a household. This allows us to analyse more effectively the impact of family structures and intra-family money transfers on saving and investment, which are important factors for growth and poverty reduction. The study revealed that these family structures can lead to opportunistic behaviour which can hinder saving and investment within households, but they also often provide the only social protection for some of the poorest people. The challenge for politicians will be to anticipate the response from households faced with policies to increase saving and investment when they are subject to such strong institutional constraints.