A number of studies have found that girls may be experiencing puberty earlier than girls did in the past, a phenomenon called the secular trend. Although it may be natural to think about biological factors that cause precocious puberty, in fact many psychological or psychosocial factors may contribute to a girl experiencing puberty early.
Precocious Puberty and Dad's AbsenceIf a girl doesn't have her biological dad living with her, she is likely to reach puberty earlier than girls who have their dad present. In fact, some studies suggest that the longer dad has been absent, the sooner puberty will begin. Some researchers believe the presence of a stepfather or boyfriend in the house is even more important to precocious puberty than biological dad's absence. In other words, it doesn't matter if dad is gone, but rather whether someone has taken his place. They theorize that unrelated males create pheromones - odorless airborne chemicals that may affect hormonal functioning. These pheromones might cause a girl to develop more quickly. This theory has been supported in animal studies, and there is some evidence of it in humans, as well.
Family ConflictThe more a family fights, the earlier a girl in that family is likely to reach puberty. Although researchers are not quite sure why this occurs, prolonged stress of any kind - physical, social or psychological - seems to speed up girls' maturation. Therefore, conflict between parental figures (whether married or not), dysfunction in the family as a whole and less warmth in a family have all been found to be related to precocious puberty in girls.
Parental Mental DisordersIf a parent - particularly mom - has a mental disorder, there is some evidence that her daughter will be likely to have an earlier puberty than her peers who have a mentally healthy mom. Why might the mental health status of a parent matter? Like family conflict, having a parent with a mental disorder can be a source of intense, chronic stress. If stress causes faster maturation, it follows that parental mental health could indeed affect timing of puberty in a child.
Ellis, Bruce J., and Garber, Judy. Psychosocial antecedents of variation in girls' pubertal timing: Maternal depression, stepfather presence, and marital and family stress. Child Development. 2000. 71: 485-501.
Walvoord, Emily C. The timing of puberty: Is it changing? Does it matter? Journal of Adolescent Health. 2010. 1-7.