Preadolescent development: case studies in twins.

The authors studied eight sets of healthy twins, ranging in physical maturity from prepubertal to late pubertal, and their parents, to assess psychological changes in early adolescence. A phase-specific psychosocial regression correlates with the biological onset of puberty. We present preliminary findings related to twinship, parental reactions, and longitudinal development, based on detailed case studies. Parents experienced increased conflict, detachment, and pride in response to their twins'' psychosocial development at puberty. The more advanced child in a pair tended to lead in all spheres of adjustment-school success, heterosexual interest, peer friendships, and independent behavior. Although the more advanced child began to break away from his twin, in general the twin relationships remained close. Differences in personality traits and rate of psychosocial development within a twinship tended to remain consistent or to become accentuated. They were occasionally narrowed but rarely reversed from early childhood through early adolescence. Some of the differences were fostered by dissimilar patterns of identifying with the parents.     

Personal experiences of bereaved twins, parents of twins, and their carers.

We invited bereaved twins, parents of twins and carers to describe some of their personal experiences. We are grateful to all of them for their brave candour. We gave extra space to Kathy's story about the impacts of the loss of her own twin because it vividly illustrates the profound connections twinship can generate. Similarly we thought the cruelly long and complex saga of the life and death of Maxine's twins could help understanding of the distressing repercussions that often attend the low birth weight and prematurity that are so common amongst multiple births.     

Psychobiological and evolutionary perspectives on coping and health characteristics following loss: a twin study.

An analysis of coping, grief and health characteristics is reported for a bereaved monozygotic (MZ) and dizygotic (DZ) same-sex twin sample. The data were examined with reference to psychobiological and evolutionary perspectives on behavior. A Coping Scale, included as part of a comprehensive Twin Loss Survey (TLS), assessed coping with daily responsibilities and activities 1-2 months before the co-twin's death, 1-2 months following the co-twin's death and currently. A Grief Intensity Scale obtained judgments of grief 1-2 months following the loss, and currently. Information on physical symptoms was available from the Somatization Scale of the Grief Experience Inventory. Psychobiological and evolutionary perspectives specified hypotheses for two twin groups: one model was specified to reflect bereavement experiences immediately following loss of the co-twin (retrospective twin group); a second model represented present bereavement response (current twin group). Consistent with psychobiological theory, twins' social closeness showed a positive association with grief intensity which, in turn, affected somatic symptoms and coping efficacy in predicted directions. With respect to evolutionary psychological theory, the effect of zygosity on current grief implicated correlates of genetic relatedness as factors in the bereavement process.     

The Norwegian Institute of Public Health Twin Panel: a description of the sample and program of research.

The Norwegian Institute of Public Health in Oslo has an ongoing program of twin research using population-based cohorts of twins. The current database includes information on twins identified through the Medical Birth Registry of Norway and born from 1967-1979, altogether 15,370 twins. This is a longitudinal study with a cohort sequential design whereby new cohorts are recruited into the study at 5-6 year intervals. Sub-samples of these twins have participated in questionnaire studies and clinical assessment sub-projects. These projects include national and international collaborations. Our primary areas of interest include mental health and psychological well-being, obesity, asthma and allergies, health behaviors and health perceptions, comorbidity, and perinatal influences on health outcomes. This paper provides a brief overview of the data, sample, and the various research projects associated with this twin program of research.     

The effect of losing the twin and losing the partner on mortality.

Several studies have explored the impact of marital bereavement on mortality, while increasing emphasis has recently been placed on genetic factors influencing longevity - in this paper, we study the impact of losing the spouse and losing the co-twin, for twins aged 50 to 70. We use data from the Danish Twin Registry and the Population Register of Denmark for the period 1968 through 1999. Firstly, we use survival analysis to study mortality after the death of the spouse or the co-twin. We find that the risk of dying is highest in the first year after the death of the spouse, as well as in the second year after the death of the co-twin. We then use event history analysis techniques to show that there is a strong impact of the event 'losing the co-twin' even after controlling for age, sex and zygosity and that this effect is significantly higher in the second year of bereavement. The effect is similar for men and women, and it is higher for monozygotic twins. The latter confirms the influence of genetic factors on survival, while the mortality trajectory with a peak in the second year after the death of the co-twin is consistent with the existence of a twin bereavement effect.     

