I walk into a restaurant with my family. The heads start to turn. People walking past do double takes, and I'm thinking, "Why did I have to wear a black dress, too?" Finally, after we're seated, the question comes from the wide-eyed waitress: "Are the two of you sisters — I mean, twin sisters?" Invariably, more questions follow: "I always wanted a twin. What was it like? Did you always dress in matching outfits? I bet it was fun. Did you play jokes on people? Trick your parents, teachers — maybe even a date or two? What about ESP — do you always know what your twin is thinking?" I can pretty much tell what she's thinking right now, because it's the same as me: "Why do people find twins so fascinating? And, thank goodness we're not triplets!"
Whatever the reason, there's no denying that people have been, are, and probably always will be fascinated by twins and other multiples. And the numbers of twins seems to be on the rise. But what's the reason for that? What causes a woman to conceive twins and why are some identical while others are fraternal?
In this article, we'll look at how twinning happens, whom it's most likely to happen to and what types of twins are out there. We'll also tackle some of the interesting topics that surround twins — like ESP, twin languages and more.
You take it for granted that you are a totally unique person, different from everybody else on Earth. And you understand that everybody else is also unique.
On a most basic level, identical twins are fascinating because they challenge this truth. They are unique people, of course, but they're eerily like each other.
Consider the extreme case of the "Jim twins." Identical twins Jim Lewis and Jim Springer were only 4 weeks old when they were separated; each infant was taken in by a different adoptive family. At age 5, Lewis learned that he had a twin, but he said that the notion never truly "soaked in" until he was 38 years old. Springer learned of his twin sibling at age 8, but both he and his adoptive parents believed the sibling had died. The two were finally reunited at age 39. The similarities the twins shared not only amazed one another, but researchers at the University of Minnesota as well. The very fact that you had twin siblings separated at birth bearing the same name, both 6 feet tall and weighing exactly 180 pounds is pretty incredible. But there's more.
In her book "Entwined Lives," Nancy Segal lists the following shared characteristics:
- As youngsters, each Jim had a dog named Toy.
- Each Jim had been married two times — the first wives were both called Linda and the second wives were both called Betty.
- One Jim had named his son James Allan and the other Jim had named his son James Alan.
- Each twin had driven his light-blue Chevrolet to Pas Grille beach in Florida for family vacations.
- Both Jims smoked Salem cigarettes and drank Miller Lite beer.
- Both Jims had at one time held part-time posts as sheriffs.
- Both were fingernail biters and suffered from migraine headaches.
- Each Jim enjoyed leaving love notes to his wife throughout the house.
Of course, before you start thinking about science fiction movies with pod people, the Jims, like other identical twins, are not carbon copies of each other. Some obvious differences were discovered during their participation in the "Minnesota Study of Twins Reared Apart," including:
- Each styled his hair differently; one Jim wore it combed straight, hanging down over his forehead (think Beatles circa 1961) and the other Jim wore it combed back and sported sideburns (think Johnny Cash circa 1957, but with longer 'burns).
- One Jim more clearly conveyed himself through speech, while the other was better suited to writing.
- While both Jims had been married twice, one Jim had taken vows with a third wife (called Sandy)
While not all as eerily similar as the Jim twins, many more instances of uncanny likenesses can be found among twins who were raised apart. The CBS news show "48 Hours" once interviewed five sets of identical twins who were raised apart, including the following:
- Also known as the "giggle twins" (because they "laugh and fold their arms the same way"), Barbara Herbert and Daphne Goodship spent the first four decades of their lives apart. In the time following their reunion, they've discovered some remarkable parallels in their lives — both had miscarriages followed by the birth of two boys and then one girl.
- Identical twins Tom Patterson and Steve Tazumi had very different upbringings. Raised in a Christian family by two janitors in rural Kansas, Tom still managed to choose the same career as his brother. Steve, who lives in Philadelphia, was raised in a Buddhist household. Both men own body building gyms.
- Debbie Mehlman and Sharon Poset were also raised by families of different faiths. But nature versus nurture prevails in some interesting ways with these women. They both have the unusual habit of crossing their eyes when they get excited.
The Minnesota study even included a set of triplets. Although raised separately, Bobby Shafran, David Kellman and Eddy Galland shared similar personalities. According to the book "Entwined Lives," all were described as, "intelligent, extraverted and slightly rambunctious." Bobby and Eddy were the first to meet, reunited by one of Eddy's college friends. Upon seeing a newspaper photo of his brothers, David immediately contacted his siblings and the triplets were fully reunited.
