Any differences in future development of monozygotic twins can be attributed only to "environmental factors".
Twins can be either monozygotic or dizygotic. monozygotic refers to the identical twins which means that they develop from one zygote, which splits and forms two embryos. And dizygotic refers to fraternal twins, which means that they develop from two different eggs. In fraternal twins, each twin is fertilized by its own sperm cell.
