Beginning with determination of sex by genetic and/or environmental factors, humans and other organisms proceed down different pathways of differentiation as they grow and develop.

[11] The early stages of human differentiation appear to be quite similar to the same biological processes in other mammals—and the interaction of genes, hormones and body structures is fairly well understood.

In the first weeks of gestation, a fetus is anatomically indistinguishable as male or female and lacks production of any particular sex hormones.

Sexual differentiation in humans includes development of different genitalia—and the internal genital tracts, breasts, and body hair—and plays a role in gender identification.

Two major pathways in gonochores exist: one with a nonfunctional, undifferentiated phase leading to delayed differentiation (secondary), and one without (primary), where differences between the sexes can be noted prior to hatching.

[4] Secondary gonochorists remain in the bipotential phase until a biotic or abiotic cue directs development down one pathway.

Primary gonochorism, without an intersex phase, follows classical pathways of genetic sex determination, but can still be later influenced by the environment.

[4] Differentiation pathways progress, and secondary sex characteristics such as anal fin bifurcation and ornamentation typically arise at puberty.

[16] Studies on gynandromorph chickens showed that the mosaicism could not be explained by hormones alone, pointing to direct genetic factors, possibly one or a few Z-specific genes such as double-sex or DMRT1.

[16] The most intensively studied species, such as fruit flies, nematodes, and mice, reveal that evolutionarily, sex determination/differentiation systems are not wholly conserved and have evolved over time.

Some simultaneous hermaphroditic organisms, like certain species of goby, have distinctive male and female phases of reproduction and can flip back and forth, or "sex reverse", between the two.

[15][23] Sometimes alternative morphs are produced by genetic differences, and in other cases, the environment can be involved, demonstrating some degree of phenotypic plasticity.