It's obvious from these twins' stories that genetics are certainly a factor in shaping who we are. In the next section, we'll see how such close genetic matches actually form.
The key to human reproduction is fertilization. The optimum time for fertilization to take place is during ovulation, which occurs about mid-way through a woman's menstrual cycle. If the one egg released during ovulation is fertilized by a male's sperm, here's what happens next:
- Sometime around day one-and-a-half, conception occurs; the fertillized egg, now called a zygote divides one time, resulting in two cells.
- By approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst.
- By the sixth day after fertilization, the blastocyst implants itself in the uterine wall where it will continue to develop for approximately nine months.
This content is not compatible on this device.
Initially after implantation, the developing baby is called an embryo. Around week nine of development, it is called a fetus. The developing baby is suspended and protected by the fluid-filled amniotic sac and it receives nourishment and oxygen and eliminates waste via the umbilical chord and placenta.
Now that you know how a singleton (single baby) is created, let's look at how twins form.
There are two basic types of twins: monozygotic (also known as "identical") and dizygotic (also known as "fraternal").
There's no doubt that identical twins stand out more and are more easily recognized. However, they are not the most prevalent type of twins. In fact, you might be surprised to find that two thirds of all twins are dizygotic. Dizygotic twins occur when two sperm fertilize two distinct eggs. In this instance, rather than releasing one egg during ovulation, the mother has released two. (For higher multiple births, more eggs are released - for example, in trizygotic triplets, three eggs are fertilized by three sperm.)
Dizygotic (DZ), or fraternal twins, can be one of three combinations:
- Two males (represents about one-fourth of all DZ twin sets)
- Two females (represents about one-fourth of all DZ twin sets)
- One male and one female (represents one-half of all DZ twin sets)
The fertilization of two eggs by two sperm results in two separate embryos — each with its own chorion (outer membrane of the amniotic sac), amnion (inner membrane of the amniotic sac) and placenta. In some instances, if the embryos implant very nearby each other along the uterine wall, the placentas can become fused.
Because they are formed from two distinct eggs and two distinct sperm, DZ twins are genetically no more similar than regular siblings are, sharing 50 percent of their DNA.
Monozygotic (MZ) twins are often referred to as "identical twins." MZ twins happen in the same way a single birth happens — one sperm fertilizes one egg. However, shortly after fertilization, instead of the blastocyst remaining as one cell group, it splits in two. Exactly when this split happens actually determines how the twins will implant in the uterine lining. The earlier the split, the more independent the embryos will be. For example, if the split occurs very early — say sometime during day two — then the embryos will have separate chorions, amnions and placentas. This scenario represents approximately one-third of identical twins. (Although, like with DZ twins, the placentas could become fused.) Other possible scenarios are:
- The twins have separate amnions and separate placentas but share one chorion.
- The twins have separate amnions but share one chorion and one fused placenta.
- The twins share one chorion, one amnion and one placenta (rarest combination).
The placenta provides the developing baby with the nourishment and oxygen it requires. Therefore, it's preferable for the MZ twins to have separate placentas (even if they are fused). In some instances, MZ twins who share a placenta will not develop equally because one twin actually receives more nourishment via the placenta than the other twin does. This is known as fetal growth restriction. MZ twins who share a placenta are also at risk for a serious and potentially fatal condition known as Twin-to-Twin Transfusion Syndrome (TTTS).
In addition to the shared placenta, twins suffering from TTTS also share some circulation, thereby enabling the transfusion of blood from one twin to the other. The donor twin can become anemic and will be unusually small for his gestational age. Meanwhile the recipient twin will have an over abundance of blood supply and will be unusually large for his gestational age. Additionally, the donor has a dangerously low amount of amniotic fluid, while the recipient twin has too much. This abundance of fluid can cause the mother continuous discomfort, difficulty sleeping and pain. It is possible to treat some cases of TTTS using amniocentesis to remove the excess fluid or laser surgery to close the circulation connection between the twins. For more information regarding TTTS, visit the Twin-to-Twin Transfusion Syndrome Foundation.
Because MZ twins come from the same fertilized egg, they share identical genetic material. MZ twins can only be same-sex, meaning that there are two possible combinations:
- Two males
- Two females
If splitting occurs very late and is incomplete, conjoined twins result. Let's take a closer look at this type of twin.
