Fascinating Facts the World Knows Thanks to Twin Studies

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Fascinating Facts the World Knows Thanks to Twin Studies

By Trista - April 30, 2021

Out of every 100 births, three are twins. Twins offer a valuable source of information for both health and psychological research. Their unique relationship gives researchers the ability to pull apart and examine genetic and environmental influences. The findings from twin studies have been extremely influential in detecting and treating various diseases and psychological disorders. Twin studies allow researchers to examine the overall role of genes in developing a trait or condition.

Furthermore, twin studies suggest that a specific characteristic or disorder has a genetic component, although the studies do not necessarily provide information about the gene’s location. Twin studies are incredibly informative because identical twins share nearly 100 percent of their DNA, whereas fraternal twins only share roughly 50 percent. Studying twins allows scientists to dive into a vital mystery of nature versus nurture. Due to their genetic makeup, twins enable researchers to control one of the main variables in so many factors, our genetics. From there, they can then drill down to the impact that environmental factors play throughout our lives. The studies of both fraternal and identical twins have long been used to understand and unwind the influences of genes and the environment on particular traits.

Fascinating Facts the World Knows Thanks to Twin Studies — *A human’s level of happiness is linked to their genetic makeup. Photo Credit: Monkey Business Images/Shutterstock*

20. Twin studies have revealed that happiness is genetically influenced.

There are three genetic variations for happiness. There are two that are responsible for differences in symptoms of depression and one linked to happiness. It has also been discovered that an astonishing 20 areas on the specific genome have been linked to happiness. Through studying both identical and fraternal twins, scientists have been able to identify that about one-third of the variation of happiness is linked to genetics. Scientists were able to focus on one gene in particular, that works to transport serotonin. This specific neurotransmitter works by reducing stress hormones.

Although your level of happiness is linked to your particular genetics, external environmental factors still play a factor. The external environmental factors influence how the genes exhibit themselves. Knowing the exact location of specific genes opens up questions about the ability to modify someone’s genetic makeup. However, it is nearly impossible to alter their makeup to enhance their full happiness because a few thousand genetic variants are linked to happiness. Therefore, it will be way too complicated to start altering that much DNA. Twin studies have helped us to learn about the genetics of happiness and how the environment affects that could be the key to better health and education. Although scientists realize that individuals are genetically different, we can begin to customize more aspects that we already do.

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19. There is increased recognition that transgender feelings start very early and are incredibly consistent.

The search for gene variants that underlie any particular trait usually begins with twin studies. Studies showing identical twins are much more likely to be concordant, meaning that both are transgender or both are cisgender compared to siblings or fraternal twins. However, this may also be underestimated that one twin may not wish to come out as trans, therefore minimizing the concordance. It suggests a vital genetic component. More recently, particular genes have been studied in great detail in transwomen and transmen. One study focused on the association between being trans and specific variants of some genes in the hormones.

The findings showed that trans women had a high frequency of specific DNA variants of four genes that would alter sex hormone signaling while developing in utero. There may be other genes that contribute to either a masculine or feminine sexual identity. However, they are not all concerned with sex hormone signaling. Some of these genes might impact brain function and behavior. Sexual identity genes do not have to be on the sex chromosomes. Studies have shown that gender identity is separate from biological sex. The next step is to compare the entire genome sequence of cis and transsexual people. Researchers review the whole genome epigenetic analyses, looking at the molecules that affect how genes function in the body might also pick up differences in genes’ action. Likely, as many as hundreds of genes work together to produce a range of sexual identities.

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18. Science has shown that our overall spirituality level is shaped by a mix of family environment and genes.

Some people who are skeptical of the twin studies might suggest that showing a similarity for belief is just displaying some cultural or family influence that wasn’t addressed adequately for the study. However, in one study of adopted twins, researchers looked at religious belief in several adopted twins raised apart. While it may be surprising to you, they found the same results. There was a more remarkable similarity in identical twin pairs, even if they were raised apart. The conclusion was unavoidable in that faith is influenced by genes.

