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Douglas BaileyThoroughly researched, annotated, and footnoted, Entwined Lives is impressive in the breadth of its study...
— Boston Globe
Identical and Fraternal
Twins: Living Laboratories
I feel a rush of excitement waiting in my laboratory for nine-year-old twins, Rocky and Tony Novak, and their mother, Linda, to arrive for their research visit. For the next two hours the four of us will be players in a great discovery game, a search for how and why people grow and develop as they do. I wonder how alike Rocky and Tony will look to one another, how similarly they will respond to problems and questions, and how much they enjoy being twins. It will be fascinating to learn how their answers and test scores, and those of other twins, illuminate areas of human development that remain in the dark.
Occasionally, I am asked to recall a "favorite" pair, or a pair that was especially memorable in some way. I always answer that it is the pair I last studied in my laboratory whose particular personalities and preferences still linger in my mind. Each individual twin pair is a fresh and fascinating take on what it means to be human. It is, however, the collective story, repeated countless times and in different ways by each set, that tells us who we are and how we got there. It was extraordinary to discover that the "Jim twins," the first pair of reunited identical twins studied at the University of Minnesota, had sons with the same name, enjoyed woodworking and bit their fingernails to the nub. It was also striking to find other examples of highly matched idiosyncratic behaviors in many other reared apart identical pairs, and to find them so infrequently in reared apart fraternal pairs. Theparallel paths of identical twins, and the diverging paths of fraternal twins, strongly suggest that human development proceeds according to a plan that is largely guided by our genes. Twins are truly living laboratories.
Twins offer a natural experiment for studying how behavioral and physical traits are shaped by nature and by nurture. In fact, when twins are not included in human developmental research, the findings may actually be suspect. The classic twin method was first described in 1875 by Sir Francis Galton, a gentleman-scholar from London. According to Galton, "It is, that their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture." Greater resemblance between identical than fraternal twins, in height or IQ, shows that genetic factors affect the development of that trait. This conclusion is reasonable because the members of identical and fraternal twin pairs are the same age and were raised together in the same home, so the only critical difference is their biological relatedness. Nature has provided science with a convenient assembly of human subjects that allows control over factors that may affect the way we mature and develop.
If the entire twin chronicle were so tidy we could stop here, but twin research has endured a mixed reception in behavioral science research. Throughout history, the twin method has been treated as both honored guest and social outcast. Supporters have reveled in the rich body of knowledge that only twin studies provide concerning the development of intelligence, special abilities, personality, psychopathology, diseases and virtually any human trait that can be measured and studied. However, detractors have pointed to "flaws" in the research design, and misuse of findings. For example, some people have argued that identical twins are so alike because they are treated more alike than fraternal twins, violating the equal environments assumption of the twin design, and that their more similar genetic makeup has little to do with it. Other people have worried that gene-behavior relationships will be used to discriminate between different groups of people or to deny the usefulness of programs designed to improve child care or educational opportunities. I will return to these important issues in the last chapter. Actually, it is only in the last two decades that the twin method has achieved its stunning comeback. This is partly due to greater appreciation for what twin studies can accomplish and to improved methods for interpreting results.
What produces twins, who has twins and how twins differ from singletons, or nontwins, are among the vital questions that have fueled debates over the role of twins in scientific investigation. The biological events responsible for twinning are a good place to begin to sort through these issues. Identical (monozygotic or one-egg) twins result when a single fertilized egg, or zygote, divides between the first and fourteenth day after conception. Further splitting can lead to identical triplets, quadruplets, quintuplets or sometimes more. The identical Dionne quintuplets, born in Canada in 1934, resulted from a single fertilized egg that split four times. Based on the quints' relative likenesses in palm prints, hair whorl, handedness, facial form and ear shape, the University of Toronto research team constructed a "continuous chain of similarities," suggesting the position of each sister in the succession of divisions: Annette, Yvonne, Cécile, Marie and Emily.
What causes the fertilized egg to divide remains a mystery. One early theory linked identical twinning to delayed implantation of the fertilized egg in the uterus. More recently, Dr. Judith G. Hall from the University of British Columbia, Canada, suggested that the early developing mass "senses" an error, attempts to "rid itself of" the abnormality, and may produce identical twins in a minority of cases. Other possible explanations include time of fertilization and different inactivation patterns in female twins' X chromosomes. Some of these theories continue to be explored.
