Asex ratio is theratio of males to females in apopulation. As explained byFisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproducesexually.^[1][2] However, many species deviate from an even sex ratio, either periodically or permanently. These includeparthenogenic andandrogenetic^[3] species, periodically mating organisms such as aphids, someeusocialwasps,bees,ants, andtermites.^[4]

In most species, the sex ratio varies according to the age profile of the population.^[5]

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 mature organisms
  • The tertiary sex ratio is equivalent to theadult sex ratio (ASR), which is defined as the ratio of adult males to females in a population.^[6][7]
  • Theoperational sex ratio (OSR) is the ratio ofsexually active males to females in a population, and is therefore derived from a subset of the individuals included when calculating the ASR.^[7] Although conceptually distinct, researchers have sometimes equated the ASR with the OSR, particularly in experimental studies of animals where the difference between the two values may not always be readily apparent.^[8]
• quaternary sex ratio — ratio in post-reproductive organisms

These definitions can be somewhat subjective since they lack clear boundaries.

Sex ratio theory is a field of academic study which seeks to understand the sex ratios observed in nature from an evolutionary perspective. It continues to be heavily influenced by the work ofEric Charnov.^[9] He defines five major questions, both for his book and the field in general (slightly abbreviated here):

1. For adioecious species, what is the equilibrium sex ratio maintained by natural selection?
2. For asequential hermaphrodite, what is the equilibrium sex order and time of sex change?
3. For asimultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
4. Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
5. 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 sexallocation 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 (1930)^[1] explains why in most species, the sex ratio is approximately 1:1. His argument was summarised byW. D. Hamilton (1967)^[2] as follows, assuming that parents invest the same whether raising male or female offspring:

1. Suppose male births are less common than female.
2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
4. Therefore the genes for male-producing tendencies spread, and male births become more common.
5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.

This means that the 1:1 ratio is theevolutionarily stable strategy.^[10] This ratio has been observed in many species, including the beeMacrotera portalis. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.^[11]

Thehuman sex ratio is of particular interest to anthropologists and demographers. In human societies, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth^[13] and bysex-selective abortion andinfanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well.^[14] As of 2024, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls).^[15] By old age,^{[clarification needed]} the sex ratio reverses, with 81 older men for every 100 older women;^{[For this statistic to be meaningful, it is necessary to define the age range that is meant by "old age".]} across all ages, the global population is nearly balanced, with 101 males for every 100 females.^[15]

Spending equal amounts of resources to produce offspring of either sex is anevolutionarily 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 genusWolbachia cause skewed sex ratios in somearthropod species as they kill males. Sex-ratio of adult populations of pelagiccopepods 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.^[16]

Several species of reptiles havetemperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. In theAmerican alligator, for example, females are hatched from eggs incubated between 27.7 to 30 °C (81.9 to 86.0 °F), whereas males are hatched from eggs 32.2 to 33.8 °C (90.0 to 92.8 °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.^[17]

In birds, mothers can influence the sex of their chicks. Inpeafowl, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.^[18]

Dichogamy (sequential hermaphroditism) is normal in several groups of fish, such aswrasses,parrotfish andclownfish. This can cause a discrepancy in the sex ratios as well. In thebluestreak 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 wrasses 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.

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 bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.^{[citation needed]}

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),^[19][20]Silene alba (Caryophyllaceae),^[21][22]Cannabis sativa^[23] andHumulus japonicus (Cannabinaceae).^[24]

Incharadriiform birds, recent research has shown clearly thatpolyandry and sex-role reversal (where males care and females compete for mates) as found inphalaropes,jacanas,painted snipe and a fewplover species is clearly related to a strongly male-biased adult sex ratio.^[25] Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males,^[25] which in some cases can reach as high as six males per female.^[26]

Male-biased adult sex ratios have also been shown to correlate withcooperative breeding in mammals such asalpine marmots and wildcanids.^[27] This correlation may also apply to cooperatively breeding birds,^[28] though the evidence is less clear.^[25] It is known, however, that both male-biased adult sex ratios^[29] and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as inAustralia^[30] andSouthern Africa. It is also known that in cooperative breeders where both sexes arephilopatric like thevaried sittella,^[31] adult sex ratios are equally or more male-biased than in those cooperative species, such asfairy-wrens,treecreepers and thenoisy miner^[32] where females always disperse.

• Evolution of sex
• Operational sex ratio
• Sex allocation
• Trivers–Willard hypothesis
• XY sex-determination system

Humans:

• List of countries by sex ratio
• Bride kidnapping
• Groom kidnapping
• Demographic transition
• Sex selection
• Sex-selective abortion and infanticide
• Youth bulge

• CIA listing of sex ratios for individual countries (including age divisions)
• A review of sex ratio theory

• Sex ratio
• Population
• Selection
• Biostatistics

• Articles with short description
• Short description matches Wikidata
• Wikipedia articles needing clarification from January 2025
• All articles with unsourced statements
• Articles with unsourced statements from May 2024