Age at menarche
Mean age at menarche in the studied human populations is in the range of 12–16 years (). Age at menarche demonstrates relatively low variability with standard deviations ranging from 1.1 to 2.1 years and coefficient of variation ranging from 8.2 to 13.1%. In a population with a history of poor nutrition (North Korean refugees), all measured characteristics – the mean age at menarche (16 years), its standard deviation (2.1 years), and coefficient of variation (13.1%) are particularly high compared to other populations [29
]. It is likely that women in the prehistoric population had a higher variation of this ontogenesis characteristic than is observed now.
Mean age at menarche and its standard deviation (SD) and coefficient of variation (CV) in human populations
In addition to age at menarche, anthropologists have studied other characteristics of sexual maturity. For example, a study of sexual maturity among boys based on the NHANES III data demonstrated variability of age at complete maturity of genitalia, which was similar to variability of age at menarche for girls: range of 1.2–1.9 years for standard deviation [46
Age at onset of menarche is an example of a characteristic determined by the ontogenesis program and is used as a marker of sexual maturity. Studies show that this developmental characteristic has a significant genetic component with heritability equal to about 0.49 [47
]. On the other hand, the age at menarche demonstrates environmental plasticity: in the last two centuries, age at menarche has decreased in several European populations [31
]. Similar changes were observed in other countries [28
], although recently further decline of the age at onset of menarche has stopped [51
]. Age at menarche is negatively correlated with urbanization, socio-economic status, and body mass index [27
]. Earlier age of menarche is associated with higher total mortality as well as mortality from stroke and ischemic heart disease at adult ages [52
Age at natural menopause
Mean age at natural menopause in the studied human populations is in the range of 45–50 years (). Age at menopause has higher values of standard deviation compared to the age at menarche: 3.6–5.5 years. However, the relative measure of variability (coefficient of variation) is almost identical to the same measure for the age at menarche: 7.3–11.1%. Thus, both ages of starting reproduction and ages of cessation of reproduction in human populations show similar values of relative variability.
Mean age at natural menopause, its standard deviation (SD), and coefficient of variation (CV) in human populations
Age at natural menopause is a hallmark of the end of reproduction related to the aging of the reproductive system [53
]. Age at natural menopause shows slight increase over time. For example, in women in the United States the mean age at natural menopause increased by 17 months for those born between 1915 and 1939 (49.1 vs. 50.5 years; p
= 0.001) [50
]. Heritability of the age at menopause is similar to heritability of the age at menarche and is equal to 0.49 [54
]. Lower age at menopause is associated with higher total mortality [55
] and mortality from cardiovascular diseases, cancer, and external causes [36
Study of the representative sample of the United States population
Study of the MIDUS sample has an advantage of estimating both ages at menarche and menopause using the same group of women. The MIDUS sample consists of predominantly white women (over 85%). We used a subgroup of women aged 60 years and over, so that most of them were menopausal by the time of survey. Women reporting past hysterectomy were excluded from the analyses to avoid cases of surgical menopause.
shows the distribution of women by reported age at menarche in the MIDUS sample. This distribution shows positive skewness equal to +0.56 with only a few cases having menarche below age 10 years. A positive skewness indicates that the tail on the right side of the distribution is longer than that on the left side, and the bulk of the values lie to the left of the mean value of 12.9 years. This may indicate that there are many environmental and genetic factors delaying sexual maturation, while it is far more difficult to accelerate it.
Fig. 1 a) Distribution of age at menarche for women participating in the MIDUS study (United States). b) Distribution of age at natural menopause for women participating in the MIDUS study (United States). c) Distribution of age at death for women in the United (more ...)
On the contrary, the distribution of age at natural menopause demonstrates negative skewness equal to −0.72 with few cases reporting menopause at ages over 60 years (). A negative skewness indicates that the tail on the left side of the distribution is longer than that on the right side, and the bulk of the values (including the peak modal age) lie to the right of the mean of 49.7 years. This may indicate that there are many environmental and genetic factors accelerating reproductive aging, while it is far more difficult to delay it.
Distribution of age at death () also demonstrates negative skewness similar to the distribution of the age at natural menopause. In this case, the skewness is equal to −1.81 (−1.26 for deaths at ages 10+ years).
compares variability measures for distribution of age at menarche, age at menopause, and age at death for women in the United States. Note that coefficients of variation for ages at menarche and ages at menopause are quite similar. The coefficient of variation for age at death is two times higher than that for age at menarche ().
Variation for characteristics of human ontogenesis and aging (women in the United States)
presents results of a comparative study of ages at menarche and death for women in Europe and the United States. Note that the standard deviations for ages at menarche are about 10 times lower than the standard deviations for ages at death. However, the coefficients of variation for ages at onset of menarche and ages at death for contemporary populations are of the same order of magnitude ().
Variation of an ontogenesis characteristic (age at menarche) and a characteristic related to aging (age at death) for women in Europe and the United States
compares mean values (horizontal axis) and standard deviations (vertical axis) for all three studied characteristics: age at menarche, menopause, and death. This is a convenient way to visualize all the data on one graph and to infer relative variability (coefficient of variation) from the slope of the line connecting the plotted data with the origin point (the point at which the axes of a coordinate system intersect). Note that the relative variability of the measures characterizing both development and aging of the reproductive system is surprisingly the same (both sets of data are on the same line with zero intercept when extrapolated to zero mean value; the slope coefficient of this line is equal to the coefficient of variation). Relative variability of total lifespan (coefficient of variation) is about 1.5–2.0 times higher (this set of data is far above the line). However, we need to remember that distribution of total deaths includes so-called non-natural deaths from external causes (accidents, suicides, etc.). Variability is also increased when deaths of infants and children are taken into account. Demographers now use standard deviation at age 10 years to evaluate the distribution of lifespan at older ages. The age of 10 years represents the age when mortality starts to increase, and hence this age indicates the beginning of manifestation of the aging process. If we use estimates of standard deviation at age 10 (SD10), then we get lower values for standard deviation, but not for coefficient of variation ().
Fig. 2 Comparison of mean ages at menarche (1), menopause (2), and death (3) as well as their standard deviations for studied human populations. Mean values are presented on the horizontal axis and standard deviations on the vertical axis. Data are taken from (more ...)
Thus, coefficients of variation (measures of relative dispersion) for an ontogenesis characteristic (menarche) and aging characteristics (menopause and death) are rather close to each other. This is particularly true for characteristics related to sexual maturation and reproductive aging: ages at menarche and ages at menopause have surprisingly similar relative variability. Some gerontologists believe that a programmed process should include stereotypical steps [1
]. Interestingly, both ontogenesis and aging of the female reproductive system in humans can be considered as examples of rather stereotypical behavior.
To summarize, this study has found a surprising precision of a “non-existent” aging program [7
]: relative variability for reproductive aging (menopause) is the same as for “programmed” sexual maturation (menarche).
We would like to thank participants of the international scientific workshop “From Homo sapiens to Homo sapiens liberatus” (May, 2010, Moscow State University, Russia) for their useful comments and suggestions made in response to our related presentation there.