They arise from a single zygote that divides early in development, producing two genetically identical — or nearly identical — embryos.

The genetic signatures of monozygotic twins in ancient DNA analysis are distinguishable from those of fraternal twins or other close relatives: the coefficient of relatedness approaches one, rather than the approximately 0.5 expected for first-degree relatives like full siblings.

The baseline rate of monozygotic twinning in human populations is roughly 3 to 4 per 1,000 births, largely consistent across populations and largely independent of environmental factors.

This is in contrast to dizygotic (fraternal) twinning, which varies significantly between populations and is influenced by factors including maternal age and genetics.