To discover in more detail what belief was genetic, two academic twin experts partnered together. They attempted to separate what is referred to as the three B’s – belonging, behaving, and believing, which make up religion. They asked a range of questions to get an idea of the individual differences in spirituality. It was estimated that the heritability of spirituality was around 40 to 50 percent. Although our senses of spirituality vary, our innate, inherited sense affects how we perceive the world, ourselves, and the universe. That is separate and independent from our traditional religious beliefs that are strangely independent of family influence. Studies show no evident genetic influence or difference from their parents in their practice for twins living at home. However, genes do play a role once the twins move from their childhood home. After leaving, children with the right predisposition can often switch religions.

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17. Studies suggest that social behavior begins in the womb.

Twin studies have helped to come to this conclusion. Researchers utilized ultrasonography to track the motion of five pairs of twin fetuses in daily 20-minute sessions. The scientists found that the fetuses began reaching for their siblings by the 14th gestation week. Over the following weeks, the number of movements towards themselves decreased. Instead, the number of actions towards their womb mate increased more and more. By the 18th week of gestation, they had spent more time contacting their partners than themselves or the uterus’ walls. Roughly 30 percent of their movements were directed towards their siblings. The movements included stroking the head or back and started to last longer. They were also more accurate than their self-directed actions.

The studies’ results revealed that twin fetuses are aware of their counterparts in the womb and prefer to interact with them. They also respond to them in special ways. The contact between the pair appeared to be planned rather than a result of a lack of space. The conclusion is that social behavior begins in the womb. The idea that fetuses can control their actions in the womb is not a surprise. Researchers have shown that fetuses display skilled movements by the fifth month of gestation and that the presence of a twin in the womb can accelerate motor development. The womb is a critical starting point to developing a sense of self and a sense of others.

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16. Two of every five common diseases are at least partially influenced by a person’s genetics.

Nearly 40 percent of 560 different diseases have a genetic component, whereas 25 percent are driven by environmental factors shared by twins growing up in the same household. Brain disorders were most strongly influenced by genetics, with four out of five cognitive diseases having a genetic component. On the other hand, eye diseases and respiratory infections were most likely to be influenced by the environment in which twins were raised. The twin studies could serve as a guiding point for individuals interested in studying the causes of any of the 560 diseases.

Twin studies are valuable because identical twins share 100 percent of their genetics. Conditions that tend to strike identical twin pairs at a higher rate than fraternal twins or siblings who share about 50 percent of their DNA are probably influenced by genetics. Diseases that occur in sibling pairs, regardless of whether they are twins, tend to be strongly influenced by environmental factors. Genetics plays a role in at least 40 percent of the diseases that were investigated. To ensure a thorough study, researchers also used zip codes in the database to estimate environmental factors, including socioeconomic status, climate conditions, and air quality, on diseases in the siblings. The findings determined that 20 percent of the diseases were affected by temperature changes, 25 percent by socioeconomic status, and six were influenced by air quality.

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15. The search for genes associated with common epilepsy has been intensive in the past few decades.

Our understanding of the genetic background of common epilepsy has improved considerably, and the current genetic studies have optimized the knowledge for the future. It has been well understood that people with epilepsy are more likely to have family members with seizures. The familial component of epilepsy provides evidence that heritable genetic factors play a role in disease susceptibility. It is believed that a single mutation in the coding region of an epilepsy gene is believed to cause the disease and can predict the occurrence of the disease. Firm evidence for a genetic cause of epilepsy susceptibility is provided by twin studies that compare the disease correspondence between identical and fraternal twins to estimate the heritability.

By doing this, the environment’s impact can be canceled, assuming both twins have the same environment. That leaves the difference in shared genetic variation to explain any difference in disease concordance. Any difference in disease occurrence can be attributed to genetics. Long ago, it was thought that epilepsy was an acquired disease. It was believed to be caused by injuries to the head or difficulty during birth. Twin studies were fundamental in determining that there was a genetic basis to specific forms of epilepsy. The reviews helped identify which of the many forms of epilepsy had a genetic basis and didn’t. Researchers were then able to determine how to best target their gene discovery and disease treatment.