Conjoined twins are monozygotic twins who are joined at some region of their bodies. The many different types of conjoined twins are classified by the area where the attachment is located:
Terata Catadidyma refers to twins joined in the lower portion of their body, or they may appear to be two bodies on top and one body on the bottom.
- Pygopagus - back-to-back, joined at the rump (about 19 percent)
- Ischiopagus - joined sacrum to sacrum (about 6 percent)
- Dicephalus - one body with two separate heads
- Diprosopus - single body and head, but bearing two faces
Terata Anadidyma refers to twins with one single upper body with a double lower half or twins who are connected by a single body part.
- Cephalopagus - connected at the head (about two percent)
- Syncephalus - connected in the facial region
- Cephalothoracopagus - connected in the facial region and at the thorax
- Dipygus - one upper body with two lower bodies (including the abdomen, pelvis and legs)
Terata Anacatadidyma refers to twins who are joined somewhere along the midsection of the body.
- Thoracopagus - joined at the chest; may share a single heart or have some cardiac connection; some organs in the abdominal region may be malformed (about 40 percent)
- Omphalopagus - joined at the chest (about 33 percent)
- Rachipagus - back-to-back, joined along the spine above the sacrum
There are also other forms of conjoined twins. Parasitic twinning is when one conjoined twin is much smaller and perhaps not as fully formed or developed as the larger twin. In rare instances, this results in limbs located in bizarre areas of a twin's body. For example, what appears to be a singleton birth arrives with an arm joined at his back or an additional leg joined at the hip region. The Internet Movie Database reports that actor Andy Garcia was born with a conjoined parasitic twin. The small malformed twin was surgically removed shortly after birth.
Another even more rare form of parasitic conjoined twinning exists. Fetus in fetu is when a malformed twin is discovered inside the body of a host twin — a living child or adult. Although it would be difficult to know the exact rate of incidence (as there may be cases that go undiscovered for long periods of time), it is believed that fetus in fetu occurs once in every 500,000 live births. In "Fetus In Fetu: A Case Report," Phatak, Kolwadkar and Phatak state: "Masses containing bones, cartilage, teeth, central nervous system tissue, fat and muscle may be found in the abdomen of newborns and children termed `Teratomas'. They are defined as fetus in fetu if there is a recognized trunk and limbs." [ref]
How and why conjoined twins are formed isn't definitively agreed upon. The main theory proposes that it happens when a fertilized egg that is going to split into a monozygotic set of twins doesn't fully separate. This is principally a matter of timing. As we discussed earlier, the sooner the split occurs, the more independent the MZ twins will be in relation to having separate or shared chorion, amnion and placenta. With conjoined twins, it is believed that the separation takes place very late (somewhere around day 12 or thereafter), so that it is never fully complete. Thereby leaving the twins physically connected to each other.
Conjoined twins happen very rarely and the survival rate is quite low. While they happen about one time in every 40,000 births, they make up only one in every 200,000 live births. Research indicates that about 40 to 60 percent of conjoined twins are stillborn (deceased at birth). And, another 35 percent remain alive for only one day after birth. Surviving twins are more often female than male. Even though more male pairs are conceived, the females lead the males 3:1 in making it to a live birth. There are probably less than 50 sets of non-separated conjoined twins living in the world today.
Ever heard of twins who celebrate their birthdays months apart? What about twins who have different biological fathers? This seems impossible, but it turns out that both of these things happen. These incidents are rare, but real.
As we mentioned earlier, some women can release more than one egg during ovulation. In some exceptional instances, these eggs are released at different times. Perhaps at 12 or even 20 days apart. Now, let's say both of these eggs are fertilized shortly after they are released. The woman conceives twins, but on two separate dates. Since the dates of conception differ, so will the dates of delivery. The twins could be born a couple of weeks or a month apart. This is known as superfetation.
When a mother releases more than one egg during ovulation and has intercourse with more than one male during that time, it may result in superfecundation twinning. Two distinct sperm from two separate individuals fertilize the two eggs the woman ovulates. Obviously, the twins resulting from superfecundation are dizygotic.
Now that we know how twins happen and what kinds of twins there are, let's see who is likely to have them.
