Sex allocation is the allocation of resources to male versus female reproduction in sexual species. Sex allocation theory tries to explain why many species produce equal number of males and females. In dioecious species, where individuals are either male or female for their entire lifetimes, the allocation decision lies between producing male or female offspring. In sequential hermaphrodites, where individuals function as one sex early in life and then switch to the other, the allocation decisions lie in what sex to be first and when to change sex. Animals may be dioecious or sequential hermaphrodites. Sex allocation theory also applies to flowering plants, which can be dioecious, simultaneous hermaphrodites, have unisexual plants and hermaphroditic plants in the same population, have unisexual flowers and hermaphroditic flowers on the same plant or to have only hermaphroditic flowers. Fisher’s principle and equal sex allocation R.A. Fisher developed an explanation, known as Fisher's principle, of why sex ratios in many animals are 1:1. If there were 10 times more females in a population than males, a male would on average be able to mate with more partners than a female would. Parents who preferentially invested in producing male offspring would have a fitness advantage over those who preferentially produced females. This strategy would result in increasing numbers of males in the population, thus eliminating the original advantage of males. The same would occur if there were originally more females than males in a population. The evolutionarily stable strategy (ESS) in this case would be for parents to produce a 1:1 ratio of males and females. This explanation assumed that males and females are equally costly for parents to produce. However, if one sex were more costly than the other, parents would allot their resources to their offspring differentially. If parents could have two daughters for the same cost as one male because males took twice the energy to rear, parents would preferentially invest in daughters. Females would increase in the population until the sex ratio was 2 females: 1 male, meaning that a male could have twice the offspring a female could. As a result, males will be twice as costly while producing twice as many offspring, so that males and females provide the same proportion of offspring in proportion to the investment the parent allotted, resulting in an ESS. Therefore, parents allot equal investment of effort in both sexes. More generally, the expected