Which single factor more than all others conditions how our personality forms. Some say the parents, (particularly mothers), some believe it is our genes, or maybe our friends or maybe birth order. Ever more researchers think that the dark matter behind how we become are our siblings. Husbands and wives arrive relatively late and parents at some point leave us, but siblings are with us for the whole journey. From birth on they are collaborators, play mates, tormentors, advisers, protectors, a reason of envy and anger. They teach us to fight, make peace, look after a friendship. Brothers teach sisters something about boys and sisters teach brothers something about girls. This gym makes us the persons we are as adults. Researchers would like to know if having lived with brothers and sisters helps the husband – wife relationship later on, if work related conflicts are overcome more easily when people come from families with many kids, if someone who was brought up in a family where parents had a preference for the brother or sister, has special resources to deal with similar situations at work later in life. Siblings who live together between the ages of 2 and 4 years on average argue every ten minutes; after this age and until 7 years, they argue every fifteen minutes. After the age of eleven years siblings spend about a third of their free time together and thus much more time than with their parents, friends or teachers. If we argue with our friends, we can leave slamming the door. Not so with siblings. Sooner or later we will have to make peace and this is where we learn to negotiate and reach compromises. The permanence of siblings is what makes them become teaching instruments. Agreement/disagreement with our own kind, with whom we often share a room, gives lessons for the future when the respective situations have changed radically. The house where we grow up is not always an oasis of peace. There are fights, envy, alliances, grudges, loyalties and everything changes from day to day. Sometimes a sibling’s behavior is so blatantly wrong that it teaches the others how not to behave. If a parent dies prematurely older brothers and sisters may step in and take responsibility for the smaller ones. Even if in middle age brothers and sisters are no longer so close (particularly in the case of two sisters), when the husband or wife dies, they often reunite. These are just a few of the situations that occur between siblings. Parents bring us up, siblings form us.