Contrary to what is commonly believed (Refs. 1,2), Sir R.A. Fisher did not stipulate that the sex ratio ought to be one. Instead, he wrote that "the sex ratio will so adjust itself, under the influence of Natural Selection, that the total parental expenditure incurred in respect of children of each sex, shall be equal" and that "in man, the males suffered a heavier mortality during the period of parental expenditure...the condition toward which Natural Selection will tend will be one in which boys are the more numerous at birth, but become less numerous, owing to their higher death rate, before the end of the period of parental expenditure...the actual sex-ratio in man seems to fulfill these conditions somewhat closely" (Ronald A. Fisher, The Genetical Theory of Natural Selection, Dover, 1958, p. 159).

Indeed, the 2000 U.S. Census data suggests that the male/female ratio for children under 18 is 1.0518 (an excess of males, as Fisher suggested). By ages 18-64, the male/female ratio has dropped to 0.989.

References

1: Nature 424, 616-617 (7 August 2003) | doi: 10.1038/424616b

The struggle for sexual inequality

By Göran Arnqvist

"...Ronald Fisher took up this challenge in 1930, noting that every offspring has one mother and one father. Therefore, the sexes on average fare equally well in passing their genes on to the next generation under an even sex ratio..."

2: Matt Ridley: The Red Queen, p. 124.