Twins have always fascinated us. In some parts of the world, they are considered a symbol of good fortune, while in others they are said to bring bad luck, but they always receive special attention. But how special are twins really? How are they formed and how identical are they? Does this invisible bond exist between them?
They are born together and are particularly similar to each other – especially if they are identical twins. Although we already learn in school how monozygotic and dizygotic twins are formed, the exact mechanisms of twin formation and their causes are still poorly understood. It is also still unclear why the number of twins keeps increasing, and research is also constantly gaining new insights into the subject of similarity between twins. There is even a whole branch of research, twin research, dealing with the genetics and psychology of twins.
How twins are formed
The emergence of life is one of the wonders of nature. For a new human to come into being, it takes the maternal egg and a paternal sperm. The eggs mature in the mother’s ovaries until they reach the fallopian tube through ovulation, where they can be fertilized by a male sperm. This results in the so-called zygote, a cell that now contains the genetic material of the father in addition to the maternal DNA in its nucleus.
The fertilized egg cell migrates via the fallopian tube into the uterus, where it settles and grows. During this migration, the zygote begins to divide, creating more and more connected cells, growing into a compound of two, then four, eight, sixteen, and more and more cells. After about seven days, the cells begin to specialize into different cell types. At this stage, called a blastocyst, the compound of cells passes from the fallopian tube into the uterus and nests there to continue growing.
Or even two
By chance, it can sometimes happen that this group of cells does not “stick together” properly on its way to the uterus and splits into two different clumps of cells. This can happen right at the beginning, after the first cell division of the zygote, up to the blastocyst stage. In very rare cases, even later. After this division, the two new cell clumps continue to migrate and grow separately. In this way, two living beings are created from one fertilized egg cell, which thus possess identical genetic information. The result is monozygotic twins.
Dizygotic or fraternal twins, on the other hand, are formed when two different eggs mature at the same time, are fertilized in the fallopian tube by different sperm in each case, and then migrate independently of each other into the uterus. Because of the different eggs and sperm, these twins do not have identical genetic material, but are only as closely related as normal siblings.
Siamese and semi-identical twins
In very rare cases, when identical twins develop, it can happen that the cell compound splits “too late”, only about twelve days after fertilization, and therefore cannot separate completely. As a result, these twins continue to develop together and are born “grown together”. Such conjoined twins are then joined at the sternum in most cases, but there are also numerous other possibilities such as a connection via the head, hip, or rump.
As long as they do not share vital organs, conjoined twins can be separated by surgery. However, they sometimes share only one heart, lung, or brain, making separation impossible. Siamese twins have existed as long as there have been humans. But they got the name only from a pair of twins born in Siam in 1811, which became known worldwide as a circus attraction.
Even rarer and an absolutely special case among identical twins, are the so-called semi-identical twins. Normally, eggs are designed by a protective mechanism to be fertilized by only one sperm. This is the only way to ensure that the chromosome set is correct and the zygote is viable. In the formation of semi-identical twins, on the other hand, fertilization of one egg with two sperm takes place.
The fact that the zygote is nevertheless viable is only possible if it divides by chance in such a way that the chromosome set is correct again. This is why this form of twinning is extremely rare: So far, only two cases are known worldwide. These twin pairs have 100 percent of the same maternal genetic information since they were created from the same egg cell like normal identical twins, but they have received different genetic information from the father through the two sperm cells.
When twins grow in the womb, they must share their protective and nurturing domicile. In this process, various forms of prenatal “shared housing” can occur. Normally, a baby grows in an amniotic sac made of two layers of skin and is nourished by a placenta. In twins, on the other hand, they may sometimes share these spaces and resources in different ways. However, this depends on their formation and development.
When twins each have their own placenta and amniotic sac, this condition is called dichorionic-diamniotic because the placenta arises from what is called the chorion and the amnion is the inner skin layer of the amniotic sac. This is how all fraternal twins and one-third of identical twins grow.
However, it is different for monozygotic twins, which separate later. By the time of separation, the chorion, and rarely the amnion, has already begun to form. Depending on when the separation takes place, these components are therefore no longer formed twice, but only once for both twins before separation. Most commonly, monochorionic-diamniotic twins are formed, separating four to seven days after fertilization. They then share a placenta, but each develops its own amnion and thus its own amniotic sac. In one percent of cases, twins separate so late that the chorion and amnion have already developed and the twins must therefore share a placenta and amniotic sac. Monochorionic-monoamniotic twins result.