There is little evidence that the tendency to produce identical twins is tied to the genes. Identical twinning seems to occur randomly so such twins might show up in virtually any family at any time. The fairly constant rate at which identical twins occur worldwide (approximately 1/250 births) is also evidence against a genetic influence on this type of twinning. Nevertheless, the birth of more than one identical pair in some families has not gone unnoticed, and a closer look at the distinguishing features of these unusual families is worth doing. Identical twinning and left-handedness appear to occur together among the relatives of some identical twins, although increases in birth abnormalities have not been observed in twins from such families. A few investigators have proposed that a single gene whose expression varies may be responsible for identical twinning in some families. These observations, while intriguing, are tentative at best.
Fraternal (dizygotic or two-egg) twins result when two different eggs are fertilized by two different sperm cells so that, genetically speaking, their relationship is the same as ordinary brothers and sisters. These twins come in two varieties: same-sex and opposite-sex. Fertilization of three, four or even five eggs at the same time is possible, although these "higher order multiples" occur less frequently than twins. Many combinations of identical and fraternal "supertwin" partners are possible.
A great deal more is understood about what causes fraternal than identical twinning, and some of the more surprising factors might well affect family planning decisions once they are more widely known. Fraternal twinning happens most often to women who bear children between the ages of 35 to 39 years. The catalogue of other characteristics linked to fraternal twinning includes:
· Maternal height and weight. Mothers of fraternal twins are taller and heavier, on average, than mothers of identical twins and mothers of singletons. Height and weight, per se, may not influence fraternal twinning, but may reflect hormonal or other factors.
· Oral contraceptives. Evidence of links between oral contraceptive use and fraternal twinning is mixed. One study found that the chance of having fraternal twins increased if conception occurred within two months after contraceptive medication was withdrawn. Other studies showed that oral contraceptives decreased fraternal twinning or had no effect.
· Social class membership. There is some evidence that fraternal twins occur more often in lower social classes, partly because family sizes are larger. Mothers with many children may be likely to express twinning tendencies because repeated pregnancies may accentuate these tendencies. This trend is not constant across studies, partly due to variable classification of families and exclusion of illegitimate births.
· Birth order. Fraternal twins are likely to be among the last-born children in the family. This is because the chance of fraternal twinning increases with parity (number of previous children). One study found that because older mothers tend to have more children than younger mothers, the parity effects vanished when age was controlled.
· Genetic transmission. A genetic predisposition for fraternal twinning occurs in families, but it is difficult to precisely predict who will bear twins. It is also unclear if the tendency to produce twins is passed only through the mother's family, or if the father's family may be involved. The genetic factor may involve hyperstimulation by follicle-stimulating hormone (FSH), suggested by elevated FSH levels in premenopausal mothers of fraternal twins.
· Nutritional and food supply. The dietary habits of mothers of fraternal twins do not differ from those of other mothers. There is, however, speculation and suggestive evidence that the Nigerian Yoruba tribe's high frequency of fraternal twinning (about 1/22 births) is explained by yams, a food with estrogenlike substances. (Some studies have challenged this particular theory.) Higher and more constant food sources may explain the increased fraternal twinning rate in southwest Finland's archipelago of Åland and Åboland, relative to the mainland.
· Intercourse following periods of sexual abstinence. There is some evidence that fraternal twins are likely to be conceived following periods of sexual abstinence. This effect has been observed when soldiers return home to their wives following wartime service. It has also been observed in orthodox Jewish families whose religious practices prescribe intercourse after two weeks' abstinence during menses and the next seven days.
· Frequency of sexual intercourse. A link between more frequent sexual intercourse and fraternal twins has been observed, based partly on a high incidence of fraternal twins conceived illegitimately and before marriage. It is possible that, in some women, intercourse stimulates ovulation, and that frequent ovulation leads to the release of more than one egg. Increased fraternal twinning has also been associated with an increased number of marriages which may offer more opportunities for intercourse and ovulation.
