There are 2 basic types of twins: Identical (result when a single egg splits) and Fraternal.
Multiple births are the most common in African American cultures. Multiple births are least common in Asian cultures.
In the United States, over 150 identical twins are married to identical twins. Genetically their children are brother and sister, but legally their children are cousins.
The scientific study of twins is called gemellology.
Studies have shown that identical twins tend to live longer than fraternal twins; researchers believe this is due to their close communication.
Identical twinning is not considered hereditary because the rate appears to be random.
Fraternal twinning is considered hereditary because if you, your mother, or your mother's mother is a fraternal twin, then your chances of having fraternal twins are as high as 1 in 17 births. If you are a mother of fraternal twins, your chances to have another set of fraternal twins are 1 in 12 births.