Peer reports of adaptive behavior in twins and singletons: is twinship a risk or an advantage?

We compared twins to their gender-matched singleton classmates in peer-assessed behavioral adjustment. Our samples include 1874 11- to 12-year-old Finnish twins (687 monozygotic, MZ; 610 same-sex dizygotic, SSDZ; 577 opposite-sex dizygotic, OSDZ) and their 23,200 non-twin classmates. Data were collected using a 30-item Multidimensional Peer Nomination Inventory containing three factors and their subscales. We found twin-singleton differences: classmates rated twin girls and boys higher than gender-matched singletons in Adaptive Behaviors (constructive, compliant, and socially active behavior), and those effects were particularly evident among OSDZ twins for assessments of social interaction, popularity, and leadership. We found no evidence that individual twins differ from singletons in Externalizing (hyperactivity-impulsivity, inattention, aggression) or Internalizing Problem Behaviors (depressive symptoms, social anxiety). Nor did we find systematic differences between MZ and SSDZ twins. Among both twins and singletons, boys exceeded girls in Externalizing, and girls exceeded boys in Internalizing Problem Behaviors. Results suggest that a twinship forms a positive developmental environment for socioemotional behavior, particularly among OSDZ twins.     

Inter-twin relationships and mental health.

We evaluated dominance-submissiveness between co-twins and its relationship to mental health in a cohort study of 419 twins followed from pregnancy to 22-30 years of age. Dominance-submissiveness between co-twins was assessed from three separate perspectives: physical dominance, psychological dominance, and verbal dominance. Depressive, nervous, and psychosomatic symptoms were analyzed in different twin groups. In the physical domain, males were more commonly dominant than females at school age and in adulthood. Before and at school age, girls were more dominant than boys in the psychological and verbal domains, as well as in total dominance. These differences disappeared in adulthood, and 81% of adult twins felt themselves equal to their co-twin in total dominance. Submissiveness in the psychological domain seemed to be associated with increased depressiveness, nervous complaints and psychosomatic symptoms in males of male-female twin pairs. Verbally submissive males in same-sex twin pairs had more depression and psychosomatic symptoms. Among females of same-sex twin pairs, submissiveness in the psychological domain was most clearly associated with depressive symptoms, whereas psychological or verbal dominance-submissiveness among females from male-female twin pairs was not associated with symptoms. Psychologically dominant males and females of same-sex twin pairs expressed greater nervousness than did their co-twins. We conclude that being submissive, especially in the psychological domain, to a female twin partner seems to be stressful, whereas it is easier, especially for females, to be submissive to a male twin partner.     

Mortality among twins after age 6: fetal origins hypothesis versus twin method.

OBJECTIVE--To test the validity of the fetal origins hypothesis and the classic twin method. DESIGN--Follow up study of pairs of same sex twins in which both twins survived to age 6. SETTING--Denmark. SUBJECTS--8495 twin individuals born 1870-1900, followed through to 31 December 1991. MAIN OUTCOME MEASURES--Mortality calculated on a cohort basis. RESULTS--Mortality among twins and the general population was not significantly different except among females aged 60-89, in whom mortality among twins was 1.14 times (SE 0.03) higher than in the general population. Mortality among female dizygotic twins was 1.77 times (0.18) higher than among monozygotic twins at age 30-59. Otherwise, mortality for monozygotic and dizygotic twins did not consistently differ after age 6. CONCLUSION--According to the fetal origins hypothesis the risk of adult morbidity and mortality is heightened by retardation in intrauterine growth. Twins, and in particular monozygotic twins, experience growth retardation in utero. The findings in the present study suggest that the fetal origins hypothesis is not true for the retardation in intrauterine growth experienced by twins. Furthermore, the data are inconsistent with the underlying assumption of a recent claim that the classic twin method is invalid for studies of adult diseases. The present study is, however, based on the one third of all pairs of twins in which both twins survived to age 6. The possible impact of this selection can be evaluated in future studies of cohorts of younger twins with lower perinatal and infant mortality.     