· Seasonality. A British study reported a higher number of fraternal twins and triplets born in December than in June. However, evidence of seasonal effects on twinning is inconclusive at best.
· Emotional stress. Stressful events may lower sperm count and sperm motility in males, cause menstruation to stop in females, and curb sexual desire in both sexes. These factors may have reduced fraternal twinning between the 1930s and 1970s in Western nations.
· Environmental pesticides. There is speculation that fraternal twinning rates are partly linked to the presence of chemical agents in the environment. They would have the effect of reducing sperm count, thus decreasing the frequency of fraternal twinning.
Whenever I review these points with parents of twins the response is a blend of recognition, amusement and bewilderment. It is the bewildered parents, those whose family characteristics do not match those listed above, who need explanation. The key concept here is that, while certain background factors may be important, fraternal twinning is influenced by a complex mix of events. Therefore, it is hardly expected that all parents of twins will conform to a certain type. An especially interesting question was raised by one mother who puzzled over how her triplets could include an identical female pair and a fraternal male cotriplet, since these two events are apparently unrelated. One answer is that because identical twinning occurs randomly, it may be simply a matter of chance when it occurs together with fraternal twinning. However, Charles Boklage, a biologist at the University of East Carolina and the father of twin girls, believes that the biological origins of the two types of twins may have a lot in common, a finding that twin researchers view with interest. This view, while not proven, is partly based on the high rates of lefthandedness among parents of both identical and fraternal twins.
Misconceptions concerning the causes of fraternal twinning abound. Fraternal twinning does not "skip" generations, although this might occur in some families. Unfortunately, this area of research is plagued with difficulties because of questionable reports concerning twin births and twin types. Many couples I know pay attention to twins in their family trees only after the birth of their own twins, and twin type in these cases is often just a guess. Furthermore, family-planning decisions, such as restricting the number of children to two, or postponing the child-bearing years, may conceal natural reproductive patterns. A practice worth examining is how often people limit their families after delivering twins, especially if the twins' birth rank exceeds three or four. Some mothers whose first pregnancy produced twins yearn for a singleton child, believing they missed the intimacy that evolves from caring for a single infant. Several years ago, I received a surprised and somewhat distressed telephone call from a mother of fraternal twins who, on the advice of her physician, became pregnant in hope of having a nontwin child—but age, parity and genetics combined to produce her second twin pair.
Twins occur in only one out of eighty births in Western nations, yet the recent frenzy of scientific and media attention has persuaded us that they are everywhere. Among Caucasians, identical twins represent only about one-third of all twins, or 1/250 births. Fraternal twins represent the other two-thirds of the twin population, or 1/125 births. (These conventional estimates are being revised due to the new reproductive technologies, as I discuss in Chapter 10.) The identical twinning rate is fairly constant across populations and ethnic groups, while the fraternal twinning rate is as low as 1/330 among Asian populations, and as high as 1/63 among African populations. Twins are celebrated among Western Nigeria's Yoruba tribe, in which one in eleven people are part of a pair. One study found that fraternal twins comprised over 95% of Yoruban twins. Among interracial couples, the chances of fraternal twinning are usually decided by the woman's background so that a black mother and a white father would be more like to produce fraternal twins than a white mother and a black father.
In 1996, the number of live births in the United States was 3,914,953; the number of infants born as twins was 100,750; the number of infants born as triplets was 5,298; the number of infants born as quadruplets was 560; and the number of infants born as quintuplets or more was 81. I have estimated that there are 73,456,348 twin pairs, or 146,912,695 individual twins in the world.
Why aren't twins more frequent than they are? An evolutionary perspective suggests that the birth of same-age infants unduly burdens families in terms of parental care, especially when there are other children in the family. Moreover, because twins are often born prematurely and show early health problems, they may press parents' emotional and financial resources to difficult limits. Multiple pregnancies may also pose serious health hazards for mothers. Nature sometimes offers "solutions" to such problems, for example, the spontaneous abortion of one twin, although researchers disagree over whether twins are eliminated more often than nontwins. The "Vanishing Twin Syndrome" occurs when one twin fetus is reabsorbed by the mother during the first trimester, resulting in a singleton pregnancy. This event may be another natural remedy when a multiple pregnancy becomes life-threatening. The rate of twin "disappearance" ranged from zero to 78% at medical centers in which women pregnant with twins received a series of ultrasounds. This finding suggests that the human twinning rate is actually much higher than one in eighty births. Many more of us start out as twins and parents of twins than we know.
