20 surprising facts about identical twins

• Identical twins… are not identical
• Identical appearance, distinct fingerprints
• Dogs can tell them apart
• Twin language
• Connection in the womb
• If one twin develops cancer, the other is also at risk of developing cancer
• Same cardiovascular and brain health, but..
• Identical twins help advance science
• Twins are rare
• Twins can have different genders
• Mirror twins
• Telepathy and identical lives?
• Similar death?
• They don’t need to be separated in school!
• Identical twins become less and less alike with age
• Twins are twice as likely to be left-handed
• Identical twins have the same tastes and eating habits
• Identical… but not when it comes to COVID-19!

Humans have always been fascinated by identical twins. The idea of having a duplicate lies at the origin of many myths and beliefs. They are often represented as mysteries of nature, sometimes inseparable, and occasionally enemies. While advances in science have disproven most of the rumours, a few surprising facts about identical twins yet remain.

Identical twins… are not identical

Identical twins are born from a single egg, which divides only after fertilization is complete. That’s why they’re called monozygotic twins. They may share the same DNA, look like the same person, and be able to swap places without being detected, but there are still little details – invisible to the naked eye – that make them unique. A recent study by Icelandic researchers, published in Nature Genetics, looked at gene mutations in monozygotic twins during embryo development. The study found that the number of mutations can vary considerably from one twin to the other.

These genetic mutations are instrumental in determining human development and follow individuals throughout their lives. In short, from the very beginning of their development, identical twins have distinguishing features. There is no such thing as a perfect copy!

Identical appearance, distinct fingerprints

While twins do share the same genetic heritage, they do not have identical fingerprints. The reason for this is quite simple. Genetic material is only partly responsible for our fingerprints. Specific factors, such as thumb sucking or umbilical cord blood pressure, also help determine what our fingerprints look like. Furthermore, fingerprints continue to change slightly into early childhood depending on the way children use their fingers.

Dogs can tell them apart

We’ve listed a few differences among identical twins, but it can still be quite tricky to tell them apart. Their genetics are so closely linked that they can outsmart the most sophisticated DNA tests on the market. This makes it pretty hard for authorities to discover the truth when twins dabble in crime. Apart from fingerprints, there’s another way to distinguish identical twins: body odour. Our scent is influenced by external factors, such as the food we eat and the places we visit, and only dogs have a keen enough sense of smell to differentiate them.

Twin language

Much to the chagrin of parents and fascination of linguists, twins often communicate with each other using a unique dialect that no one else can understand. Most twins develop their own language code and expression. Let’s not get carried away, though. Twins don’t actually invent their own language. They simply spend so much time together as children that they understand any strange sounds or words the other twin makes. In other words, one twin can easily understand the other because they tend to make the same type of mistakes. Still, it can be quite impressive to witness.

Connection in the womb

Twins are known to have a uniquely complicit relationship that begins long before they are born. According to one Italian study, twins begin to interact and socialize from 14 weeks’ gestation onwards. Researchers came to these conclusions after observing fetuses interacting in the uterus. One may think that in such a small space, twins touching and pushing each other to make room for themselves would naturally explain most of their movements. In many cases, however, twins voluntarily seek out contact. They cuddle and touch the other’s hands and eye area. Of course, they sometimes bicker and push each other away. Still, those ultrasounds must be very moving.

If one twin develops cancer, the other is also at risk of developing cancer

In 2016, researchers from Harvard and Norwegian and Danish universities conducted a study on monozygotic and dizygotic twins with cancer. In both types of twin, they concluded that if one develops cancer, the risk of the other twin also developing the disease increases significantly. The risk of developing a cancerous tumor increases to 37% for dizygotic (fraternal) twins and 46% for identical twins.

Same cardiovascular and brain health, but..

Monozygotic twins are genetically identical, both inside and out. That’s why researchers often study identical twins to understand the role of nature vs. nurture in humans. A US study looked at the genetic relationship between cardiovascular and cognitive health by comparing identical twins (one egg) and fraternal twins (two eggs). Results showed that heart and brain health is largely determined by factors related to family environment and socioeconomic status. In short, people’s life choices play an important role in their health.

Identical twins help advance science

American astronauts Scott and Mark Kelly are a great example of how identical twins can contribute to medical and scientific advances. The twin brothers are both astronauts, but only one of them (Scott) was sent on a space mission. Researchers carefully analyzed the differences between two identical bodies: one that spent a year in space and another that remained on Earth. Scientists not only found changes at the molecular level; they discovered that Scott’s telomeres were longer than Mark’s. Telomeres are supposed to shorten with age, but they showed significant growth after a stay in space. In short, this experiment is one of the most accurate studies of the changes a human body undergoes after a trip to space.