I'm an identical twin. My best friend from college is the daughter of an identical twin and she has identical twin nieces on her husband's side of the family. While working here, at one time I worked with another twin (he has a fraternal twin sister), a co-worker who has younger twin brothers and HowStuffWorks' founder was expecting identical twin boys. It's a curious thing when you seem to be surrounded by something like that. You begin to ponder, "Why do certain people have twins?" Seriously — what made my mother, my friend Melinda's grandmother and sister-in-law, Kevin's mom, Karim's mom and Marshall's wife have twins? And, if you're like me, or one of my twin-related friends, you might wonder, "Since I have a twin-family connection, am I more likely to have twins?"
Of course, there are all sorts of personal theories, stories and the like that claim to understand how twins happen. For example, you may have heard that twinning is hereditary. Maybe you've heard that if you're a twin, you can't have twins because it skips a generation. Perhaps you've even heard that certain foods will increase your chances of conceiving twins. Much like an urban legend that you hear repeatedly, so much so that "it just has to be true" - many people believe these statements because they've heard them all their lives.
Twinning is actually a bit more mysterious than that — well, monozygotic twinning is. Right now, we really don't know what causes MZ twins to occur. What we do know is that the birth rate of MZ twins is consistent throughout the world; for every 1000 births, approximately four MZ twin sets are born.
While MZ twins remain a mystery, scientists do have a better handle on what might cause dizygotic twinning. In fact, there are several key factors, including:
- Maternal age
- Maternal weight
- Ethnicity or race
- Infertility treatments
- Health Care
In order to produce DZ twins, the mother has to release more than one egg. The ability to hyperovulate (release more than one egg at a time) could be an inherited trait received from your mother. Furthermore, a DZ female twin is almost twice as likely to mother DZ twins herself as any other female. Increased ovulation can also be due to an amplified level of follicle-stimulating hormone (FSH). Studies indicate that levels of this hormone increase among heavy women and older women. With the U.S.' trend toward growing waistlines and older parenting, it's not surprising that the twinning rate has risen 38 percent since 1990, and 76 percent since 1980 [ref]. As of 2014, 33.9 twins were born for every 1,000 Americans.
National and international research suggests that ethnic origin or race is also a reliable factor in the occurrence of twinning. The highest rate of twinning is found among women of African decent. For example, here in the United States, the rate of DZ twins among the African American community is 25.8 per 1,000 live births. Furthermore, the rate of twinning among the Yoruba tribe of the West African country Nigeria is an incredible 45 per 1,000 live births. Remaining twinning rates here in the United States, from highest to lowest are: Alaskan natives 24.9 per 1,000 live births; Caucasians 19.6 per 1,000 live births, Native American descent 18.8 per 1,000 live births, Japanese descent 17 per 1,000 live births, Hawaiian descent 15.3 per 1,000 live births, Filipino descent 13.2 per 1,000 live births and Chinese descent 11.2 per 1,000 live births.
Certain environmental factors also have an impact. Improved health care and nutrition certainly play an important role in the survival of twin fetuses. Another important factor in the increased rate of DZ twinning could be attributed to infertility treatment. Women who have trouble conceiving due to ovulation difficulties are treated with ovarian stimulation. This process can cause the woman to hyperovulate. Other treatments can be conducive to DZ twinning as well. For example, during in vitro fertilization, eggs that have been fertilized outside the woman's body are placed back inside her uterus. There is no guarantee that a fertilized egg will successfully implant itself in the uterine lining, so usually more than one fertilized egg is placed for possible implantation. If more than one fertilized egg does implant itself, multiple births will occur.
Children develop language skills in stages. From birth up to about six months old, your baby is like a sponge soaking up all the sounds in his or her environment. At first, the level, tone or quality of a person's voice is most striking. But by about the fourth month, your baby will start to hone in on certain sounds and will start making similar sounds himself. The gurgling, cooing and babbling you hear over the next five or six months will not be recognizable, but that doesn't mean your baby doesn't understand what you're saying. Many babies begin to associate words with the objects they represent long before the babies can articulate the words themselves.
After your baby turns one, language development can quickly move into high gear. Over the next couple of months, your child suddenly understands more than the simple word here and there. Full sentences and concepts are digested on the spot. As soon as you say, "We're going to the park," your little one starts to head for the front door. When you say, "It's about time for lunch," he makes his way to the kitchen. As quickly as his understanding of your language is growing, he's beginning to master several words himself. As birthday number two approaches, your baby's vocabulary will probably comprise 40 to 50 words. While this seems to be the norm for many singletons, it's not always true of twins. (Reportedly, about one child per 10 to 15 children has difficulty comprehending or developing language skills.)