A Conversation with Bakhtin : On Inquiry and Dialogic Thinking

Two different traditions in psychology emphasize the study of twin-twin and mother-twin relationships. One line of research, the twin development approach, has compared parenting styles and sibling relationships in families with twin and singleton offspring. This research tradition is based on the assumption that twins, relative to singletons, may be at risk for developmental delays in motor, cognitive [1-3], linguistic [1,4,5], and social-emotional functioning [6,7] because of their increased rate of prematurity, lower birth weight [8-11], increased parenting demands that result in less interactive maternal speech [12-18], shared peer time [19], and other experiences that distinguish twins from singletons [7,8,19-21]. Although recent studies have not replicated the developmental concerns [22-26] or concerns about specific birth defects [27] that emerged from earlier studies comparing twins and singletons, there continues to be interest in the relational differences that may be present in twin and singleton families and in the need to address educational and social policies that promote twin development [28,29]. Within the framework of this approach, distinctions between twins and singletons have received more attention than distinctions between fraternal, dizygotic (DZ) and identical, monozygotic (MZ) twins.     

Gestational duration, and fetal and infant mortality for twins vs singletons.

The aim of this research was to study fetal and infant mortality in Sweden between 1973 and 1996 in twins vs singletons in relation to gestational duration. Analysis was of fetal and infant mortality based on the number of pregnancies at risk as the denominator rather than the number of deliveries each week. The analysis was based on information stored at the Medical Birth Registry (MBR), the National Board of Health and Welfare, Stockholm. The MBR keeps records on virtually all pregnancies (> 99%) regarding delivery and neonatal information, and for infant mortality up to 1 year of age. During the study period, 2,206,738 singleton and 52,658 twin births were registered. Risk evaluation was made as odds ratio (OR) with a 95% confidence interval. The material was stratified according to parity, maternal age, year of delivery, and delivery unit. Results showed the OR for twin births before 34 weeks gestation was 6 to 8-fold increased compared with singletons. The OR for fetal mortality was increased in all gestational weeks, and like-sexed twins had a consistently poorer prognosis compared to unlike-sexed. Between 1989-96, unlike-sexed twins had a fetal mortality approaching that of singletons. In conclusion, real progress in reduction of infant mortality in twins may be impossible until the high incidence of preterm births can be decreased. Hypothetically, about 100 twin labors would have to be induced to avoid one fetal death in like-sexed twin pregnancies.     

Monozygotic and dizygotic twins' retrospective and current bereavement-related behaviors: an evolutionary perspective.

The present study compared bereavement responses of 325 monozygotic (MZ) and 176 dizygotic (DZ) adolescent and adult twins following the loss of their co-twins. A subset of twins completed the Grief Experience Inventory using a retrospective time frame, while a second subset completed it using a current time frame. It was hypothesized that MZ twins (in both retrospective and current groups) would report higher levels of grief-related behavior than DZ twins, consistent with Hamilton's (1964) concept of inclusive fitness. Discriminant function and profile analyses yielded supportive findings, but only for the retrospective MZ and DZ twin comparisons. Females in both groups expressed higher levels of bereavement-related behavior than males. Findings are discussed with reference to theoretical aspects of grief and mourning.     

Potential bias regarding birth weight in historical and contemporary twin data bases.