These division ratios of amniotic sac and placenta are investigated in every twin pregnancy, as they can pose various risks. For example, if the placenta is shared, one twin may be undersupplied and develop worse. In a monochorionic-monoamniotic pregnancy, there may also be a risk of conjoined twins or umbilical cord entanglement. Last but not least, this examination can also provide clues as to whether the twins are monozygotic or dizygotic.
How common are twins?
How often are twins born? Overall, they are probably rather rare. About 3.2 percent of births in the United States are twins. This means that there are 32 twins for every 1,000 births. Identical twins are much rarer than fraternal twins; only about a quarter of all twins are identical. They have been occurring unchanged for decades and have a worldwide probability of about 0.04 percent.
Fraternal twins, on the other hand, are becoming more common. According to studies, the number of twin births worldwide increased by one-third from the 1980s to 2010, although the number of identical twins always remained the same. But what is the reason for this? What influences the formation and thus the frequency of identical and fraternal twins?
Why it comes to the emergence of identical twins and what could influence this emergence, researchers puzzle over until today. Up to now, it has often been assumed that this is simply a coincidence that causes the cells to separate spontaneously. However, research is trying to get to the bottom of the cause of this “coincidence” in more detail, so that nowadays a large number of different hypotheses exist.
The latest findings from 2021 show that identical twins carry certain epigenetic markers on their DNA, by means of which a person can be identified as an identical twin even in the absence of his or her twin. These attachments to the genome influence gene activity and thus contribute to the formation of different cells and tissues in the embryo despite the same DNA.
Scientists suspect that the formation of these epigenetic structures may be the cause of identical twins. Thus, genes for cell cohesion are also affected by epigenetic markers, which could explain why cell cohesion splits. So far, evidence for this hypothesis is lacking.
Another possible cause is provided by the so-called cell rejection hypothesis. Thus, every embryo normally rejects “damaged” cells in which mutations have occurred. The theory now is that these rejected cells nevertheless develop successfully and become the second twin. However, genetic factors are not completely ruled out either, as very rarely does a dominant predisposition for identical twins also occur within families.
Likewise, environmental factors are investigated due to a conspicuous number of identical twins in certain cities, or a connection with artificial insemination is suspected. However, despite the numerous theories, the exact cause of the split is far from clear.
Double ovulation and its causes
In contrast, double ovulation as the cause of the development of fraternal twins can be well explained scientifically today: Basically, the follicle-stimulating hormone FSH causes eggs to mature and ovulation to occur. Ovulation, in turn, is then caused by the luteinizing hormone (LH). If these hormones are produced in increased amounts, this can lead to double ovulation and subsequently fraternal twins.
Various factors can be considered as triggers for this hormone overproduction. One of these is heredity. Since fraternal twins occur frequently in some families, it has been clear for some time that genetic factors play a role. Scientists have already been able to identify two gene variants in mothers of fraternal twins that are linked to the production of FSH and the sensitivity of the oocytes to this hormone. However, the exact inheritance has not yet been fully clarified.
However, the age of the mother also plays a role, since the FSH concentration in the blood increases with age, as does hormone treatment in cases of unfulfilled desire to have children. Artificial insemination also favors fraternal twins because more than one embryo is usually implanted.
The size of the mother could also have an influence on twin pregnancies. Gynecologist Gary Steinman in New York found in a study that mothers of fraternal twins were always taller than the national average and attributed this to the growth hormone IGF (insulin-like growth factor), which is produced more by tall people. This is supposed to have an influence on the sensitivity of the ovaries to the hormone LH and thus promote double ovulation. For the same reason, diet is also said to be able to cause twin pregnancies. The scientist suspects that growth factors from foods such as meat or milk could promote the development of fraternal twins.
These potential triggers could also explain why the number of fraternal twins has increased in recent decades. On the one hand, women are having children later and later, and on the other hand, more and more couples are undergoing fertility treatments such as hormone therapy or artificial insemination. In addition, medical progress is also increasingly preventing miscarriages in high-risk twin pregnancies.
How do the differences in identical twins arise?
While fraternal twins are no more alike than any other normal sibling and share only about 50 percent of their genes, identical twins were long considered genetically absolutely identical – clones. Because they are created from one and the same fertilized egg, they would have to match 100 percent of their genome. What’s more, they look virtually the same and often share similar interests and tastes.