The idea that twins develop entirely fabricated secret languages that only they use and only they can understand has long been a source of fascination for scientists and lay people alike. This is known as twin language, idioglossia or cryptophasia. In most cases, however, what the twins are speaking is not an entirely new or separate language. It's actually a matter of delayed or poor speech development in either one twin or both. Here's a possible scenario: Twin A has difficulty articulating certain sounds and thereby certain words. Although twin B is better able to articulate these sounds and words, he chooses to mimic or repeat the manner in which Twin A speaks. So, the two continue to talk this way, understanding what the other is saying but meanwhile it sounds like gibberish or some concocted language to you.
Why does this happen? There are a number of theories. For one, delayed speech in general is related to low birth weight and premature births. Statistics indicate that nearly 60 percent of twins and over 90 percent of higher multiples are born premature. Reportedly, the length of gestation decreases with each additional baby. An average single-birth pregnancy lasts 39 weeks. For twins that number drops, on average, to 36 weeks. And for triplets, quadruplets and quintuplets, it can range from 32 to 29 weeks respectively [ref]. Other factors include restricted one-on-one communication time with parents and twins' keen ability for non-verbal communication skills. Sometimes twins have more one-on-one communication time with each other, rather than with a parent or guardian. So, it seems reasonable that they would continue to foster close communication with each other - even if it entails using incoherent modifications of real speech.
So, is there really no secret language between twins? While research does indicate that the phenomenon most people think of as a secret twin language isn't what it seems, that doesn't mean there aren't twins and other siblings out there making up their own private words or codes.
Why bother with secret languages anyway? Can't twins read each other's minds? Next, we'll look at twin telepathy.
Consider the following story: My identical twin sister, nicknamed "Fred," was hiking in Sabino Canyon while visiting our aunt for two weeks in Arizona. In the middle of the hike, she slipped and suffered a sprain, a few cuts and some scrapes and bruises. It took a little more than an hour to hobble back to the car and head home. Meanwhile, across the country in North Carolina, I was interviewing new wait staff at work. In the middle of an interview, I had an intense feeling of worry for no apparent reason. I was thinking of my sister and could not resist the urge to call and check on her. I excused myself from the interview to make the call, but got her voicemail. For another hour and a half, I couldn't shake the worried feelings. Later that evening, when Fred finally called me back, we discovered the "incidents" were within minutes of each other.
Now, this story: One day I was reminded of a college friend I hadn't talked to in two years. When I went home that evening, I mentioned this to my husband. He smiled and asked, "Have you checked the voicemail today?" when I did, I was surprised and pleased to hear a message from the friend I had been thinking about all day.
How do you explain these incidents? And why do I share them with you now?
One question that constantly surrounds twins has to do with the existence of a possible special connection the pair might share. Can they read each other's minds? Do they finish each other's sentences because they are truly telepathic? Do twins possess some shared form of extrasensory perception that the rest of the world doesn't?
Extrasensory perception (ESP) is a collective term for various hypothetical mental abilities. All of these abilities are based on the idea that human beings can perceive things beyond the scope of known bodily senses. ESP believers around the world have different ideas of how these abilities manifest themselves. Some people believe everybody possesses these abilities, and we involuntarily experience moments of ESP all the time. Others say only a handful of psychics, shamans or mediums have the special power, and that they can only access this power when they put themselves into a special mental state. And, there are those who believe this can be a special ability that is shared among twins, especially identical twins.
Now, let's consider the stories above. In the first story, the people involved are identical twins. In the second story, the individuals are not even related. So, are these random coincidences? Many scientists would say they are. In fact, results of various experiments conducted on twins testing for telepathy show no real indication that twins are any more connected in this manner than others are.
For example, in one experiment, one twin — the "sender" — was asked to select a particular card out of a group of cards. The sender was then told to concentrate on the image (a target) displayed on the card and try to communicate it telepathically to his twin. In a separate location from the sender, the "receiver" twin was then asked to pick a card. The results indicated that the receiver accurately picked the target card about 50 percent of the time. The test was readministered, but this time a research assistant picked the actual card from the deck and asked the sender twin to try to transmit the target image to the receiver twin. Again, the receiver twin was asked to pick a card. This time, however, the receiver accurately picked the target card only about 25 percent of the time.
