Twins are categorised into either maternal or paternal twins.
Maternal twins are formed when an embryo splits into two (or more sometimes) in the early stages, and grow separately. In these cases, the two genetically identical babies sometimes even share a placenta. This fact makes them very useful in a research context (Nazi Germany used them extensively in unethical biomedical experiments).
As paternal twins come from separate egg and sperm, they are born as normal brothers and sisters. Because of this, it is also possible to have a boy and a girl (maternal twins have the same gender).
In a standard population, less than 5% are twins. However, according to some researches, it is posited that almost 25% of all pregnancies are twins. They hypothesise that out of all these twins, many cases involve one of the twins dying prenatally. If this happens, the corpse is absorbed by the mother and the other baby to recycle the nutrients. Sometimes a half-absorbed foetus is found embedded in the placenta (vanishing twin), or found in the body of the other twin years after birth (fetus in fetu).
A more interesting hypothesis from this research is that in these cases, one twin is left-handed while the other is right-handed.
Therefore, left-handed people may have absorbed their right-handed twin in the womb and survived.
This theory is validated by cases from nature. Many animals tend to birth two offsprings, then help the stronger one to survive. It is essentially nature’s insurance system.
Another interesting fact related twins is about a place called Candido Godoi, Brazil. This place is famous for being “the twin capital of the world”, due to having 18 times the rate of identical twin births compare to anywhere in the world. This means that one in five pregnant couples can expect double the bundle of joy.
The strange fact about this place is that there is a theory that the unnatural twin-birth phenomenon may be linked to a Nazi doctor, Josef Mengele (a.k.a. the Angel of Death) who fled to this town around 1963. It was around then when the phenomenon started occurring. However, it is still uncertain whether he was involved in any way.