The parents would not only look out for their self-interest, but also the interest of its children. Rousseau uses the family analogy when it suits his purpose in the civil state, but conveniently leaves out when it does not in the state of nature. Without these ideas taken into consideration, Rousseau believes that the state of nature will not work out and the civil state is a superior alternative. .
A civil state is superior because it "substitutes justice for instinct in man's behavior." (pg 69) There is an agreement or contract of rules and morals in a civil state which turns him "from a limited and stupid animal into an intelligent being and Man." (pg 69) These contracts must be fair and just mutual agreements where there is a give and take process in order for both parties to benefit in some way. Man does have to give up the advantage of being able to take whatever one desires in natural liberty. In return, he gains ownership of property and protection from being dominated by physical brutality.
Rousseau uses the family as an analogy for a natural society. When a child is born into the world the members of the family look after the child and the child has to follow by necessity and need. When the child comes of age and no longer needs the parents to care for them, they have a free choice, as in a state to have a mutual agreement with the family and choose to be part of the family. This free choice is in the best interest of both parties and is not out of necessity any more but is a deliberate choice. .
This analogy of the family provides a real basic example almost everyone can relate to. It draws strong emotional ties while you are reading and can lead to Rousseau persuading you towards his reasoning unjustly. In this analogy, Rousseau makes the assumption that a child is not equal to his parents from birth. It can be argued each member of the family has absolutely equal authority from birth.