Twins​—How Close Are They?

In November 1985 Mary suffered a painful attack that prompted her to see a doctor. After several visits and a series of tests, the doctor determined it was her gallbladder.

Approximately three weeks later, Mary’s sister Martha, who then lived some 1,500 miles [2,400 km] away, became seriously ill. She, like her sister, suffered a series of painful attacks. The symptoms were identical. The problem​—her gallbladder.

Jeanette and her sister, Jeanne, always dressed exactly alike. A mutual friend had an idea on how they could double the size of their wardrobe. Since they are the same size, they could each buy their own distinctive outfits and then wear each other’s clothes.

Convinced it was a good idea, they selected a major department store and went their separate ways to shop, agreeing to meet back at a specified time to compare the items they had selected. When they rendezvoused several hours later, to their surprise, both had chosen the identical outfit!

BIZARRE, you say? True, most people will agree that such experiences are certainly rare between family members. However, the individuals named above are not ordinary siblings. They are twins. Research shows that experiences like these occur more frequently among twins, particularly when they are identical. ‘But why?’ you may ask. Why do twins often share traits and characteristics that are not shared by two children born separately in a family? Just how close are they? Let’s find out.

The Making of Twins

It is estimated that there are 50 million pairs of twins around the world. Since 1960, the percentage of multiple births has climbed to its highest level. In the United States alone, it is estimated that one birth in every 50 is a multiple birth.

Twins occur when two fertilized ova, or eggs, instead of one are produced in a woman’s womb. When babies develop from two eggs and two sperm, they are called fraternal twins. They may be no more alike than nontwin babies.

However, when twins originate from a single fertilized egg that has divided shortly after conception, they are called identical. These twins always belong to the same sex and share the same genetic makeup. Identicals may occur one fourth to one third of the time in twin births. Worldwide, identical twins occur approximately once for every 250 to 350 births.

Determining at birth whether twins are fraternal or identical is not as simple as once thought. For years, doctors concluded that a single placenta (afterbirth) for a pair of same-sex twins proved they were identical, while two placentas were proof of their being fraternal. Doctors now realize that it is possible for the placentas of fraternal twins to become fused together as one, while each embryo of an identical twin can have its own placenta, as well as its own water sac and umbilical cord.

Not surprisingly, many twins in years past were misdiagnosed by midwives and doctors. Some were told they were fraternal when they were really identical or that they were identical though actually fraternal.

‘But you can just look at a pair of twins and tell that they are identical​—isn’t that proof enough?’ Not really. While it is true that most identical twins will be look-alikes, this does not prove that they are identical. The term “identical twins” really means that the hereditary factors of the twins are identical, not their appearance. For example, Wade and Wayne are fraternal twins who are so similar in appearance they are often mistaken for identicals. What accounts for this?

In his book Twins and Supertwins, Amram Scheinfeld explains: “Some fraternal twins may have a high degree of resemblance if they have an unusual proportion of matching hereditary factors​—that is, while fraternals . . . on the average have about 50 per cent of their genes in common, some may have many fewer, and look very unalike, and some may have many more, and look sufficiently alike to be mistaken for identical twins.”

Testing for Identicals

How, then, does one know if twins are truly identical? A number of hereditary traits are always alike in identical twins. For example, Scheinfeld notes that “since each special type of blood substance is inherited, all of the blood substances must be exactly the same in identical twins.” When any of these substances are different, “the twins must be fraternal.”

However, in a small percentage of cases, blood tests may not be sufficient to establish twin types. So doctors may test for other substances that, because of heredity, are alike in identicals. Body chemicals like those found in sweat and in saliva secretions are always alike in identical twins. This explains why police dogs are easily confused by the similar body scents of identicals. Usually, the dogs must be given special training to distinguish the two.

Eye color and hair provide additional sources for comparison. In identical twins, heredity also produces fingerprints that are almost exactly alike. These too can be helpful in tests because they are considerably more alike than those of fraternal twins.

Probably the most precise method for identifying twin types, though, involves the skin graft. This test is successful only with identicals. Explaining why, Scheinfeld says: “Since identical twins are completely alike in the hereditary makeup of all the tissues of their bodies and in all hereditary blood and chemical substances, it is possible to take skin or flesh from one and graft it onto or into the other twin, with complete assurance that the graft will ‘take’​—just as if it were from one part to another part of the same body.”

How Close Are They?

But how does all of this explain the strange experiences shared by twins like Jeanette and Jeanne or Martha and Mary? For one thing, we have noted how close identical twins are genetically. To some extent, this genetic bond seems to account for many of the similarities shared by twins, such as in taste and dress.

To illustrate, Dr. Magdalena Krondl, associate professor in the Department of Nutritional Sciences at the University of Toronto, studied the eating habits of both identical and fraternal twins. To determine whether there is a genetic basis for a person’s choice of diet, she selected twins who had lived together as children but who were later separated so that “their own food preferences would emerge.” Her research revealed that “the diets of the identical twins were significantly more alike than the diets of the nonidentical twins.”

This helps us to understand why the doctors told Martha after her sister Mary’s gallbladder attack that she should also expect to have gallbladder problems. She explains: “The doctors said that if my twin sister, Mary, had already experienced problems with her gallbladder, heredity and the similarity of our eating habits made me a likely candidate.”

