Nancy L. Segal Ph.D.


Pair of Twin Conferences

A pair of conferences draws more than twins

Posted Aug 26, 2010

It may not be surprising that twin conferences come in pairs.  In May and June of 2010 I attended two truly excellent meetings organized around twin studies. The first one, held at the University of California, Los Angeles was targeted to the general public, while the second one, held in Seoul, South Korea was aimed at research professions. Summaries of these meetings will be published in a forthcoming issue of Twin Research and Human Genetics, the flagship journal of the International Society for Twin Studies.

“Everything You Wanted to Know About Twins

UCLA Center for Society and Genetics

May 18, 2010

A May 2010 twin panel, hosted by UCLA’s Center for Society and Genetics, was moderated by Abigail Pogrebin, former  60 Minutes producer and the author of One and The Same, reviewed in TRHG.  Pogrebin was introduced by the center’s Dr. Norton Wise. Here are Pogrebin’s thoughts about the event, in her own words:

“So many experts, so little time. That was my anxiety going into the unprecedented panel discussion on twins.  It was actually Dr. Nancy Segal who planted the seed for this special event.  We were having coffee in New York on one of her trips to the East Coast, and I was telling her about an interview series I moderate at Manhattan’s Jewish Community Center called “What Everyone’s Talking About.” She said, “You should put together a panel on twins in L.A.” The proverbial light bulb went off, and faster than I thought possible, six of the top experts in the field of twins (all of whom I’d interviewed for my book, all of whom taught me more than I could ever include) had agreed to talk about twins from every angle in front of an audience of 200-plus.  Fortuitously, these specialists are all based in the same neighborhood, and even more fortunately—and generously, UCLA agreed to host.

My goal in the discussion was to weave together several different disciplines, all in the service of understanding twins more expansively – psychologically, developmentally, genetically, epigenetically, and even emotionally.   The umbrella question came down to identity: how do we become who we are?” 

The panelists included Dr. Thomas Mack, Dr. Eileen Pearlman, Dr. Laura Baker, Dr Eric Vilain, Dr. Joan Friedman and myself. I asked each of them to send me a short summary of their remarks and most of them did.

Dr. Thomas Mack, from the Department of Preventive Medicine at the University of Southern California described the value of twins for studying factors affecting disease predisposition. His study of 79 MZ twin pairs discordant for multiple sclerosis (MS) showed that childhood sun exposure was associated with an increased risk of MS. In a related study of 400 MZ and 380 DZ twin pairs, Mack examined MS concordance in twins as a function of zygosity, sex and latitude.  Twins from Canada and adjacent US states (at or above 41-42 degrees north) were labeled “northern.” He found that the heritabilty of MS was the same for males and females (concordance ratios were 2.9 and 2.6, respectively). Latitudinal variation in MZ co-twin concordance rates was influenced by both genetic and environmental factors; MS was diagnosed two years earlier in MZ twins from northern regions.

Mack believes that MZ twins should inform their physicians of their twinship status because illnesses or other health-related conditions in their co-twin may increase their own risk. Interestingly, being born a twin or a non-twin has never been included in the medical history forms that patients routinely complete prior to examinations.

Dr. Eileen Pearlman, Director of Twinsight in Santa Monica and co-author of Raising Twins From Birth Through Adolescents, stressed that the psychological developmental process differs for twins and non-twins.  In addition to learning how to separate and individuate from their parents, twins must take an additional step: Twins have to learn to separate and individuate from each other.  The separation/individuation process becomes either more or less intense throughout the different stages of twins’ lives.  For example, it is more intense during early childhood, adolescence, and young adulthood, when there are changes in twins’ lives. Other factors which contribute to psychological developmental differences between twins and non-twins are that twins share the intrauterine environment and hear their co-twin’s heart beat in addition to their mother’s heartbeat while also interacting with one another.  The quality and quantity of time is also different for twins whose parents spend more time with both twins than do parents of singletons, but spend less time with each twin individually. Twins often become each other’s transitional object, e.g. an object like a stuffed animal, which is used to soothe and comfort the infant when his caregiver is not available.  These differences are important to address when examining the twin relationship.

Dr. Joan Friedman, a Los Angeles psychotherapist, noted that little attention has focused on adult twin pairs who might experience developmental challenges resulting from growing   up as half of a dyad. Her research has highlighted the finding that older twins are often reticent about seeking help. They feel a sense of guilt, shame, and inadequacy if their relationship does not reflect the stereotypic norm of best friends and soul mates. Along these same lines, there is a need for well-trained mental health practitioners and educators who can understand and manage emotional concerns specific to the multiple birth population. Friedman’s upcoming book will discuss these issues in greater detail, providing insight and practical solutions to the difficulties and discrepancies affecting twin relationships.

I reviewed research from studies of twins reared together and apart showing that there is genetic influence on most measured traits. I also discussed research on twin relationships, virtual twins and twin loss. Most studies show that MZ twins share closer social relationships than DZ twins, regardless of their rearing status. This is a finding that teachers and educators need to consider when deciding the placement of twin children at school; see the news story below. I am in the process of studying social relations within virtual twin pairs (VTs or same-age unrelated children reared together since birth), and I expect they will show reduced closeness relative to MZ and DZ twins. One of my recent studies showed that VTs were less successful on a joint problem-solving task than MZ and DZ twins, with MZ twins being the most successful (Segal, McGuire, Miller, & Havlena, 2008).

Twin loss continues to be an important topic, given that it is highly overlooked by bereavement specialists. I now have nearly 700 twins enrolled in the ongoing Fullerton Twin Loss Project. The finding of increased grief intensity among surviving MZ than DZ twins is robust. A recent twin study of suicidal predisposition, based on a subset of these twins, indicated a genetic component (Segal, 2009).

