One was the belief that the woman had intercourse with her husband and with a deity, resulting in a demi-god and a mortal. If the deity was appreciated or feared, that could be a great benefit to the twins who would be given special favors or respectful distance. These cultural traditions may have led to selection for or against twinning. In Japan dizygotic twins are rarer than monozygotic twins and I suspect a past history of destroying twins in ancient Japanese culture. These twins are almost always dizygotic twins.
Identical twins have more risks during pregnancy than dizygotic twins.
Dizygotic twins always have two separate membranes around each of the embryos or fetuses. Monozygotic twins in eighty percent of the cases, have a common outer sac and two inner sacs. The other twenty percent are indistinguishable from dizygotic twin membranes. Contrary to popular belief, most identical twins do not come from a divided egg that separates into two cells. They usually arise after implantation and involve the splitting of the preembryonic mass into two chunks.