Monozygotic twins are produced from the same zygote, that is, a single zygote divides to give rise to two embryos. They are usually identical. Dizygotic twins are formed when a female releases two eggs that are fertilized by two different sperms and result in the formation of two zygotes, and eventually two embryos.
Twin studies allow researchers to examine the overall role of genes in the development of a trait or disorder.
Hellin's law is an empirical observation in demography that the approximate rate of multiple births is one n-tuple birth per 89n-1 singleton births: twin births occur about once per 89 singleton births, triplets about once per 892, quadruplets about once per 893, and so on.