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14. It was discovered that despite growing up separately, their lives had turned out shockingly similar.

The male twins had both married and then divorced a woman named Linda. Their second wives were both named Betty. Both twins named their first son James with the same middle name, although with different spellings. One was called Allan, whereas the other son was Alan. If that wasn’t interesting enough, both male twins had a dog named Toy. As they grew up, both twins went into law enforcement and worked as part-time deputy sheriffs in their Ohio towns that were only 70 miles apart. They vacationed at the same, three-block-long beach in Florida, and both traveled there and back in a Chevrolet. Their drinking and smoking patterns were nearly identical. It has long been known that identical twins have been known to have remarkably similar brain waves, which can contribute to the perception that twin minds think alike.

The probability of two people independently being given the same name is not that rate. However, when you start to add up the coincidences between their two lives, it becomes implausible quickly. Although there is limited information on this fact, this 1979 study revealed that genetics could impact name preferences as displayed by these twins and their similarities. It is incredible that aside from four weeks, they lived their lives separate and independently and still had wives with the same names as well as sons. That study set the stage for even further twin studies to view the connections of twins reared apart. Although the chances of twins are three out of every 1,000 births, only one in three twin births produces identical twins.

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13. Variations due to genetic vs. environmental factors has been investigated in twin studies using structural and functional MRIs.

Heritability is defined as the proportion of variation that arises from genetic influences. Environmental influences explain the other part of the variation and are divided into shared common environmental influences and non-shared environmental influences. To disentangle the genetic and environmental factors on the variation, the data is derived from twin studies. The majority of heritability studies have been performed in twins as the twin design allows for the most precise estimation of genetic and environmental effects. Identical twins share all their genes. One current study focuses on brain morphology and function across different ages. The purpose is to provide an overview of how genetic factors influence other brain structures and functions during normal development. It was reviewed from childhood to late adulthood.

Comparing identical and fraternal twins allows researchers to learn which genes and lifestyle factors cause cognitive resilience or decline later in life. Studying older twins gives researchers the ability to identify what causes the brain to age slower or faster. There is a thorough review of their experiences throughout their lifetime, including their diet and exercise. Also, if they drank or smoke. The conclusion that can be determined is that genetic influences play an important role throughout the different phases of brain development. If the traits are heritable, the next step is to identify the genetic variants associated with brain morphology and function. Identifying genes might provide some clear insight into the underlying biological mechanism and help researchers understand why specific individuals have typical or atypical brain development.

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12. A twin study suggests that genetic makeup has a strong influence on whether or not your marriage will last.

Researchers compared marriage and divorce rates in identical and non-identical male twins. Their results showed that identical twins who share the same genes were more likely to follow the same patterns of divorce than non-identical twins, who only share approximately half of their genes like ordinary siblings. That suggests that there is a genetic influence on divorce. However, there was no difference between identical and non-identical twins when it came to whether they were married or not in the first place. That suggests getting married in the first place is entirely determined by your environment. The thought is that the environment trumps any genetic influence on whether someone gets married but that other factors have a role once they are married.

The theory suggests that the genetic influence on divorce is related to drug abuse, depression, and alcoholism, all of which have a genetic component. For instance, the study showed that twins who were pathological gamblers were two-and-a-half times more likely to get divorced in comparison to others. Almost any kind of psychopathology is going to make staying married more difficult. The study is critical because it helps to reveal the roots of behavior patterns and human nature. It is said that divorce is what is called a heritable trait, meaning that genes influence it. The genes that contribute to divorce are also involved in personality traits that are related to divorce. Based on this pattern of facts, researchers can determine that genes contribute to the transmission of divorce from parents to children instead of the rearing environment.

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11. A twin study that was done examined identical twins reared in different families found eery similarities.

Since 1979, more than 350 pairs of twins have gone through six days of extensive testing, including analysis of blood, brain waves, intelligence, and allergies. The results on personality were reviewed. For the majority of the treats that were measured, more than half of the variation was due to heredity. That leaves less than half determined by the influence of parents, home environment, and other life experiences. Even though the findings strongly point to the impact of heredity, the family still does shape the broad suggestion of personality. For example, a family might tend to make an innately timid child more or less shy.