But this is deceptive. Even identical twins are never completely identical. This starts with the fingerprint and the pattern of the iris, which are never completely the same even in identical twins, as is usually the case with body size. Also, moles develop in different places and one twin can be left-handed, the other right-handed. In addition, the character does not have to be similar at all, completely different personalities with completely different interests and inclinations can develop, just like health development can differ partly strongly.
Different activity of the genes
Researchers believe that epigenetics is the cause of this difference in development. This is a mechanism that controls the activity of our genes by attaching molecular structures to the DNA, so that even identical twins can develop differently despite having the same DNA.
Unlike the genetic code of DNA, this epigenetic pattern is not rigid, but can be altered by environmental influences. These include diet, athletic or mental activity, stress, even air quality, and much more. Therefore, as twins age, even identical twins can become increasingly different depending on their lifestyles. Thus, given the same genetic condition, for example, a mutation that increases the risk of breast cancer, only one of the twins may actually develop the disease and the epigenetic pattern may even be revealed as the cause.
Epigenetic divergence can even begin in the womb. Already here, the embryos are subject to slightly different environmental influences, for example in the position in the abdomen or the composition of the umbilical cord blood. For a long time, researchers attributed the differences between identical twins only to epigenetics. However, it is now known that actual differences also exist at the DNA level.
No genetic clones
Thus, identical twins also differ genetically to a much greater extent than previously thought and by no means have completely identical genetic material. A team of researchers from the Icelandic company DeCODE genetics has now discovered that identical twins begin to diverge at the DNA level at a very early stage due to mutations. On average, they already differ in the early embryonic stage by 5.2 mutations, 15 percent of the twins even considerably more. The researchers were able to identify up to 100 mutations.
On the basis of these mutations, twins can now even be clearly distinguished genetically. What was long considered impossible is now possible thanks to increasingly sophisticated sequencing methods that determine the exact sequence of bases in DNA. The days when paternity or criminal acts of identical twins remained unexplained are thus over.
These subtle gene differences could also explain the rare cases in which only one twin develops a purely congenital disease due to a mutation. This is an important aspect also for so-called twin research. For this is actually based on the assumption that identical twins are genetically completely identical and, on this basis, investigates the influence of environmental factors.
Genetics or environment
Research has been very interested in twins for a long time. Already about 150 years ago Sir Francis Galton, the cousin of Charles Darwin, investigated the distribution of behavior and certain abilities within and between families and especially between twins, which is why he is considered the founder of twin research. To this day, twin research attempts to answer this question of “disposition or environment?” That is, whether certain characteristics are due to genes, environment, or perhaps both.
Therefore, especially identical twins are interesting for research. Environmental influences can be studied particularly well in them because of their almost identical genetic information. When evaluating twin studies, however, it must be kept in mind that even identical twins are not 100 percent genetically identical, but only about 99.99 percent, as is known today.
How does twin research work?
However, not only identical twins are interesting for twin studies. It also applies to fraternal twins, because although their genetic information does not match as closely as that of identical twins, they are also born at the same time and have been subject to the same or at least very similar environmental influences since their conception. Therefore, they serve as a comparison group in classical twin research.
The researchers then examine the so-called concordance, the similarity of certain characteristics of identical twins compared to fraternal twins. If identical twins resemble each other more than fraternal twins in this characteristic, this is an indication of a genetic influence. If, on the other hand, identical twins differ strikingly in certain characteristics, this could be an indication of the influence of environmental factors. Of particular interest in this regard are identical twins who were separated after birth. Here, twin researchers investigate how similar the twins still are despite the separation and what makes them different.
And the findings?
In this way, twin researchers have been studying every conceivable human characteristic for years. From height, weight, favorite food, or athleticism to political beliefs, religiousness, sexual orientation, and choice of partner to predisposition to mental illness, cancer, or aggressiveness.
A very popular question that has been asked for a long time is the heritability of intelligence. With the help of twin studies, researchers have come to the conclusion that intelligence is inherited up to 70 percent of the time. This means that 70 percent of deviations from the average intelligence of the population are due to genetic factors. However, this value only refers to the average, not to a person’s individual intelligence.
Conversely, twin researchers determined that social environment and upbringing are more decisive than genes in the development of personality. For example, traits such as fearfulness or openness are said to be only about 30 percent genetic, and the genetic influence on political or moral attitudes is probably also very small.
