The psychic being at the time of death chooses what it will work out in the next birth and determines the character and conditions of the new personality. Life is for the evolutionary growth by experience in the conditions of the Ignorance till one is ready for the higher light. <ref>http://incarnateword.in/cwsa/28/rebirth#p1</ref>
The psychic being's choice at the time of death doesn't work out the next formation of personality, it fixes it. When it enters the psychic world, it begins to assimilate the essence of its experience and by that assimilation is formed the future psychic personality in accordance with the fixation already made. When this assimilation is over, it is ready for a new birth.