The human sex ratio is of particular interest to anthropologists and demographers. In human societies, however, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth, and by sex-selective abortion and infanticide. The CIA estimates that the current world wide sex ratio at birth is 107 boys to 100 girls, though during the late 1990s there was concern that the ratio of males to females was declining too rapidly. In 2010, the global adult sex ratio was 986 females per 1,000 males and trended to reduce to 984 in 2011.
Types of sex ratio 
In most species, sex ratio varies according to the age profile of the population. It is generally divided into four subdivisions:
- primary sex ratio — ratio at fertilization
- secondary sex ratio — ratio at birth
- tertiary sex ratio — ratio in sexually active organisms.
- Also called adult sex ratio and abbreviated to ASR. ASR is defined as the proportion of adults in a population that are male.
- quaternary sex ratio — ratio in post-reproductive organisms
Measuring these is a problem since they lack clear boundaries.
Sex ratio theory 
The theory of sex ratio is a field of study concerned with the accurate prediction of sex ratios in all sexual species, based on a consideration of their natural history. The field continues to be heavily influenced by Eric Charnov’s 1982 book, Sex Allocation. He defines five major questions, both for his book and the field in general (slightly abbreviated here):
- For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
- For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
- For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
- Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
- When does selection favour the ability of an individual to alter its allocation to male versus female function, in response to particular environmental or life history situations?
Biological research mostly concerns itself with sex allocation rather than sex ratio, sex allocation denoting the allocation of energy to either sex. Common research themes are the effects of local mate and resource competition (often abbreviated LMC and LRC, respectively).
Fisher’s principle 
Fisher’s principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher’s argument in his 1967 paper on “Extraordinary sex ratios” as follows, given the assumption of equal parental expenditure on offspring of both sexes.
- Suppose male births are less common than female.
- A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
- Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
- Therefore the genes for male-producing tendencies spread, and male births become more common.
- As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
- The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
Examples in non-human species 
Environmental and individual control 
Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategy: if the general population deviates from this equilibrium by favoring one sex, one can obtain higher reproductive success with less effort by producing more of the other. For species where the cost of successfully raising one offspring is roughly the same regardless of its sex, this translates to an approximately equal sex ratio.
Bacteria of the genus Wolbachia cause skewed sex ratios in some arthropod species as they kill males. Sex-ratio of adult populations of pelagic copepods is usually skewed towards dominance of females. However, there are differences in adult sex ratios between families: in families in which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); in families in which females can produce eggs continuously after only one mating, sex ratios are strongly skewed towards females.
Several species of reptiles have temperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. In the American Alligator, for example, females are hatched from eggs incubated between 27.7 to 30 °C (82 to 86 °F), whereas males are hatched from eggs 32.2 to 33.8 °C (90 to 93 °F). In this method, however, all eggs in a clutch (20–50) will be of the same sex. In fact, the natural sex ratio of this species is five females to one male.
In several different groups of fish, such as wrasses, parrotfish and clownfish, dichogamy — or sequential hermaphoditism — is normal. This can cause a discrepancy in the sex ratios as well. In the Bluestreak cleaner wrasse, there is only one male for every group of 6-8 females. If the male fish dies, the strongest female changes its sex to become the male for the group. All of these wrasse are born female, and only become male in this situation. Other species, like clownfish, do this in reverse, where all start out as non-reproductive males, and the largest male becomes a female, with the second-largest male maturing to become reproductive.
Domesticated animals 
Traditionally, farmers have discovered that the most economically efficient community of animals will have a large number of females and a very small number of males. A herd of cows with a few prize bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.
Dioecious plants secondary sex ratio and amount of pollen 
It was found that the amount of fertilizing pollen can influence secondary sex ratio in dioecious plants. Increase in pollen amount leads to decrease in number of male plants in the progeny. This relationship was confirmed on four plant species from three families – Rumex acetosa (Polygonaceae), Melandrium album (Cariophyllaceae), Cannabis sativa and Humulus japonicus (Cannabinaceae).
Polyandrous and cooperatively breeding homiotherms 
In charadriiform birds, recent research has shown clearly that polyandry and sex-role reversal (where males care and females compete for mates) as found in phalaropes, jacanas, painted snipe and a few plover species is clearly related to a strongly male-biased adult sex ratio. Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males, which in some cases can reach as high as six males per female.
Male-biased adult sex ratios have also been shown to correlate with cooperative breeding in mammals such as alpine marmots and wild canids. This correlation may also apply to cooperatively breeding birds, though the evidence is less clear. It is known, however, that both male-biased adult sex ratios and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in Australia and Southern Africa. It is also known that in cooperative breeders where both sexes are philopatric like the Varied Sittella, adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens, treecreepers and the Noisy Miner where females always disperse.
See also 
- Bride kidnapping
- Demographic transition
- Evolution of sex
- Human sex ratio
- List of countries by sex ratio
- Operational sex ratio
- Sex allocation
- Sex selection
- Sex-selective abortion and infanticide
- Trivers–Willard hypothesis
- XY sex-determination system
- Youth bulge
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