Dramatic cases involving infanticide of twins, usually the weaker twin or the female in opposite-sex pairs, have occurred in some societies when removal of the child improves chances for the survival of parents or other siblings. In these societies twins are not at greater risk of infanticide than singletons because the motivations are the same in both cases. The disturbing findings that child abuse is elevated in families with twins urges serious rethinking of support services available to new parents. A conversation with a mother of several young children and infant twins is burned indelibly into my brain. One of the twins was handicapped, and given the tensions of child care, she admitted to being "this close" to becoming abusive. We will return to this topic in the final chapter.
Most people assume that identical twins occur more frequently than fraternal twins, but this is true only in Asian populations where the fraternal twinning rate is low. If identical twins appear to be more plentiful than fraternal twins, it is because their physical likeness makes them easy to spot. In contrast, many fraternal twins blend into the background with their twinship virtually hidden from view. I was amused to learn that when seven-year-old fraternal female twins, Abigail and Rebecca Moore, attended a Girl Scouts meeting they were "introduced" to one another by a member who assumed they were unrelated. Their difference in appearance is one of the most dramatic I have ever seen because Abigail was a tall, sturdy blonde and Rebecca was a petite, "pixie" redhead. Seeing them together with their older brother, Adam, who resembled Rebecca, gave me the impression that the "wrong" people had been born together. In contrast, some fraternal twins look very much alike. I recall the intensity with which I stared at one young pair, Katie and Becky Mitzuk, searching for a distinguishing feature, since I knew that a blood test confirmed their fraternal twinship.
The fact that some fraternal twin pairs look quite different, while others look quite similar, highlights a very interesting, often overlooked, feature of fraternal twins, namely that they vary on a "spectrum" of genetic relatedness. Fraternal twins share half their genes on average, but members of some pairs share genes for a greater number of traits than others. It is simply a matter of chance if fraternal twins inherit the same genes from each parent: Each person carries two forms of each gene and passes one of them to each child with a 1/2 probability. Fraternal twins, therefore, have a 50% chance of resembling or not resembling one another. This same process affects every one of our genes, and also explains why some siblings resemble one another in many traits or in only a few.
Assortative mating, the tendency for "like to marry like" in traits such as height, intelligence and personality, can affect fraternal twin similarity. If traits are influenced by genetic factors, then fraternal twin similarity in those traits may increase beyond the expected 50%. This is because when parents have the same genes for certain traits, they pass on the same genes to each child. Assortative mating would not affect identical twin resemblance, because identical twins share 100% of their genes, a situation unaffected by their parents' mating patterns.
Behavioral geneticists distinguish between discrete and continuous traits. Discrete traits such as Huntington's chorea (progressive brain disease beginning in middle age) and Tay-Sach's disease (metabolic disorder causing blindness, mental retardation and death in infancy) are associated with single genes. They are considered to be "either-or" traits because people have them or they do not have them. The transmission of these traits in families follows the laws worked out by a Moravian monk, Gregor Mendel, in 1866. Huntington's chorea is a dominant trait, so only one copy of the gene (either from the mother or from the father) appears in affected individuals. Tay-Sach's disease is a recessive trait, so two copies of the gene (one from the mother and one from the father) are required for its expression. Continuous traits such as verbal skill, extraversion and depression are associated with many genes whose combined effects affect the appearance of the trait. The separate genes are, however, inherited, according to Mendel's laws.
Identical twins are not identical in every way and, in fact, show differences in virtually every trait that has ever been studied. The myriad of differences between identical twins is explained only by differences in their environments or by unusual events occurring soon after fertilization, a topic explored more closely in Chapter 2. The variations among both types of twins sometimes challenge the ability of scientists, family members and even twins themselves to know for sure if they are identical or fraternal.