However, this study determines that the family would be unable or unlikely to make the child brave because the innate personality trait is to be bashful. Among the 350 pairs of twins that were studied, some were raised apart. Among those separately reared twins were 44 pairs of identical twins and 21 pairs of fraternal twins. By comparing twins raised separately with those raised in the same house, researchers could decide the relative importance of heredity and the environment in their development. Although some twins attempt to emphasize differences between them, most identical twins are very much alike in their personalities. Among the traits that were most strongly determined by heredity were leadership and obedience to authority. No single gene is believed to be responsible for any one of these traits, but that a combination of many genes determines each characteristic. Hence, the pattern of inheritance is intricate and indirect.

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10. Twin studies have shown that about 40 percent of twins invent their own languages.

Scientists after the last few decades have been working on building their supporting research into the twin language. Twins are more likely to maintain an invented language because they spend a lot of time together and are on the same developmental schedule. The twins imitate and reinforce each other’s early inventions and weaken each other’s incentive to learn the real language. Twins have been shown to spend less time communicating with their parents and other adults than non-twins. In some cases, children do develop an entire language of their own. Twin languages tend to be simple, and they might even freely mix subjects, verbs, and objects. The universal twin-language structure might be easy to use in a one-on-one conversation.

However, it would be difficult to evolve to be useful to a larger community. It is also known that kids generally stop using private languages spontaneously or after a low-key intervention. It has been suggested that cryptophasia, or the language developed by twins that only the two children can understand, may harm children if gone on for too long. Cryptophasia has also been thought to hamper socialization. They rely heavily on their counterparts rather than socializing with other friends. Twins often generally get less intervention in comparison to non-twins. It is often assumed that it’s normal for twins to have a funny, unique language, and so they don’t usually make an assessment and diagnosis as quickly as others might. Inventing their own language further tightens their close bond. Their communication is more enjoyable than others have experienced, much like a secret communication, which is one of the most essential parts of every intimate relationship.

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9. A twin study suggested that genetics contribute to 40 to 70 percent of obesity.

The modernization of our society has contributed to higher rates of obesity through an environment that promotes increased calorie intake and decreased physical activity. It was discovered that more than 50 genes are strongly associated with obesity. While changes in the environment have significantly increased obesity rates over the last 20 years, the presence or absence of genetic factors protect us from or predispose us to obesity. Most commonly, obese individuals have multiple genes that predispose them to gain excess weight. One gene is the fat mass and obesity-associated gene (FTO) found in 43 percent of the population. In the presence of food, those with this particular gene may have challenges limiting their caloric intake. The presence of the FTO gene in combination with other genes can cause increased hunger levels, increased caloric intake, reduced control overeating, and an increased tendency to store body fat.

The classic twin study completed in 1990 looked at the importance of genes for body-fat storage. A dozen lean young male twins were housed and overfed by 1,000 calories a day for three months to do this. Although every participant was heavier by the end of the experiment, the weight gained ranged from nine pounds to 29 pounds. Weight gain within pairs was much more similar than weight gain between different twin pairs. The twins in each pair tended to gain weight in the same places, whether in the abdomen or thighs. This similarity among twins suggests that genetics play a role in the likelihood of obesity and where body fat will be stored. Understanding your family history can help you better understand your risk for obesity or obesity-related conditions. While your genes might predispose you to obesity, it does not have to be your destiny.

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8. Twin studies have been conducted to show that the environment in the womb sets the tone for your future health.

It has long been suggested that a mother’s stress during her pregnancy can impact the brain of her developing child. Twin studies have long been used to estimate the unique contributions of genetic and environmental influences on variation in human traits. The prenatal environment is a specific and crucial environmental influence on many human characteristics. Slow growth in utero may be associated with increased allocation of nutrients during development and may result in accelerated weight gain during childhood. Many factors can contribute to the environment in the womb. These include the mother’s age and how many pregnancies she has had.