Twins are rare

A twin pregnancy (being pregnant with twins) is already quite rare, but having identical twins is even rarer (20% of twin pregnancies). Researchers, however, have noted a considerable increase in the number of twins worldwide over the last 40 years. The number of twin pregnancies, in particular, is on the rise in Western countries. Some age-related factors may partly explain this phenomenon. After a certain age, women are more likely to become pregnant with twins. According to some statistics, the increase in fertility treatments is also behind the recent twin boom.

Twins can have different genders

Such twins are actually called sesquizygotic or semi-identical twins. In 2019, an Australian woman’s ultrasound showed a single fertilized egg and a single placenta containing two fetuses, just as you would see with a case of monozygotic twins. However, after 16 weeks of pregnancy, it was clear that there was one male and one female fetus. Doctors currently support the hypothesis that the same egg was fertilized by two spermatozoa rather than one. This phenomenon has only been observed twice in history.

Reproduces

Sadly, the mortality rate of newborn twins is higher than that of single births, primarily due to often difficult and premature childbirth. After sharing a small space and the same amount of food during gestation, twins tend to be born smaller in size and weight. In Africa, twin mortality rates are up to seven times higher than singleton births.

Mirror twins

When the separation of the egg occurs later than usual, the identical fetuses develop diametrically opposed traits. One twin will be right-handed and the other left-handed. If one has a mole on the left side, the other has it on the right side. They’re called mirror twins because when they stand face to face it’s like looking at a reflection in the mirror.

Telepathy and identical lives?

This myth is so widespread that researchers at the University of Cambridge have taken a scientific interest in it. In addition to being genetically similar, identical twins generally have the same educational, social, and family background. Equipped with the same points of reference and similar reflexes, they tend to make similar choices and follow the same path, even if they live apart. Some stories verge on the paranormal, though, despite the numerous scientific and rational explanations for twin similarities. One such case is the tale of brothers Jim Lewis and Jim Springer, identical twins separated at birth. After 39 years apart, they finally found each other and discovered that they had lived nearly identical lives. They had even the tiniest details in common, drinking the same kind of beer or giving their respective sons and dogs the same names!

Similar death?

We now understand that most twins have nearly identical life experiences due to living in the same environment. In addition to resembling each other physically, they may have the same friends and passions and, therefore, go through the same ordeals. This doesn’t mean that they’re destined to live identical lives, though, and that’s what makes the story of these American twin sisters so unsettling. Martha Williams and Jean Haley died in the same way at the age of 97, just minutes apart. One of the twins fell down the stairs, and when the other tried to help her, she tripped over the carpet. Doctors determined that the cause of death was exactly the same.

They don’t need to be separated in school!

Many believe that it’s best to put twins in different classes or even different schools to help them fully develop and gain independence, but this can be a logistical nightmare for parents. Happily, keeping twins in the same school doesn’t seem to affect their academic performance. In fact, there are no hard and fast rules when it comes to personal development. Parents should focus on what’s best for their twins… just like any other child!

This strange phenomenon only occurs when two pairs of monozygotic twins have children. The probability that two identical twins will have children who are also identical twins is just one in a million, but it has actually happened. In Ohio, in the US, a pair of twins married twin brothers. The two couples are genetically identical, as are their children. Strangely, this means that their children are genetic siblings, but cousins geneologically.

Identical twins become less and less alike with age

They don’t just look alike, parents of monozygotic twins also tend to dress them the same way. They often go to the same school, share the same interests, etc. As they get older, however, twins start to develop their own looks. While they continue to share similar features and characteristics, they eventually start to develop independently of each other. Their career choices, the food they eat, and the places they visit all affect their physical appearance.

Twins are twice as likely to be left-handed

Not only is it more likely that one of them will be a leftie; it’s nearly twice as likely that both will be left-handed compared to the rest of the population. Mirror twins are, of course, an exception to this trend!

Identical twins have the same tastes and eating habits

Researchers from King’s College London tested more than 2,000 twins on food preferences and habits and found that identical twins are more likely to share the same preferences as fraternal twins. This led researchers to conclude that food preferences have a genetic component. It makes sense, then, that twins with identical genetic backgrounds are more likely to develop the same tastes in food.

Identical… but not when it comes to COVID-19!

In Italy, 60-year-old twin brothers who live together and work at the same place both contracted the COVID-19 virus. They even caught the virus from the same person. However, one twin was hospitalized for just a few days and managed to make a full recovery, while the other was in intensive care and had to stay in the hospital for more than a month. Fortunately, both brothers are now doing well.

This virus is definitely forcing us to reexamine everything we think we know!