That Jeanette and Jeanne found their taste in clothing so similar is likewise not surprising. Other identical twins have shared similar experiences. For example, Bruce received several items from his twin brother, Brian, living some 2,500 miles [4,000 km] away. The items had been mailed inside a shoe box. When Bruce opened the wrapping, he noticed that the shoe box resembled the one in his closet, prompting him to make a comparison of the two. Thinking it strange that the boxes were identical, he called his brother to ask about it. As he suspected, they had both purchased shoes that were identical in color, size, and style!

Similarities in intelligence are also common among identicals. A study conducted at the University of Minnesota of more than 350 pairs of twins, many of whom had been separated since birth, showed that genetics seem to play a definite role in both intelligence and personality development. Diane told Seventeen that when she and her twin sister, Karen, graduated from high school, they “had the exact same grade point average, had received the same grades on tests, and, despite sitting on opposite sides of the room, had even missed the same test questions.”

Effects of Environment

Many twin studies only serve to fuel the continuing debate among researchers over what exercises greater influence in the lives of twins​—heredity or environment. Yet, researchers admit that both play some role.

In her book Identical Twins Reared Apart: A Reanalysis, Susan Farber, assistant professor of clinical psychology at New York University, describes the case of Harry and Alfred. These identical twins were so unalike in appearance that blood tests were necessary to prove they were truly identicals. She notes that “Harry was three-and-one-quarter inches [8.3 cm] taller and 62 pounds [28 kg] heavier than Alfred. Alfred, the twin from the poorer environment, suffered from anxiety, fainting spells, and a psychogenic symptom of pain in his heel so severe that it required medical and psychiatric treatment. Harry had no such symptoms.”

After carefully reevaluating combined data from 121 published case studies, Farber concluded that, although the studies revealed “remarkable​—sometimes unnerving—​similarities” among identical twins, often such studies did “not include many of the identical twins who turned out to be most unlike each other.” The reason? As she explains, the identicals usually preferred by researchers “were originally chosen for study just because they were so similar.”

Raising Twins

Whether twins are remarkably alike or totally dissimilar, raising them can pose a unique challenge to parents. Various authorities feel that an important feature of each child’s development is the fostering of his ability to make decisions independently of the other.

The book The Care of Twin Children, published by the Center for the Study of Multiple Gestation, points out that “there are many ways to foster individuality without destroying the special bond of twinship.” The book notes that many parents of twins choose names for their children that are “unlike-sounding” and call “both children by their names more frequently than normal to reinforce their individuality in their own minds.”

Arranging for special time alone on occasion with each twin is recommended, as is taking pictures of each child “separately as well as together.” Rather than parents’ treating twins as a “unit,” the Center reasons that it is better that parents help the children recognize their own individuality and separateness. Each child should be encouraged to pursue his or her own special interests. This will help to create circumstances wherein the twins will be “called on to make independent decisions affecting them personally.”

The Center discourages comparing the performances of each child because “one twin may begin to measure himself against the other, feeling he lacks some quality the other supposedly possesses.” Such unfavorable comparisons could easily incite feelings of jealousy and result in conflicts between the two.

A Unique World

It is not surprising that researchers are fascinated by identical twins. Psychology Today describes them as being “among the most sought-after subjects for psychological and medical research.” Explaining why, David T. Lykken, former president of the Society for Psychophysiological Research, said: “Almost any experiment that one might think of doing with human subjects will be more interesting and yield more valuable results if one does it with twins.”

Yes, the presence of so many similarities within the genetic makeup of identical twins places them in a truly unique world all their own. For many, it is a world filled with much happiness and pleasure. As one twin wrote: “There are two laughs for one joke, two thrills for the same joy. . . . It is a lot of fun to be a twin. . . . It is happiness just to live when one is born a twin.”

[Box/​Pictures on page 24]

Twin Conception

Identical Twins

A single sperm unites with one egg

Egg divides, producing genetically identical twins

Fraternal Twins

Two sperm unite with two eggs

Each becomes a genetically different twin

[Box on page 27]

Why Siamese Twins?

The term “Siamese twins” became popular when the world discovered 19th-century twins Chang and Eng. Born in Siam (now Thailand) in 1811, these twins were joined at the chest by a band of tissue five and a half inches [14 cm] long and seven and a half inches [19 cm] around. They became internationally famous as the “Siamese Twins,” traveling with P. T. Barnum’s circus. Eventually, the twins left the circus, married two sisters from North Carolina, U.S.A., and fathered 22 children between them. They are survived by more than a thousand descendants.

Conjoined, or Siamese, twins occur when a single fertilized egg that begins to divide in the production of identical twins fails to separate completely. Such twins may be physically linked together at any part of the body and will occasionally share one or more vital organs. It is estimated that births of Siamese twins occur approximately once out of every 100,000 births worldwide.

[Picture on page 25]

Identical twins, produced from the same sperm and egg, are always of the same sex. On the other hand, fraternal twins, produced from two sperm and two eggs, may be of different sexes, such as the twins seen here

[Picture on page 26]

One twin is often a mirror image of the other