The scientists believed the initial 50 percent accuracy rate was based on shared preferences rather than telepathic ability. The 25 percent accuracy rate during the second half of the test supports that theory. Because the "sender" actually selected the card during the first half of the study, the fact that the "receiver" twin accurately selected the same card 50 percent of the time simply could be based on the fact that the two would both be attracted to similar cards due to a shared preference for certain shapes, colors, patterns, etc. During the second half of the test, the research assistant selected the cards, not the "sender" twin — so that initial action of selecting something because of a particular preference was gone. The accuracy rate of 25 percent is no more significant than similar studies done using non-related individuals.
In another study conducted by Lindon Eaves and Krystyna Last, 34 sets of twins — including both MZ and DZ sets — were seated in separate rooms from their twins. Each twin was given an opinion survey and directed to answer the survey in the manner they believed their twin would respond. Once they completed the first survey, they were given the exact same survey, but this time were asked to fill it out with their own opinions. The results showed no conclusive support for telepathic ability. The twins were no more apt to predict the correct response than an incorrect one.
Even though these experiments show no indication of a special bond, there are other experiments that do indicate otherwise. These relationships are not limited to twins, but rather include siblings, mothers and their children, and spouses. This would lead us to believe that these connections are not really paranormal activity at all but instead normal results of living together and coming to know one another very well. In fact, what may appear to be a psychic connection could merely be a heightened ability of keen observation.
Your five senses are constantly noticing information, and your brain is constantly processing this information on an unconscious level. So, after living with someone for an extended period of time, you have probably stored countless amounts of information about that individual. For example, you might know — on a conscious or unconscious level — that your brother chews on his lip and scratches his nose right before he's going to ask you for a favor. So, if he approaches you and does those things without saying a word, and you immediately say something like, "What is it that you need me to do?" An observer might think, "Wow, she's reading his mind!" But, what you're really doing is simply noticing visual cues and running those against a database of information you have stored in your mind.
Still, much like the incidents described earlier, there are countless anecdotes of psychic or paranormal phenomenon between twins. Entire books are dedicated to the subject. For example, Playfair's Twin Telepathy includes accounts of everything from shared thoughts to shared tactile sensations (one twin is having a heart attack and the other has chest pains) to actual physical manifestations (like bruising) of what is happening to the other twin. But the biggest problem with anecdotes like these is that they are just that. Someone is telling another person about an event that happened previously. Short of watching a set of twins 24 hours a day, 7 days a week, indefinitely waiting for something like this to happen, there's no way to manage actually documenting such an incident. So, no matter how credible the source of the story, there will always be room for doubt. In fact, even though my twin and I have experienced some pretty amazing things — how can I know that it's singularly attributable to the fact that we're twins?
For more information about twinning, twins and related topics, check out the links on the following page.
Originally Published: Sep 22, 2005
More Great Links
These are some sources that we found useful in researching this article:
- Stedman's Medical Dictionary, 27th edition
- The Concise Science Encyclopedia, Kingfisher Publications
- Entwined Lives: Twins and what they tell us about human behavior by Nancy L. Segal, Ph.D.
- Twins and What They Tell Us About Who We Are by Lawrence Wright
- Twins: An Uncanny Relationship by Peter Watson
- Twins! Pregnancy, Birth and the First Year of Life by Connie Agnew, M.D., Alan Klein, M.D. and Jill Alison Ganon
- The Art of Parenting Twins by Patricia Maxwell Malmstrom and Janet Poland
Articles, Journals and Periodicals
- National Vital Statistics Reports
- "Fetus In Fetu: A Case Report" by SV Phatak, PK Kolwadkar, MS Phatak: Ind J Radiol Imag 2003 13:1:93-94
- "Larger Women More Likely to Have Twins" by Emma Baines, GP: Obstet Gynecol 2005
- "Twin Language: A Risk Factor for Language Impairment?" by D.V.M. Bishop, S.J. Bishop: Journal of Speech, Language & Hearing Research, February 1998, Vol. 41, Issue 1
- CBS News, 48 Hours http://www.cbsnews.com
- Internet Movie Database http://www.imdb.com
- The Twin to Twin Transfusion Syndrome Foundation Inc.www.tttsfoundation.org
- The Minnesota Twin Family Study http://www.psych.umn.edu/psylabs/mtfs/default.htm