For various reasons, however, many species deviate from anything like an even sex ratio, either periodically or permanently.Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as Polistes fuscatus and Polistes exclamans, bees, ants, and termites.

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.

This ratio has been observed in many species, including the bee Perdita portalis.

A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.

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.

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.