In this study we examine the hypothesis that monozygotic (MZ) twins in historical databases are less discordant for birth weight due to negative selection of severely discordant MZ twins. Furthermore, we test the hypothesis that MZ twins are less discordant for birth weight when comparing a volunteer based twin registry with a population based twin registry, due to selective registration. Data were available on 3927 twin pairs from the volunteer Australian Twin Registry born before 1964, 3059 volunteer twin pairs from the Netherlands Twin Register born 1987-1989 and 454 Belgian twin pairs from The East Flanders Prospective Twin Survey born 1987-1989. Intrapair relative birth weight differences (RBWD) were computed for MZ and dizygotic (DZ) twins from each twin registry. Comparing birth weight differences between MZ and DZ twins provides support for the hypothesis that MZ twins are subject to a negative selection in historical databases. Furthermore, Australian MZ twins have a lower RBWD compared to Dutch MZ twins when corrected for the RBWD of Australian and Dutch DZ twins, indicating circumstances which only affect MZ twins. Our hypothesis that MZ twins are less discordant for birth weight in a volunteer based twin registry compared to a population based twin registry had to be rejected. We suggest that investigators using historical databases to test the fetal origins hypothesis should be aware of this increased likelihood of selective exclusion of individuals with extreme morphometric parameters at time of birth.     

Twins and non-twin siblings: different estimates of shared environmental influence in early childhood.

Twin studies typically indicate shared environmental influence for cognitive abilities, especially in early childhood. However, across studies, DZ twin correlations tend to be greater than non-twin sibling correlations, suggesting that twin estimates of shared environment are to some extent specific to twins. We tested this hypothesis in a sample of more than 1800 MZ and 1800 same-sex DZ pairs from the Twins Early Development Study (TEDS), a study of twins born in England and Wales in 1994 and 1995. For this analysis, we obtained comparable data from more than 130 same-sex younger siblings of the twins. Twins and their younger siblings were assessed for language, cognitive abilities and behavior problems by their parents at 2 and 3 years of age. For language and cognitive measures at both 2 and 3 years, but not for behavior problems, estimates of shared environment were more than twice as large for twins as compared to non-twin siblings. We conclude that about half of twin study estimates of shared environment for cognitive abilities in early childhood are specific to twins. Although many possibilities exist for explaining the special shared environment effect for twins, we suggest that cognitive-relevant experiences that are not shared by siblings are shared by twins because they are exactly the same age.     

Establishing a twin register in Sri Lanka.

Nearly all twin registers are based in developed countries and there is no twin register in the developing world. Our objectives were to initiate the process of establishing a nationwide twin register in Sri Lanka by starting a volunteer register first and working towards a population-based register. Regular newspaper advertisements, feature articles, radio talks, and television programmes were used to publicise a competition for twins, their parents/relatives and friends requesting them to participate by sending in details of twins. The competition ran from 28 March 1997 for a period of 3 months. It offered prizes for three winners selected by drawing lots. Advertisements highlighted the objective of the competition as establishing a twin register for future research and emphasised that informed consent would be obtained for individual research projects. Those who registered comprise 4602 twin pairs (same sex: male--1564, female--1885; different sex--1153), 80 sets of triplets (same sex: male--17, female--31; different sex--42) and two sets of quadruplets (different sex). The oldest twins, triplets, quadruplets are 85, 46, and 5 years old, respectively; 88.0% of twins are less than 30 years old. Although others have previously used media publicity to enrol twins in twin registers, we believe this to be the first time that twins have been enrolled through competition. We have more young twins, and our gender and zygosity proportions after applying Weinburg's rule do not match the proportions expected from a volunteer twin sample. Establishing a twin register for research purposes has proved possible in a developing country.     

The personalities of twins: just ordinary folks.

Twin studies have demonstrated that personality traits show moderate genetic influence. The conclusions drawn from twin studies rely on the assumptions that twins are representative of the population at large and that monozygotic and dizygotic twins are comparable in every way that might have bearing on the traits being studied. To evaluate these assumptions, we used Multidimensional Personality Questionnaire (MPQ) data from three samples drawn from the Minnesota Twin Registry (totaling 12,971 respondents) to examine the effect sizes associated with mean differences on the 11 MPQ scales and 3 higher-order MPQ factors for singletons versus twins and MZ twins versus DZ twins. The singletons in the samples were family members of the participating twins. We also used ratios of scale variances to examine the significance of variance differences. The only mean or variance difference replicated across all three samples was greater Social Closeness (about.1 standard deviation) for twins than for singletons. This difference was obtained for both males and females. It would appear that, with respect to personality, twins are not systematically different from other people. Our results also highlight the importance of replication in psychological research because each of our large samples showed differences not replicated in other samples.     