One of my favorite cartoons shows a couple preparing to go out for the evening to visit friends who are parents to a pair of twins. "Don't ask them what kind of twins they have," the woman warns her partner, "or we will be debating that all night!" It is a great surprise to many people to encounter ambivalence among some parents and twins, even physicians, in response to queries about twin type. It is unfortunate that scientific procedures for twintyping are not routinely applied following twin deliveries because mistakes lead to inaccurate accounts of how the gene-environment balance influences behavior. It is also disturbing that descriptions of methods for determining twin type are occasionally missing from scientific reports.
Knowledge of twin type is vital for individuals dedicated to the care of twins. Developmental discrepancies between twins might warrant different interpretations and treatments depending on twin type. If the musical talents of one fraternal twin foreshadowed a Beethoven or a Bach, while the twin brother or sister turned in a less polished performance, there may be small cause for alarm. This difference, while dramatic, might reflect a difference in genetically influenced ability which, in turn, may affect the amount of effort invested in musical activity. Most fraternal twin differences are natural and expected, and do not reflect "unfair" parenting. In contrast, marked differences in appearance or ability between identical twins might be the residue of a prenatal nutritional deficit, or a childhood illness, and would warrant serious attention. Knowledge of twin type can also inform emergency medical decisions involving blood transfusion or organ transplantation. It can additionally guide, but not dictate, decisions concerning classroom placement, recreational pursuits and other activities because the nature and extent of social closeness varies with twin type. Medical insurance does not cover the costs of blood-typing or DNA analysis for classifying twins, but Patricia Malmstrom, Director of Twin Services, in Berkeley, California, is attempting to reverse this policy.
It is curious that some parents prefer to remain in the dark over their children's twin type. This reluctance may come from worry that such information may affect how they treat their children. The stunned reaction of Rachalle Spears to learning that her twin daughters, Natalie and Noel, were really identical and not fraternal underlines this concern. She lamented that everyone, from teachers to camp counselors, had been told that the twins were fraternal, so she feared adjustment of their attitudes and practices toward her children. This concern is understandable, but is probably not justifiable. There is considerable research evidence that behavior toward twins is largely influenced by the twins themselves, and is generally unrelated to opinions of twin type. Even when parents of identical twins believe that their twins are fraternal, their personality ratings, warmth and criticism of their twins do not differ from those of parents who have correctly identified their children's twin types.
Apologies to parents of twins, but mothers and fathers are among the least accurate judges of twin type. Members of one of the first participating pairs in my doctoral research were, according to their mother, very different in appearance and clearly fraternal. I was to study seven-year-old Jodie and Katie on consecutive days. On the second appointed day I waited with anticipation for the arrival of twin number two, wondering how she might differ from her obedient and somewhat shy sister. Anticipation turned to bewilderment when I caught sight of the child—why was the mother bringing the first twin back? It turned out that this second twin was identical to the first, but not in her mother's mind.
That parents are often incorrect in this important part of their twins' development flies in the face of common sense since they had the children, raised them from birth, and wrestled with so many decisions affecting their health and well-being. I suspect that parental misperceptions reflect acute sensitivity to subtle behavioral and physical differences between identical twins, perhaps a slightly broader smile or more animated facial expression, which escape the initial notice of an investigator or stranger. I have also learned that older siblings of identical twins possess an uncanny ability to tell them apart.
I confirmed this lesson in a 1984 study, using forty-seven identical and six fraternal twin pairs whose twin type had already been scientifically established (see below). Based upon my first impression of physical resemblance, I correctly classified 94% of the twins, in contrast to parents who were accurate only 74% of the time. Even more striking, forty-two physicians (whose judgments were provided by mothers) ended up trailing the field with a shocking 67% accuracy. The observations of two blood-typing experts, Ronald Race and Ruth Sanger, anticipated my own experience: "For many years, Mr. James Shields of the Genetics Unit at the Maudsley Hospital [in England] has been sending us samples of blood from the twins. We find that the blood groups practically never contradict the opinion of such a skilled observer of twins."
The most common error made by parents in this study was the misclassification of identical twins as fraternal, rather than the other way around. Given the probable sensitivity of parents to small differences between twins, look-alike fraternal twins are unlikely to be confused at home, although they might be confused by teachers, friends and especially distant relatives. Some look-alike fraternal cotwins may have difficulties fulfilling the similar expectations held by others, because there is no necessary connection between physical and behavioral resemblance.
