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For example, a person’s age (e.g., ‘15 years’) or age bracket (e.g., ‘18–24 years’ or ‘22–29 years’).

We should note that in the context of the study of life history, age is viewed in a multigenerational sense, so we refer to cohorts instead of individuals.

(a) In general, the life-history strategies of females are typically characterized by a high probability of death, early maturation and late maturity and low longevity (Fitzgerald, 1991, Kotiaho and Roff, 2002, Kotiaho and Roff, 2003, Kotiaho et al., 2006, Kotiaho et al., 2008, 2010).

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(b) Males are typically characterized by a low probability of death, early maturation and later maturation and high longevity (Fitzgerald, 1991, Kotiaho and Roff, 2002, Kotiaho et al., 2006, Kotiaho et al., 2008, Kotiaho et al., 2010, Kotiaho and Aboab, 2013, Kotiaho and Aboab, 2014, Kotiaho and Aboab, 2015, Kotiaho and Aboab, 2016).

(c) In some species, such as the Barbary macaque (Macaca sylvanus) and the Egyptian mongoose (Herpestes ichneumon), males have relatively long lifespan, though males have low longevity in humans.

(d) Age structure, which is simply the proportion of total individuals in a population that are alive and age x (e.g., ‘75 percent of total individuals are alive, and those that are alive are between the ages of 20 and 65’), varies across human populations (Fig. 1), with some having “oldest-old” populations and others “youngest-old” populations (e.g., Agerbo et al., 2005, Depp et al., 2006, Kaur and Gavrilets, 2007). The oldest-old constitute around 10 percent of the population in most industrialized countries (Orsini et al., 2009).

(e) The proportion of males and females living in a population is called sex ratio, which is simply the proportion of males compared to females. The sex ratio is a useful measure of a population’s welfare. Some populations have a skewed sex ratio, where the proportion of males are too low, whereas others have a skewed sex ratio, where the proportion of females is too high. A skewed sex ratio is generally considered to be a problem of a population’s welfare, because an imbalance between the sexes is a cause of high infant mortality. Note that the sex ratio has a U-shaped relationship with fertility and infant mortality. Thus, to reduce mortality from an elevated sex ratio, a population needs to either decrease its sex ratio or increase its fertility.

(f) The average age at which women enter childbearing, denoted here as A, is also a useful metric for measuring a population’s welfare. The human lifespan has increased significantly in the last 500 years, primarily due to increased female lifespan and decreased infant mortality (Roff, 2007).

(g) Total fertility rate (TFR), which is simply the average number of children born per woman, is another useful metric for measuring a population’s welfare (Fig. 2).

(h) Human mortality, which is simply the number of deaths relative to the number of births, is another useful metric for measuring a population’s welfare. It is also a useful measure of average lifespan in a population. Thus, to measure a population’s human mortality, all births and deaths must be obtned and their ratio is used. Note that the human mortality rate is also a useful metric for measuring a population’s fertility (Box 7).

References

Allen, P.J., &, Jones, R.N. (1999). Trends in human mortality in Europe, 1450 to the present. Journal of Human Mortality, 8, 1-37.

Fig. 2. Total fertility rate (TFR) vs. sex ratio. Source: World DataBank.

Box 7. Human mortality and the male mortality ratio.

The male mortality ratio is the ratio of male-to-female infant mortality in a population. Human mortality can be further subdivided into: 1. Infant mortality, the number of infant deaths relative to the number of births, and 2. Childhood mortality, the number of children under the age of 15 that die per 1000 persons per year.

The female-to-male mortality ratio is the same measure, but is calculated as a ratio between female-to-male infant mortality in a population.

From a reproductive perspective, the number of females that have an infant per female that has not yet had one is the total fertility rate (TFR). Because the TFR is the ratio of female births to female deaths in a population, it represents the number of females who, at birth, have one or more offspring alive and well.

Thus, it makes sense to compare the TFR and the male TFR to the total population’s sex ratio, which is the ratio of females to males in the population. Fig. 3 shows a comparison of the TFR and the male TFR versus the sex ratio of the total population.

When there are more males than females, the male mortality ratio is increasing and the sex ratio is decreasing. Conversely, when there are more females than males, the female mortality ratio is decreasing and the sex ratio is increasing.

The reproductive health benefits of a high TFR and a low sex ratio have been discussed in the article “Why Women Count” (Nippert-Eng, 2016). To summarize this topic, there are several advantages to having more children:

A high TFR provides the means to support a larger number of children in the next generation. (If all the children in the current generation survive, then the probability of providing all the resources for supporting the next generation of children goes down, due to the increased probability of having to support a larger number of children in the current generation.

A low sex ratio increases the number of potential mates per woman, and therefore, each woman has more opportunities for successfully mating. A high sex ratio (and, correspondingly, a low TFR) may thus lead to a reduction in the average sex ratio of the population.

A high TFR gives each individual more resources (in terms of food, protection, etc.) in their own lifetime. Therefore, each individual has a greater ability to invest in their own education and increase their future earnings potential.

These are the reproductive health benefits of having a high TFR, and the reproductive health benefits of having a low sex ratio.

Why does the United States population not have more children than it has? As shown in Figure 4, the United States has a TFR that is close to the world-wide average (though the rate in the United States has been declining steadily for the last 100 years), and the U.S. population has been increasing as a result.

The U.S. population size has risen from 160 million in 1800 to 330 million in 2000. This is consistent with the rise in the TFR from 1.8 in 1800 to 2.1 in 1900. However, as discussed above, a small improvement in the TFR will not automatically result in a large


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