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(December 5, 2010)

Girls and Premature Puberty

We have already seen how, in our culture, young girls develop a very strong desire to have their own phones once the changes of puberty begin to repattern their brains and affect their emotions. We have also seen, generally, that young girls do not have the knowledge and experience necessary to use such technology wisely in the face of such strong desires. Since so much of what happens is an indirect result of puberty, it becomes especially important to investigate why puberty in girls is occurring earlier than used to be the case.

In the 19th century, the average age at which girls in the U.K. began puberty was 15. (For boys, it was 17 years old.)[18] In Western Europe, the average age of menarche in 1850 was 17 years old. (As we discussed in the previous section, menarche, the start of menstruation, comes about two years after the onset of puberty.) By 1950, menarche had fallen to 13 years old.[19] Since then, these ages have been slowly decreasing.[20] The most important factors seem to be better nutrition, living conditions, and hygiene.[21][22]

In the last 20-30 years, however, many different studies have found that both girls and boys, around the world, have been starting puberty at even earlier ages. For example, a Danish study published in April, 2009, found that, from 1991 to 2006, the average age at which puberty started in girls decreased from 10.9 years to 9.9 years, a difference of over 9 percent.[23]

The table below summarizes the relevant findings. As a reminder, "thelarche" refers to the beginning of breast development; "menarche" is the first menstrual cycle.

Ages at Which Danish Girls Start Puberty[23]

199110.9 years13.4 years
20069.9 years13.1 years

Three months after the Danish study was published, in August 2009, researchers in the U.S. published a research paper[24] in which they concluded:

"The proportion of girls who had breast development at ages 7 and 8 years, particularly among white girls, is greater than that reported from studies of girls who were born 10 to 30 years earlier."

The table below shows specific numbers from this particular study, which involved 1,239 girls from three different urban areas in the U.S. The definition of "starting puberty" was having experienced thelarche (the beginning of breast development).

U.S. Girls Starting Puberty (2010)[24]


Why the disparity between different ethnic groups? There are several reasons related to environmental contaminants (see the discussion below). However, there is also a genetic difference that is significant. Specifically, a gene known as CYP3A4 has been shown to affect the age of onset of puberty in young girls. Researchers have found that a greater proportion of black and Hispanic girls have the variation of CYP3A4 that leads to early puberty.[25]

When puberty starts abnormally early, we describe it by using the medical term PRECOCIOUS PUBERTY. At one time, doctors used this diagnosis only if a girl experienced thelarche (breast development) before the age of 8. This guideline has now changed. Precocious puberty is now defined as either thelarche or pubarche (pubic hair growth) before age 7 in white girls and age 6 in black girls.[26]

Think about that for a moment: Many girls are now starting puberty at 7 years old or earlier. Indeed, as you can see from the table above, by the age of 8, well over 40 percent of black American girls have entered puberty. This raises the very important question: Why has the age of puberty in girls changed so significantly?

The clue to understanding the situation is to remember that the goal of puberty is "reproductive maturity", that is, to change a young girl's body so that it becomes capable of sexual reproduction. There are a number of influences that control the onset of puberty. Let's start by discussing two of the most important: body fat and hormones.

When a girl has enough body fat and the hormones are there, she will start puberty. Within a year of starting puberty, a girl will want her own phone.

Before we get into the details, here is the summary: When a girl has enough body fat and the hormones are there, she will start puberty — and within a year of starting puberty, no matter how old she is, she will want her own phone.

Obesity: Why body fat? As every mother knows, being pregnant is a biologically demanding experience. Creating a brand new human being requires a lot of nutrients, including a huge amount of extra calories, and the mother must provide for both herself and the baby. These demands don't stop at childbirth: taking care of the newborn child after the birth — including breastfeeding — also requires large amounts of nutrients and calories.

Put simply: pregnancy and nursing are not for women who are too skinny, a rule that nature enforces strictly. If a woman doesn't have enough body fat, she will find it difficult to get pregnant. Indeed, if a woman's fat content dips too low, she won't even ovulate or have periods. (The medical name for this is AMENORRHEA). This condition is common, for example, in women who are serious long distance runners or ballet dancers, and in women who suffer from anorexia nervosa.[21]

When it comes to starting puberty, the same principle holds. Young girls who are too skinny almost always experience delayed puberty. Indeed, there is evidence that before puberty will take place, a girl must weigh no less than 105 pounds (45 kg), at least 17% of which must be fat.[21] (The exact numbers, however, are controversial.)

Conversely, a young girl who is obese, or even mildly obese, is more likely to start puberty at a younger age[21][27][28] — and I don't have to remind you that we are living in the middle of an obesity epidemic.

Hormones: There is an important link between obesity and hormones. The link exists because abdominal fat does more than simply store energy. Abdominal fat cells also produce and secrete numerous chemicals, including estrogen as well as another hormone named leptin. Indeed, until the age of puberty, the major source of estrogen in a young girl's body is her fat cells.[29] Thus, the more abdominal fat a young girl has, the more estrogen she will produce, and the more leptin she will produce.[30] Estrogen, as we discussed in the previous section, is one of the prime movers when it comes to puberty. More abdominal fat means more estrogen, which means earlier puberty.

