Usually souls choose to incarnate together in many incarnations - they exchange roles of husbands, wives, children, grandparents, friends, coworkers, bosses, subordinates, all possible relationships to teach each other lessons and learn in different scenarios. In other words, a landing party is preferentially recruited from the same soul kindred. Not only do soul kindreds incarnate together but also whole societies might reincarnate together, for specific reasons. Currently the West and the America are replaying the scenario of the destruction of Atlantis - the same people who incarnated in Atlantis during its destruction, came now to replay that scenario and get it right this time. Yet, typically people incarnate to different genders, races and continents to expand their experiences.
Beginner souls take easy incarnations and life lessons. Their initial experience of Earth could be very short and simplistic. Some beginner souls might choose a very short incarnation to be born and die in childhood. This provides them only with a short sample of earth experience. Old souls know it all, know really well how to live on Earth and come here for serving, teaching and upholding the planet's vibration.