Whenever I address gatherings of mothers of twins, I am besieged with requests to examine photographs of children and to render an opinion: identical or fraternal. I am still astonished, but no longer surprised, by the general lack of agreement between the mothers and me. Fortunately, a number of very reliable procedures are available for classifying same-sex twins as identical or fraternal. Opposite-sex twins are, of course, diagnosed as fraternal due to the sex difference.
· Examination of blood group systems. One of the most accurate methods for determining twin type is comparing cotwins' similarity in red blood cell factors. In addition to the familiar A, B, O and RH positive or negative (Rhesus) blood groups, twins may be tested for factors such as Kell, Duffy and Kidd. Cotwin comparison of eight blood group systems discriminates identical and fraternal twins with less than 2% error. Fraternal twins might match on a single blood group, so multiple comparisons are necessary to avoid their misclassification as identical.
When differences in any of the blood groups are detected the twins are diagnosed as fraternal with complete confidence. Fraternal cotwins do not have the same genetic makeup, so blood group differences always indicate nonidentical twinning. In contrast, identical cotwins have the same genes, so identical twinning is virtually certain if all their blood groups match. Identical twinning is not completely certain because, on rare occasions, fraternal twins can inherit exactly the same blood groups from their parents. Fortunately, I had seen such a pair during the course of my studies and, based upon their differences in eye color and height, which are strongly influenced by genetic factors, I reassigned them as fraternal twins. Many studies do not enable close contact between researchers and twins, but using large numbers of twins probably dilutes the effects of misclassifying a few pairs.
How accurate are the results from blood-typing? David T. Lykken, a twin researcher at the University of Minnesota, showed that when twins are compared across eighteen blood group factors, in addition to various physical measures, such as similarity in the ponderal index (height/cube root of body weight) and fingerprint ridge count, the probability of misdiagnosis is less than .001.
· Physical resemblance questionnaires. Physical resemblance questionnaires can be used for twin classification if blood-typing is not possible. These forms request information about twins' similarity in genetically influenced characteristics, such as hair color, eye color, height and weight. Other items may concern how often twins are confused by parents, teachers and friends. This method shows approximately 95% agreement with results from blood-typing analyses.
· Fingerprint analysis. Fingerprint analysis refers to the number of lines, or ridges, that are formed on the tips of the fingers during the second trimester of pregnancy. The total number of ridges across all ten fingers is genetically influenced, as revealed by the much greater resemblance between identical than fraternal twins. Identical twin resemblance is rarely perfect because intrauterine disturbances, such as viruses, may affect the formation and similarity of the ridges.
The palm and fingerprint characteristics of identical twins who differ in mental disorder are especially interesting because they may flag the effects of prenatal disturbances. Dr. H. Stefan Bracha reasoned that since fingerprint patterns and some phases of brain development occur during the same prenatal periods, identical twin differences in prenatal insult (e.g., infections or unequal blood supply, shown by cotwin differences in health or body size) may lead to differences in schizophrenia (mental disorder characterized by disordered thinking and disordered emotion) and in fingerprint characteristics in predisposed pairs, a relationship he was able to demonstrate.
Some identical twins are partners in crime, but an innocent cotwin need not fear punishment for his sibling's misdeed. He or she can count on his fingerprints.
· Molecular genetic techniques. Techniques that can reveal the identity of individuals based upon unique characteristics in genetic material are extremely accurate. A procedure called DNA Fingerprinting, developed in the early 1980s by Dr. Alec Jeffreys at the University of Leicester in Great Britain, has been used to establish twin type, as well as paternity and identity. DNA, or deoxyribonucleic acid, is present in all cells of the human body. Genes, the hereditary units that are transmitted from parents to children, are actually segments of DNA. Scientists can obtain DNA from blood samples and examine the segments to determine unique patterns for each individual.
Examining DNA profiles is ideally suited to classifying twins as identical or fraternal because it would be extremely rare for two people to show exactly the same patterns. Identical patterns indicate identical twins with near certainty, but it is theoretically possible for a fraternal pair to also show identical patterns.
