Almost every person diagnosed with Rett syndrome is female.

The MECP2 gene, mutations of which are the main cause of Rett syndrome, lies within band Xq28 on the X chromosome. A female born with a MECP2mutation on her X chromosome has another X chromosome with an apparently normal copy of the MECP2 gene. On the other hand, a male with a mutated MECP2 gene on his X chromosome has no other X chromosome, only a Y chromosome, thus he does not have a normal MECP2 gene to fall back on. Male foetuses with an MECP2 mutation are believed to die before birth. However, there are a small number of live males with a genetic change in their MECP2 gene who have developed signs and symptoms like those seen in Rett syndrome, including intellectual disability, seizures, and movement problems, some of whom have been diagnosed with Rett syndrome.

The incidence of Rett syndrome in Australia is 1:9,000 live female births. The disorder is not restricted to any race or ethnic group.

RETT Syndrome Association of Australia