Twin researchers are also very interested in life expectancy and the development of diseases. Many different diseases, such as cancer or chronic inflammatory bowel and nerve diseases, are currently being investigated for their genetic causes. Thanks to highly developed methods, however, it is now possible to examine the DNA itself so precisely that researchers can detect minimal differences between identical twins and deduce the genes that cause disease.
In addition, twin studies have provided researchers with important insights into epigenetics, i.e. environmental factors that influence gene activity and affect health individually and independently of the actual DNA.
Overall, twin researchers, today conclude that both aspects, genetics, and environment, always play a role, albeit in different proportions. However, the exact interaction of these factors is highly complex and in many cases only partially understood.
Effects of being a “twin”
Just as scientists are interested in their genetics, they are also interested in the twins themselves. What does being a twin do to them? How do they feel about their own identity? How do identical twins differ from fraternal twins or normal siblings in personality, and do they really possess that special bond among twins?
There are countless stories about the legendary bond of twins. One reads about twins who feel each other’s pain or are quasi-telepathically connected, who buy each other identical gifts or run into mirrors because they think they are spotting their counterpart there.
Is there a special connection?
But does the legendary, invisible “twin bond” exist? According to some scientists, probably not. Scientists compared identical, and fraternal twins and also normal siblings to examine their relationships and find out whether the bond of identical twins is indeed unique. It appears the less alike the siblings, the less important their relationship is to them. Surprisingly, however, the results were different.
All children found it most important to find support and backing from their siblings, regardless of whether they were twins or normal siblings. Moreover, according to the researchers, it is equally true for all types of siblings that they have a significant influence on each other’s personality development. It was concluded that the legendary invisible bond between twins does not exist per se. In many cases, they are no closer emotionally than normal siblings are.
But there are also research teams that came to other conclusions. According to these, twins could very well possess an extraordinary bond. Among others, psychologists Caroline Tancredy and Chris Fraley of the University of Illinois at Urbana-Champaign believe they have found evidence of this close bond. They examined the relationships between twins, parents, and siblings using questionnaires and found that the respective twin was always the most important caregiver, significantly more than other siblings, parents, or even partners. They were able to gain something from this relationship that no other could give them.
Considered one of the most amazing and therefore very prominent narratives is the life story of twins Jim Springer and Jim Lewis, who were separated at birth and did not meet again until 39 years later. Through their extraordinary life story, they founded the study of twins who grew up separately.
For not only were they both accidentally christened Jim by their adoptive parents. Both also loved math in school and hated spelling, they named their dog Toy and were married twice. The first time to a Linda, the second time to a Betty. They also named their first son almost the same, James Alan and James Allan. Both worked as deputy policemen for a time and bite their fingernails. They also smoked the same brand of cigarettes and drank the same beer. Their car was also the same color and make. For years, they went to the same beach in Florida on vacation and both have a round white bench around a tree in their backyard.
In studies, the two twin brothers also produced nearly identical test results; even their brainwave patterns were barely distinguishable. In this story, coincidence probably also had its fingers in the pie, but nevertheless, it shows in a vivid way how special a connection between twins can be. And that even when they don’t even know each other.
Identity development – me or us?
But there are not only advantages to being a twin. It shows a peculiarity in the development of twins that is noticeable early on: the difficulty of becoming aware of and forming one’s own identity. As babies, twins need a few months longer to recognize themselves in the mirror because they think they see their twin there. They also do not use the word “I” at first. Often twins invent a word that describes them both or even develop their own language.
Over time, however, twins also recognize themselves as individuals, and as early as kindergarten they begin to make friends of their own. During puberty, many twin siblings want to distinguish themselves more from each other and often dress or behave deliberately differently. But even here, their similarity can become an obstacle. Because no matter how different the twins may be, they are still often perceived as a “twin pack” by those around them.
Another hurdle in their personal development is the lack of comparison with others. Normally, this is important in order to be able to assess oneself and one’s abilities. However, twins are often compared to each other and even small variations in their abilities lead to them being disproportionately classified. Then one can be judged as athletic, the other as unathletic, although this would not be the case in comparison with others.
This often results in a “division of labor” in which each twin always does what they are a little better at. In identity development for twins, it is then challenging to still try out individually and to recognize in which areas one is really good. An additional problem that can make it difficult for twins to separate themselves is a temporally different need. For example, if one twin detaches earlier due to new friends or life partners, the other may feel set back as a result.
However, after such a conscious detachment and development of their own identity, many twins eventually reunite and usually have a close relationship throughout their lives.