A 2019 meta-analysis and review of the research literature from all inhabited continents found that between 1977 and 2013, the age of pubertal onset among girls has fallen by an average of almost three months per decade, but with significant regional variations, ranging from 10.1 to 13.2 years in Africa to 8.8 to 10.3 years in the United States. This investigation relies on measurements of thelarche (initiation of breast tissue development) using the Tanner scale rather than self-reported menarche (first menstruation) and MRI brain scans for signs of the hypothalamic-pituitary-gonadal axis being reactivated. Furthermore, there is evidence that sexual maturity and psychosocial maturity no longer coincide; twenty-first-century youths appear to be reaching the former before the latter. Neither adolescents nor societies are prepared for this mismatch.