Early sexual development is known as "precocious puberty." The Mayo Clinic defines girls entering puberty before age 8 and boys before age 9 as experiencing precocious puberty. However, researchers are also finding that children in the developed Western world are tending to enter puberty earlier in general. The reasons why, either for the onset of precocious puberty or as part of an overall trend, are unclear, but there are many theories.

Early sexual development is better documented and more apparent in girls than in boys, although it does occur in boys. This means there is more research into why a girl might enter puberty at a young age. Obesity has long been associated with early sexual development. Genetics may also be involved; if you went through puberty early, your children may, too. There has been some thought that overexposure to certain chemicals or light can also induce early puberty.

Some specific groups of girls have shown higher rates of early sexual development. One study in the United States found that 25 percent of African-American girls entered puberty early, as opposed to only 8 percent of white American girls. Another study has found that girls in upper-income homes with no biological father also tend to enter puberty early. Neither study was able to pinpoint an exact reason why these girls developed sexually at early ages. Some evolutionary biologists have suggested that a lack of a biological father present might indicate the presence of a high number of unrelated males to the girl, which can trigger pheromones that might in turn trigger early secretion of the hormones that bring on puberty.