A short history of twin research
Twins are unusual. They are interesting because they are so rare. They have always been a favourite subject of myths, folk tales and drama. Identical twins in particular are objects of great fascination and their similarity has caused us to ponder the deepest questions about the nature of human identity. (1) Few people are indifferent to witnessing the birth of twins. The womb, which is clearly designed to carry one baby, manages to carry two, and they are both born alive. Of course the twins we see are always in pairs. They are often, if not always, together. They seem to be bonded by a connection that goes back to their time together in the womb. A lone twin is a sad sight, for in the public eye this person surely is no longer a twin, because twins come in twos, we all know that.
The scientific study of twins and twinning, as far as we know, is relatively recent when compared to other branches of obstetrics. One of the first publications to consider in detail the biological nature of twinning in humans was published in 1883.(2) It was written by Francis Galton, who was one of those remarkable nineteenth century English thinkers who laid the basis for almost all the later theories on twinning - that is, until the development of ultrasound and artificial methods of reproduction a hundred years later. He gathered his material by “sending circulars of inquiry to persons who were either twins themselves or near relations of twins.” He wanted to discover if the obvious similarity between twins was due to “trifling accidental circumstances” or something much more profound and as yet unnoticed and un-remarked.
Galton was aware from animal studies that twins arise out of two very different events. One is where two or more are born, each developing from a separate ovum. The other is caused by the development of “two germinal spots” in the same ovum, each of which becomes baby. Galton noticed that if the ovum divides, the twins are wrapped in the same membrane – the chorion - and invariably of the same sex. He was puzzled that all twins were not alike, and that boy/girl twins in particular were never alike. He also noticed the phenomenon of the Alpha and Beta twin in many twin pairs but, lacking our modern vocabulary, he described them thus:
The one was the more vigorous, fearless, energetic; the other was gentle, clinging, and timid; or the one was more ardent, the other more calm and placid; or again, the one was the more independent, original, and self-contained; the other the more generous, hasty, and vivacious. In short, the difference was that of intensity or energy in one or other of its protean forms; it did not extend more deeply into the structure of the characters. (3)
Galton was the first to name the Nature - Nurture debate as such, and saw that twins who were very alike in their youth “…continue their lives, keeping time like two watches, hardly to be thrown out of accord except by some physical jar.” It was clear to Galton and others at that time, following the publication in 1858 of Darwin’s Origin of Species, that these twins had similar genes. The science of genetics - and its dark twin eugenics - was developing rapidly. Galton, with his great interest in studying twins, continued to study eugenics for the rest of his life, basing his research on the differences between twins, particularly those who were so identical they could not be distinguished from one another.
The use of identical twins for research later absorbed the interest of Josef Mengele, who was a doctor working in the concentration camp at Auschwitz in Poland, during the Second World War. Dr Mengele selected out any pairs of twins from the crowds arriving, and they were glad to be chosen because it seemed it would keep them alive. However the cruel and terrible experiments carried out on these twins by Dr Mengele meant that of 3000 twins selected for medical research only 157 lived to tell the world what had happened.(4)
Mengele is considered quite mad by most historians and was probably schizophrenic. In a place of death where life was cheap, he was able to hack off limbs and remove organs from his young victims, seemingly without any ethical scruples. However he was able to be kind to his twins and pampered them. It is thought he actually loved children. It was puzzling that two such separate and diametrically opposed characters were to be found within the same individual - that is, until we consider the possibility that Mengele was a womb twin survivor.
Over the next two generations, knowledge of twinning expanded, mainly due to medical studies made on the placenta, blood vessels and membranes after the delivery of twins. The concept of monozygotic (MZ) and dizygotic (DZ) twins was now clear; that some twin pairs developed from the same zygote and some from two, respectively.
By the 1950s it was becoming increasingly common for women to be delivered in hospital. As twin births are often dangerous and require hospital admission, there were plenty of twin births for doctors to study. It became clear that the arrangement of the membranes and the blood vessels in DZ and MZ twins varied considerably. It was conjectured that some kind of intrauterine influence may be at work, which made itself felt in later life and may explain some of the difference between MZ and DZ twins and between members of an MZ pair.
Bronson Price, a geneticist writing in the 1950s, had some new ideas. In a paper published in 1950, (5) he issued a warning to those who were studying the differences between twins in order to make decisions about the genetic origins of disease: they should remember that the prenatal experience of sharing the same chorion may be an important environmental influence in itself.
There is at least one kind of environmental influence that is not only peculiar to the majority of monozygotic pairs but acts before they are born. Over 98% of humans have never been exposed to this condition at all.
Sadly, as we will see, this warning has not been heeded. Simply being conceived as a twin, whether DZ or MZ, is a pre-birth experience unique to twins, yet this specific factor is not included in studies of environmental influences. Twin research studies even now still follow on the classic method of comparing identical twins with each other and noticing how they differ: if a trait is common within an MZ pair then it is considered to be inherited and therefore of genetic origin. If it is not, it is considered to be environmental in origin - but the pre-natal environment is not mentioned.
Today we know that MZ twins do not get exactly the same genes when the zygote splits (6) - which is the reason why the term “identical” twins has fallen out of favour, since identical twins are not identical. They may even be of different sexes.(7) Despite this, and because of the huge number of twins that are now available, thanks to various twins databases, twin research continues to produce useful results. Nonetheless, it still is a very uncertain science, based on some fundamental methodological assumptions that may prove eventually to be mistaken, as we shall see.
1. Wright, L (1997) Twins: genes, environment and the mystery of identity. London, Orion Publishing
2. Galton, F. (1883) Inquiries into Human Faculty and its Development Macmillan, London
3. Galton F. (1883) ibid.
4. Wright. L. (1997) op. cit P.18
5. Price, B. (1950). Primary biases in twin studies; a review of prenatal and natal difference-producing factors in monozygotic pairs. American Journal of Human Genetics 2(4): 293-352.
6. Bruder, C. E., A. Piotrowski, et al. (2008). Phenotypically concordant and discordant monozygotic twins display different DNA copy-number-variation profiles. American Journal of Human Genetics 82(3): 763-71.
7. Edwards, J. H., T. Dent, et al. (1966). Monozygotic twins of different sex. Journal of Medical Genetics 3(2): 117-23.