While many may still think of puberty as something that starts in the teen years, or middle school at the earliest, today’s girls are hitting that developmental milestone far earlier. A new study in Pediatrics aims to determine what’s behind this trend.

Researchers recruited 1,239 girls between the ages of 6 and 8 at health centers in the San Francisco Bay Area, New York City, and Cincinnati. They followed the girls through multiple visits from 2004 and 2011 to determine the age at which breast development began and what impact, if any, body mass index (BMI), race and ethnicity, and geographic location had on the result.

The researchers found that the onset of breast development varied by all of these indicators. The data show that breast development began at a median age of 9.7 for white and Asian girls, 9.3 for Hispanic girls, and 8.8 for Black girls. According to the researchers, the median age of puberty they found for white girls, in particular, was lower than what previous studies had reported. In addition, they found that BMI was a stronger predictor of early puberty than race or ethnicity. The researchers say more investigation needs to be done to determine the exact factors behind this trend, but concluded, based on the data, that the earlier onset of puberty in white girls is likely caused by an increase in obesity in this population.

Frank Biro, the lead author of the study, said in a press statement, “The impact of earlier maturation in girls has important clinical implications involving psychosocial and biologic outcomes.” He went on to explain that girls who mature earlier have been found to have lower self-esteem, higher rates of depression, and lower academic achievement. They have also been found to be more likely to break social rules or norms. Early puberty also increases a woman’s risk of high blood pressure as well as breast, ovarian, and endometrial cancer.