Beyond these factors, fetal development can be further impaired by poor placental function or maternal disease. An altered long-term risk of illness is initially induced by the fetus’s adaptive responses to the mother’s cues about her health or physical state. Fetal responses can include metabolism changes, hormone production, and tissue sensitivity to hormones that can later affect various organs’ development. The effects of subsequent environmental exposures during infancy, childhood, and adult life may be influenced by these past exposures and may condition the disease’s risk later on in life. The twin studies have helped researchers understand the impacts of lifestyle factors such as diet and stress on our health and how these are possibly passed down through the generations.

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7. Twin studies have revealed that our genetics play a role in how severe our hangover is.

Genes interact with environmental factors to influence how much we drink and our susceptibility to becoming dependent on alcohol. Researchers utilized twin studies to assist in untangling the contribution of genetic and environmental factors for a given behavior. To be accurate, researchers looked at the differences in hangover experiences in fraternal and identical twins. Identical twins share all of their genes, and fraternal twins only share 50 percent of their genes, so any differences are assumed to result from genes rather than the environment. The first study was conducted in 1972, where twins were asked questions to understand their hangover experiences better.

The results indicated that the experience of hangovers is exponentially influenced by genes, with unshared environmental factors accounting for the rest of the variation. A second study was conducted to examine genetic influences on several measures of an alcohol hangover, including frequency, resistance, and susceptibility. This study showed that genetic factors accounted for 45 percent of the difference in hangover frequency in males and 40 percent in females. Our genes might contribute to our ability to drink alcohol without falling victim to a terrible hangover. However, these studies cannot identify which specific genes contribute to the risk of a hangover. The likelihood of identical twins getting hangovers were much more similar in comparison to fraternal twins. This determination revealed that genetics play a decisive role in the chances of having a hangover.

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6. Researchers have learned that our environment influences genes through twin studies.

Twin studies have played a critical role in understanding phenotypic heritability. Furthermore, these now offer an opportunity to study epigenetic variation. Epigenetics can be more specifically defined as cellular modifications that can be heritable and modified by the environment. Studies of twins have been critical to distinguish the contribution of genetic factors to numerous complex traits. Twin studies in epigenetics have the potential to address two questions. The two questions include the extent to which epigenetic changes are heritable and how much variation there is in epigenetic heritability across the genome. Identical twins are traditionally referred to as genetically identical. Therefore, any phenotypic differences within identical twin pairs are attributed to environmental factors.

Epigenetic modifications control which genes are essentially switched on and off in a particular cell or tissue. It can be altered by many environmental factors, including diet, exercise, and smoking. Researchers looked at the epigenome of 79 pairs of identical twins. One individual had rheumatoid arthritis and the other was healthy to study if epigenetics was disrupted in rheumatoid arthritis. Although identical twins have identical genomes, they have different epigenomes. The epigenomes’ changes were uncovered to be in genes involved in pathways within a cell that controls cells’ reaction to stress. Cellular stress could have been caused by many factors, including environmental exposures such as smoking. The changes in the DNA methylation can alter how the immune cells react to stress, further sensitizing them and causing an increased immune activity. This heightened sensitivity consequently leads to the progression of the disease.

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5. Twin studies have helped to identify the genetic and epigenetic risk of breast cancer.

Through the study of twin pairs, researchers have identified which gene increases the risk of breast cancer. Epigenetic marks help to send chemical signals to DNA to tell it to switch on or off. The epigenetic marks can vary from one twin to the next. One research team was able to identify the epigenetic change in the twin who will develop breast cancer even though the other will not. The research group studied DNA methylation levels, which is the best known epigenetic mark, in the blood of 36 pairs of twins diagnosed with breast cancer or healthy. Over half a million pieces of the genome were analyzed in each twin and compared to one another.

What was uncovered was that women who have developed breast tumors showed a pathological gain of methylation in the DOK7 gene. This epigenetic alteration is associated with an increased risk of breast cancer detected in the sick twin a few years before the clinical diagnosis. The next step in understanding the whole situation is knowing the exact function of the DOK7 gene. It is thought to be a regulator of tyrosine kinases, an antitumor drug target already used to treat breast cancer. If DOK7 performs this function, new studies to test drugs with tumor chemopreventive effects in breast cancer could be planned in the future.

