are rare in humans with twins as being the most common form of this event. can arise in many different combinations of ways but the probability of giving birth to more than one child remains fairly constant when compared to the entire human race. The chances of can also vary from race and genetic background. Scientist and researchers do not know what the exact cause of these variations is but many of them feel that it is caused by hormone differences between different racial groups and/or the difference in social class. The prenatal and ....