Even after such factors as the girl's bodyweight were considered, the findings still held, the authors wrote.
Julianna Deardorff, UC Berkeley assistant professor of maternal and child health, and lead study author, said:
The age at which girls are reaching puberty has been trending downward in recent decades, but much of the attention has focused on increased body weight as the primary culprit. While overweight and obesity alter the timing of girls' puberty, those factors don't explain all of the variance in pubertal timing. The results from our study suggest that familial and contextual factors - independent of body mass index - have an important effect on girls' pubertal timing.
The results came from the Cohort study of CYGNET (Young Girls' Nutrition, Environment and Transitions) - a project that looked at patterns of health (epidemiologic project) lead by Lawrence Kushi, associate director of etiology and prevention research at the Kaiser Permanente Northern California Division of Research.
This project is part of the UC San Francisco Bay Area Breast Cancer and the Environment Research Center (BCERC), one of four centers funded by the National Cancer Institute and the National Institute of Environmental Health Sciences. Early puberty has been associated with a higher probability of developing breast cancer and reproductive cancers later in life. Kushi said:
Although the main focus of the CYGNET Study is on environmental exposures, we are also keenly interested in the social and behavioral contexts in which maturation occurs. These findings demonstrate that such factors may play important roles in the onset of puberty in girls.
The authors explain that previous research had identified a link between early puberty and the absence of the father. However, most of those studies depended on the female remembering when her first menstrual period occurred, while a few examined what impact BMI (body mass index), ethnicity or income might have.
In this latest study 444 girls aged 6 to 8 years were recruited through Kaiser Permanente Northern California, and have been monitored every year. The investigators' analysis was based on the first two years of follow-up. They considered puberty signs that emerge before the start of the first menstrual cycle (menarche). The girls were interviewed by the investigators and asked who lived in the household and what their relationship was (mother, father, brother, sister, etc.) - the girls' caregivers were present during the interviews.
At the time of recruitment 80 girls said their biological father did not live in the household. The researchers were surprised to find a link with earlier breast development in higher income families in which the biological father was absent. A higher income family in this study is one whose yearly household income is at least $50,000 per year. Earlier onset of pubic hair development was only detected in African American families of higher income.
In short - early breast development was found in higher income families across all ethnic groups, while early pubic hair development was found in African American higher income families.
Scientists cannot explain why this occurs. Some theorize that perhaps the lack of a biological father may indicate an unstable family environment, resulting in earlier female puberty.
Some have suggested that perhaps when the biological father is absent, the girls are exposed to more unrelated adult males - and their pheromones - leading to earlier puberty. In this study, however, the presence of other adult males in the household did not alter their results.
The investigators cannot explain why the absence of the biological father increases the risk of early puberty among females only in higher income families, particularly African American ones.
It's possible that in lower income families, it is more normative to rely upon a strong network of alternative caregivers. A more controversial hypothesis is that higher income families without fathers are more likely to have a single mother who works long hours and is not as available for caregiving. Recent studies have suggested that weak maternal bonding is a risk factor for early puberty.
The researchers wonder whether the girls in higher income without the biological father around might be exposed to more artificial light, which is known to speed up puberty in animal experiments. They also wondered whether African American girls might be more exposed to specific beauty products, such as hair straighteners, which have estrogenic properties that could have an impact on when puberty occurs.
A recent study involving 1,200 girls carried out by researchers at Cincinnati Children's Hospital Medical Center revealed that approximately 15% of the girls showed the beginnings of breast development at 7 years of age.
Bay Area BCERC's principal investigator Dr. Robert Hiatt, UCSF professor and co-chair of epidemiology and biostatistics, and director of population science at the campus's Helen Diller Family Comprehensive Cancer Center, said:
The hunt for an explanation to this trend is significant since girls who enter puberty earlier than their peers are not only at greater risk for reproductive cancers, they are also more likely to develop asthma and engage in higher risk sexual behaviors and substance abuse, so these studies have broader relevance to women's health.
In some ways, our study raises more questions than it answers. It's definitely harder for people to wrap their minds around this than around the influence of body weight. But these findings get us away from assuming that there is a simple, clear path to the earlier onset of puberty.
Father Absence, Body Mass Index, and Pubertal Timing in Girls: Differential Effects by Family Income and Ethnicity
Julianna Deardorff, Ph.D., John P. Ekwaru, M.Sc., Lawrence H. Kushi, Sc.D., Bruce J. Ellis, Ph.D., Louise C. Greenspan, M.D., Anousheh Mirabedi, B.A., Evelyn G. Landaverde, M.A., and Robert A. Hiatt, M.D., Ph.D.
Journal of Adolescent Health doi:10.1016/j.jadohealth.2010.07.032