Buccal cells, obtained by scraping the inner cheek with special brushes and swabs, can also be used for DNA testing. It is a much less expensive procedure than analysis of blood-derived DNA. Cell samples can be obtained at home and mailed to laboratories for examination in specially prepared kits.
· Examination of the placenta and fetal membranes. The placenta and fetal membranes provide less information on twin type than many people realize. Once the sperm has penetrated the egg, the new cell develops very rapidly. This is followed by implantation in the uterus between days six and eight, the formation of two protective fetal membranes called the amnion and chorion, and the development of the placenta. The placenta connects the mother and fetus via the umbilical cord, enabling exchange of nutrients and waste products between them. The placenta is also one of the most misunderstood structures from the point of view of classifying twins as identical or fraternal.
All fraternal twin pairs have two placentae, two amnions and two chorions, but so do one-third of identical twins. Consequently, knowing that twins have "two of everything" is not informative with respect to twin type. Approximately two-thirds of identical twins share a placenta and chorion, but have separate amnions. Some identical twins share an amnion, chorion and placenta, but this is found in only .1-4% of the pairs. When division of the fertilized egg is incomplete, the result is the rare occurrence of conjoined twins (identical twins who are connected physically), most of whom are female.
Additional complications are posed by the chance that two placentae may fuse, opening a window for misdiagnosing fraternal twins as identical. Martin G. Bulmer, author of The Biology of Twinning in Man (1970), found that 42% of fraternal twin pairs have fused placentae, hardly a trivial number. Fused placentae have also been observed among approximately 43% of two-chorion identical twins, but this situation would not lead to misdiagnosis.
I love talking to gatherings of parents of twins. These mothers and fathers have rare access to those living laboratories that are so precious to psychological and medical science. Parents know well what many professionals fail to see, namely that raising twins differs from raising singletons, and that genes explain a great deal about how people develop and why they differ. My visits to twins clubs are, by now, very predictable events: Following a thirty-minute lecture on some aspect of twinning, I field questions and comments for close to twice that time. It is at forums like these that I am privy to new bits of information and new threads of thought that sharpen my thinking about where research should be headed. One member casually noted that her identical twin daughters delighted in using their own private words and gestures to communicate, behavior sometimes called "twin language." Her inability to understand her twins was easily solved when an older child in the family proved an able interpreter. This was exciting news because a clever investigator could include older siblings in research programs to advance understanding of the nature of twins' private speech. This is an important problem to resolve, because twins, on average, perform below nontwins in general intelligence tests, especially in verbal abilities.
Fortunately for twin researchers, there is an extensive network of local, state, national and international organizations for families with multiples. The members are highly dedicated to promoting work that will advance knowledge about human behavior and parenting. There are many informative ways to use twins in research. If some of the methods sound familiar, it is probably because informal versions of them are applied in homes, schools and workplaces everywhere.
|Preface: Not Like Me|
|1||Identical and Fraternal Twins: Living Laboratories||1|
|2||Identical, but Not the Same: Differences Between Identical Twins||21|
|3||Natural Experiments: Unusual Twin Types||36|
|4||Developing in Tandem: Intelligence and Special Mental Skills||49|
|5||Unfolding Lives: Personality Traits, Mental Disorders and Atypical Behaviors||70|
|6||Friendship Extraordinaire: Twins' Special Relationship||97|
|7||Twist of Fate: Twins Reared Apart||116|
|8||Another Twist of Fate: Children Adopted Together||152|
|9||Lonesome Crowd: Loss of a Twin||169|
|10||Making Multiples: New Fertility Treatments and Beyond||187|
|11||Two-Base Hits and Triple Toe Loops: Physical Growth and Athletic Prowess||208|
|12||Noah's Ark: Twins in the Nonhuman Animal Kingdom||230|
|13||The Other Half: Noteworthy Twins||247|
|14||Double Indemnity: Twins in the Courtroom||278|
|15||Separate Minds in Shared Bodies: Conjoined Twins||295|
|16||Double Entendre: Twinship's Many Meanings||313|
|Afterword: Part of Me||329|