Basic Information
A twin is one of a pair of offspring born as the result of a single pregnancy. Separate terms ("triplet", "quadruplet" exist for larger multiple pregnancies) This applies only to species which give birth to live offspring (that is, not egg layers or similar things) and generally isn't used about those where multiple births are habitual (where the group of offspring will be termed a litter or something similar) - for example a bitch will not normally deliver twins, she will have a litter of two, whilst a mare who would normally be expected to deliver a single foal and who instead delivers two foals will be said to have had twins. For reasons of sanity, and the purposes of the board, most of what follows may be assumed to apply to humans (and/or other equivalent sapients).
The live offspring criteria applies only in the generic - if one of a pair of twins is stillborn, the survivor is still a twin.
The two most common types of twins are, of course, Fraternal Twins and Identical Twins. Fraternal twins (the term is used regardless of gender) are the result of the fertilisation of two separate ovae and are, essentially, two independent pregnancies that happen to be simultaneous and share a womb. Identical twins, by contrast, result from the spontaneous fission of an already fertilized ova (a zygote) or early stage embryo and are essentially natural clones. Frequency of fraternal twin pregnancies will be a function of the mother's ovulation, has a genetic component and tends to run in families down the distaff line, whilst identical twins appear to occur at a standard frequency across all pregnancies. It is technically possible for fraternal twins to have different fathers, but the social biology of the sort of species that are considered to bear twins tends to make this unusual.
Rarer types of Twins include:
- Mirror-Image Twins
- Chimeric Twins (see also Vanishing Twin Syndrome, and, Chimera I guess?)
- Conjoined Twins
- Craniopagus Parasiticus
- External Heteropagus Twins
- Fetus In Fetu (may be a Teratoma)
Cultural responses to twins vary greatly - some regard them as highly auspicious, others treat them with revulsion. Hunter-gather societies and others with limited resources tend to be particularly harsh on them (sometimes simply because the band can only carry so much as they migrate) and may subject one or both to infanticide … possibly dressed up as human sacrifice and accelerated promotion to ancestor. Historical attitudes have also included attaching the suspicion of adultery to multiple births (based on the misapprehension that a father only "sows" one child at a time and multiple children require multiple fathers).
Where primogeniture is an issue, twins may be a problem in the matter of birth order, requiring a reliable midwife and possibly careful marking (see, for example, the Biblical accounts of Jacob and Esau or Perez and Zerah).
Many cultures assume that twins have a special bond between them - sometimes going as far as to suggest a shared soul.
Twins in the News:
- News: Doctors baffled by Indian village of over 200 sets of twins
- News: Nazi angel of death Josef Mengele 'created twin town in Brazil’
See Also:
- Triplets, Quadruplets, Quintuplets, etc.
- Family Tropes, Sibling Tropes, Twin Tropes.
- Gemini (Sun Sign)
- Twin Paradox
Sources
Game and Story Use
- The lone survivor of a pair of twins might have the ghost of their sibling as a spirit ally.
- This may be a feature of the infanticide process in some primitive societies, creating a shaman by binding the twin that is killed to the other, meaning that the survivor grows up with a constant connection to the spirit world.
- Conflict between twins - especially, say, for a crown - may be vicious.
- Conversely, in the case of mixed gender fraternal twins, some societies (say, Ancient Egypt) might think it a good idea to marry them to one another to "enhance the purity of the sacred blood". For a given value of purity.
- The shared soul idea might lead to some form of suttee for surviving twins…