Dr. Laura Baker, from the Department of Psychology at the University of Southern California and USC Twin Project addressed questions concerning twin relationships in young twins and how parents may handle them.  Referring to her studies of aggression, she pointed out that twins who tend to be aggressive may not get along because of these genetically influenced behavioral traits. Dr. Eric Villain, from the Departments of Urology and Human Genetics at the University of California, Los Angeles, discussed his research on twins and sexuality.  We all answered questions from the audience, many about the rearing of twins and how to handle within-pair conflicts.

The UCLA event may be viewed in full on (“Everything You Wanted to Know About Twins”). The program is 88 minutes long.

13th Triennial Meeting of the International Society for Twin Studies

Seoul, South Korea

June 4 – 7, 2010

The 13th meeting of the International Society for Twin Studies was hosted by Dr. Yoon-Mi Hur in Seoul, South Korea, in June 2010. The venue was the Hyatt Regency Hotel, located near the area of Itaewon. The meeting was very well attended and included ten symposia, twelve paper sessions, five plenary talks, a presidential address and eighty poster presentations.

There were many excellent presentations so only a sampling of highlights from the perspective of this author is possible. This was the year of epigenetics, the study of chemical reactions that cause parts of the genome to become expressed or silent. A symposium, “Epigenetic Studies in Perinatal Cohorts,” included four contributions from Australian investigators. Jeffrey Craig detected methylation discordance in all twin pairs, with DZ twins showing greater variation than MZ twins. This finding demonstrated that both the intrauterine environment and genetic factors affect the neonatal epigenome. Richard Saffrey described the collection of birth specimens including blood, buccal smears and placenta from infant twins in approximately 250 pairs. These materials will enable further comprehensive investigation of factors affecting the neonatal epigenome. Eric Joo speculated that microenvironmental features of the intrauterine environment will be reflected in epigenetic marks and gene expression. B. Novakovic discussed evidence that preconception maternal alcohol consumption may be affect the epigenetic profile.

The symposium, “Twin Studies in China” was a fascinating look at the research and registries that have been developed in recent years.  Given my recent work with young Chinese twins separated at birth, I was particularly interested in this series of papers. Liming Li reported that over 8,000 twin pairs have been enrolled in the Chinese National Twin Registry. The twins come from four cities in China and will be used to study a variety of medical and psychological traits. A paper by Mingguang He described the Guangzhou Eye Study that began in 2005 and includes over 9,000 twin pairs.  Twins are followed longitudinally to assess changes in opthalmological health. Ting Wu discussed blood pressure and obesity studies using twins from the national registry. Genetic correlations between systolic blood pressure and body mass index were reported. Weili Yan described a twin study of low and non-response to hepatitis B vaccination (HPV) in infants. Factors affecting this response will help to develop techniques for improving the efficacy of HPV vaccination in low and non-responders. A paper by Tao Li offered an overview of the Southwestern Twin Registry. IQ and personality traits have been studied in twins 6 – 16 years of age. The heritability of IQ was shown to increase with age, a finding that has been demonstrated in other twin and adoption studies.

A Bulmer session was held on Sunday June 6, late in the day. Virtually every twin researcher is familiar with M.G. Bulmer’s marvelous 1970 book, The Biology of Twinning in Man. The superb papers in this session by Catherine Derom, Jodie Painter, Gonneke Willemsen, Niels Lambalk and E.A.M Kuiper examined various topics on the genetic aspects of twinning. A late entry in this session, “The Genetics of MZ Twins,” was not listed in the official program. The paper was delivered by Bruno Reversade from the National University of Singapore. Reversade began by saying that MZ twinning is genetic, not stochastic. Based on his studies of a Jordanian village that has a high monochorionic twinning rate, he claims to have found a gene common to some of the families. The gene is associated with the blastocyst, is specific to the inner cell mass of the human embryo and is critical in development. Reversade believes that if this gene is “overexpressed” the result is MZ twins.  He believes that this gene might explain the high frequency of MZ twinning in a subset of families only, with random processes explaining MZ twinning in general.

 Additional families are needed to confirm Reversade’s findings and he is pursuing studies along those lines.  It seems extraordinary that after so many years of twin research the answer to the question of what causes MZ twinning remains elusive.

Matt McGue’s presidential address was a wonderfully informative overview of where twin research began and where it has gone.  Galton, of course, was the father of the twin method in the late 1800’s. However, twin research was largely ignored because of the widespread behaviorist views of the 1920s and in the decades that followed. Eugenic movements in the United States and elsewhere during that time also undermined behavioral genetic research in the United States. I would add that horrific twin studies conducted by Mengele at Auschwitz also did considerable damage tothe status of twin studies. In the 1960s there was a growing disenchantment with environmentalist explanations, as well as accumulating evidence of behavioral heritability. Our colleagues such as Irving Gottesman and Steven Vandenberg did a great deal toward reviving behavioral genetics in general, and twin research in particular. There were critics of the twin method, but they made us more careful about our data collection and interpretation.  McGue also cited the wonderful twin research done by Dorothy Burlingham in the 1940s and 1950s.

According to McGue, twins are “an antidote to the blank slate.” They saved psychology from purely environmentalist views. He concluded by saying that twins did more for psychology than psychology did for them. I think he meant that twins deepened out understanding of behavior by adding a genetic perspective. I think he also meant that greater efforts toward understanding the unique developmental aspects of twinship are needed.

About the Author

Nancy L. Segal, Ph.D., is a professor of psychology and the Director of the Twin Studies Center, at California State University, Fullerton.

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