Leptin, on the other hand, has a more indirect effect on puberty. Leptin has a variety of roles within the body, the most well-known one is to help regulate appetite and energy expenditure by acting upon of part of the brain called the hypothalamus. In young girls, leptin has another effect. Research shows that puberty will not take place until a girl is producing a certain amount of leptin.[21][31] This may be because leptin acts upon the hypothalamus which, as we discussed in the previous section, is instrumental in initiating puberty, by signaling the pituitary gland to secrete FSH (follicle-stimulating hormone) and LH (luteinizing hormone) which, in turn, cause the ovaries to produce estrogen. And when it comes to young girls, puberty is all about estrogen.

Aside from obesity and hormones, there are several other reasons why girls are experiencing puberty so early in life. First, young girls grow up in an environment contaminated by a plethora of chemicals, some of which have been shown to create estrogen-like biochemical signals within the body. Such chemicals are called ESTROGEN DISRUPTORS, and the biochemical signals they invoke are thought to contribute to the increase of precocious puberty (as well as the incidence of breast cancer).

Second, the age of puberty is also affected by various social factors relating to the girl's family. Let's start with the chemicals, which we can classify into the following four groups.

Estrogen residues in food: Commercially raised cows and sheep are given a variety of hormones, including estrogens, to increase their growth.[32][33] In fact, it is estimated that two-thirds of the cattle raised in the U.S. are given hormones, usually testosterone or some type of estrogen.[34] Along these lines, researchers have found that young girls who eat a large amount of meat are more likely to reach puberty earlier.[22][35]

Estrogen in household products: Some household products have estrogens or estrogen disruptors. For example, a variety of hair products contain estrogen or placenta. In fact, it is thought that the use of such hair products by young black girls is one reason why they start puberty earlier than girls of other racial groups.[33][36][37][38] (The use of these products is also thought to be one of the reasons black women have relatively high rates of breast cancer.)[39]

Phytoestrogens: Some plants have naturally occurring chemicals, called phytoestrogens, that act as estrogen disruptors. For example, soybeans (tofu, and so on) contain phytoestrogens, as does processed food to which soy is added.[40]

Industrial Chemicals: There are a vast number of synthetic, industrial chemicals used in large amounts around the world. Some of the chemical that are known or suspected estrogen disruptors are: Bisphenol A (BPA),[41][42] polybrominated byphenyls (PBB), polychlorinated biphenyls (PCBs), and phthalates.[43]

The problem with these industrial chemicals is that they are widely distributed in clothing, cosmetics, toys, and so on. As such, they accumulate in the environment. For example, phthalates are ubiquitous, as they are widely used to make plastics more flexible. PCBs are used as industrial coolants and lubricants. BPA used in the manufacturing of many different plastics. In particular, BPA is found in plastic water bottles and within the lining of cans containing foods and beverages, from which it can leach into our food and drink.

The potential downside to these chemicals has not escaped scientists and governments. For example, in last November 2010, the European Union banned the use of Bisphenol A (BPA) in plastic baby bottles.[44]

Social Factors: Finally, it has been found that social factors can have a significant influence on the are of puberty. Some of the most interesting influences are related to broken families, divorce, stepfathers, and so on.

Family conflict, divorce, and the absence of the biological father are all associated with earlier puberty in girls.

For example, researchers have found that family conflict and the absence of a girl's biological father are both associated with earlier puberty in girls.[45][46] Other researchers see a similar association with the presence of a stepfather in the home or a mother who suffers from mood disorders.[47] Still other researchers have noticed earlier puberty in girls who live in homes that have suffered from divorce or separation of the parents. The cause seems to relate to multiple changes in childhood caretaking environment.[48]

One explanation is that the chronic stress brought about by such social factors modifies the girls' internal biochemical environment in a way that predisposes them to precocious puberty. Indeed, it has also been shown that broken families have other important effects on young girls. For example, researchers have found that when parents divorce or separate before the girls are 5 years old, the effects can be significant. Girls from such families tend to be younger when they enter puberty; when they have sex for the first time; and when they get pregnant for the first time. Such girls also have shorter first marriages.[48]

When parents divorce or separate, timing is crucial. If the family breaks up while the girls are very young, puberty is affected. If the family breaks up while the girls are adolescents, precocious puberty is not an issue, but there are lasting effects. For example, it has been found that such girls tend to have a larger number of sex partners.[48]

Considering, for a moment, such social factors and how they influence — not only puberty — but a girl's experiences with sex and pregnancy, who do you think is more likely to spend more time texting and talking on her phone: a girl with a happy intact biological family, or a girl whose parents are divorced, who is living with step-parents, experiencing multiple changes in her caretaking environment, and experimenting with sex at an early age?

At this point, we are now ready to discuss what the increase in premature puberty means for the young girls who grow up in a society occupied by an army of smartphones, computers, and Internet connections. In doing so, we are now able to answer the question:

Why do 10-year-old girls have such a strong desire to have their own phones?

Jump to top of page


1. Introduction
2. A Word From Our Sponsor
3. The Female and Male Brains
4. Excessive Text Messaging
5. The Female Brain and Puberty
6. Introducing The Pleasure Center
7. Why Is It So Hard To Quit
8. Girls and Puberty
9. Girls and Premature Puberty
10. Why Your 10 Year Old Daughter Wants Her Own Phone
11. A Final Note
12. References