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4. Twin studies are a helpful tool in behavioral genetics and understanding the complexity of eating disorders.

By studying twins, researchers can vary family environments and differ in genetic makeup. By comparing twins, scientists can better understand the roles of a shared environment, the impact of genetics, and possible unique environments that influence behavioral traits. Research findings from twin studies on eating disorders have found significant genetic components, including behaviors and attitudes associated with clinical eating pathology. One twin study investigated the ‘thin-ideal internalization’ to determine if genetic factors make individuals more susceptible to this outside pressure than others. Sets of twins were studied and evaluated. When one twin has a disease, the other twin’s chances of having it is called the concordance rate.

The results showed that identical pairs had a closer level of a thin idealization than fraternal twin pairs. The concordance rate for identical twins was 59 percent compared to only 11 percent for fraternal twins. The exciting findings allowed researchers to determine that genetics play a significant role in this area. It is suspected that several susceptibility genes can contribute to an individual’s risk of an eating disorder. The search for specific genetic factors is still underway.

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3. Intelligence can be challenging to study because it is measured and defined in different ways. According to twin studies, intelligence is in your genes.

Most of the definitions of intelligence include the ability to learn from experiences and adapt to changing environments. Elements of intelligence include the ability to plan, reason, solve problems, and think critically. Many studies rely on a measure of intelligence called the intelligence quotient, or IQ. Researchers have conducted many studies to look for genes that influence intelligence. Many look at the similarities and differences in IQ within families. These studies suggest that genetic factors contribute to about 50 percent of the difference in intelligence among individuals.

By studying both identical and fraternal twins, researchers found that certain brain regions were highly heritable. That included language areas and the frontal region, which plays a large role in cognition. These specific areas showed a 95 to 100 percent correlation between one twin and the other in identical twins. The frontal structure appears to be highly influenced by genes. Fraternal twins were near identical in the language areas but were less similar in other areas. The studies suggest that the environment, their own experiences, and what they learned in life play a negligible role in shaping their intelligence.

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2. Up to 75 percent of differences in children’s abilities in math, reading, and spelling could be due to genetic differences.

Twins and unrelated children were tested for reading comprehension and fluency. They then answered mathematics questions based on the national curriculum. The information collected as a result of these tests was combined with DNA data and showed a substantial overlap in the genetic variants that influence mathematics and reading. First, the similarities of thousands of twins were reviewed, and measured their tiny differences in DNA. Similar collections of subtle DNA differences are essential for math and reading.

However, it would be best if you also were mindful that our life experiences are crucial in making us better at one or the other. That is the complex combination of nature and nurture as we grow up to shape whom we become. Although the study does not point to specific genes, it does suggest that genetics influence complex traits such as learning abilities. Children differ genetically in how easy or difficult they find learning. Furthermore, it is crucial not only to recognize but respect these individual differences. Even though the twin studies found such strong genetic influences does not mean anything can be done to assist if a child finds learning difficult. It may just mean that more effort is needed from schools, parents, and teachers to help bring the child up to speed.

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1. Through recent twin studies, researchers have been able to conclude that sexual orientation is genetically influenced.

Some people claim that sexual orientation is a choice or that being gay is a conscious decision individuals make. To determine the role of genetics, if any, in sexual orientation, researchers compared identical twins to fraternal twins. The 2001 review concluded that almost all identical twins were increasingly more likely to share a sexual orientation. That means that they are both either gay or straight compared to fraternal twins, who are genetically less close. These findings suggest that genes do play a factor in a person’s sexual orientation.

Other studies have indicated that biological effects, including hormone exposure in the womb, contribute to shaping sexual orientation. These studies support that the differences in the central nervous system exist between heterosexual and homosexual individuals and that the differences are possibly related to early factors in brain development. There are also findings of physiological differences such as different inner ear shapes between homosexual and heterosexual women that contribute to the idea that biological effects play a role. These studies have been able to debunk the myth that sexual orientation is merely a choice. Instead, it relies heavily on genetic factors.

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