I'm confused. I keep hearing different answers: some say identical twins are hereditary, some say it's the non-identical twins (fraternal) that are hereditary. Which is correct?
It seems that there is weak heritability for identical twins, and stronger heritability for fraternal (non identical) twins.
This paper found weak heritability in identical twins.
Wikipedia tells us that rates of fraternal twins varies across locations, but that the rates of identical twins seems to be the same everywhere. This might be because of heritability, or it might be to do with food. For example the Yerba have the highest rate of twinning. This might be genetics or it might be a type of yam they eat which increases the numbers of eggs released.
Here is another article by a geneticist that might be interesting. Seems like geneticists are not all completely in agreement either, but it does seem like fraternal twins are more likely to be a hereditary thing.