Examining the dimensions of intimacy in twin and peer relationships.

While it is widely assumed that twinsh p involves high levels of intimacy to the detriment of other relationships, an examination of twinship using measures of intimacy has not yet been conducted. Participants were 214 individua twins (128 MZ, 62 DZ same-sex, and 24 DZ mixed-sex) between 16-73 years of age. Twins completed measures of intimacy in reference to their co-twin, their closest same-sex friend, and their closest other-sex friend. As expected, the highest level of intimacy was reported for the co-twin. Contrary to expectation, MZ twins did not report significantly higher levels of intimacy with their co-twin, or significantly lower levels of intimacy with their closest friends when compared to DZ same-sex or DZ mixed-sex twins. Furthermore, twins who reported high levels of intimacy with their co-twin did not report significantly lower levels of intimacy with their closest friends. Implications and directions for future research are discussed.     

Correlations between live and stillbirth outcomes in twin pairs

A relationship has been proposed to exist between individual outcomes (live or stillbirth) of twins in the same set. Here, we analyze this association between live births and stillbirths among individuals in different twin pairs. When national birth registers are analyzed, individuals in opposite-sex twin sets can be identified and the correlation between individual outcomes estimated. However, full information about the individuals in same-sex twin sets is not, as a rule, available, and consequently, correlation coefficients cannot be estimated, but upper and lower limits of the correlation coefficients can be obtained. The methods introduced here were applied to data from Sweden (1869-1967), the Aland Islands (Finland) (1750-1949), the Kingdom of Saxony (1881-1900), and England and Wales (1940-2003). Comparisons between the correlation coefficients among opposite-sex twins and the lower bound (minimum) of correlation coefficients among same-sex twins indicate that in all populations studied a stronger association exists between twins in same-sex rather than opposite-sex twin sets or pairs. For opposite-sex twin sets no general association between the correlation coefficient and the stillbirth rate was identified.     

The comparative constitution of twinship: strategies and paradoxes.

In both traditional and modern societies, twinship, as an unusual mode of reproduction, involves difficulties for social systems in maintaining consistent classification systems. It is proposed that the most prevalent response to twinship involves various 'strategies of normalisation' to defuse and contain the potential disruption. This proposition is illustrated and analysed in relation to ethnographic maternal drawn mainly (but not exclusively) from African communities in the twentieth century. Following a discussion of twin infanticide as the most extreme of the normalising strategies, the article concludes by identifying a number of paradoxes in the social construction of twinship.     

The context and long-term impacts of multiple birth loss: a peer support network perspective.

Following my own experience with the death of a twin baby, I founded the Center for Loss in Multiple Birth (CLIMB) Inc. During the last 13 years I have worked with nearly 7000 bereaved parents of twins, triplets or other high multiples throughout the US and worldwide. The role of a peer support organisation and the range of families with multiples who have contacted it is described. They come from diverse backgrounds with many forms of bereavement: death during pregnancy, at birth or after of both or all babies, of one twin, of one or more higher order multiples; multifetal pregnancy reduction or selective fetocide; the death of one or more multiples in childhood. Some have had more than one multiple pregnancy and loss. Many are concerned for the surviving children. A peer support organisation has an important role to play in increasing public awareness of the needs of these families as well as influencing policy on infertility treatments that carry a high risk of producing multiple pregnancies with the associated risk of death and disability for one or more of the babies.     

Evaluating 2 year outcome in twins < or = 30 weeks gestation at birth: a regional perinatal unit's experience.

With improved technology in assisted reproductive medicine, there has been an absolute increase in the numbers of twin pregnancies with an associated increase in perinatal mortality and morbidity. This increase in perinatal mortality and morbidity is largely due to a higher incidence of delivering preterm as compared to singletons. Twin pregnancies have their unique complications that include abnormal placental communication and discordant growth which are associated with perinatal mortality and morbidity. The objectives of this study were two-fold: i) to determine if the morbidity/mortality outcome at 18-24 months corrected age seen in a cohort of twins born between 24-30 weeks gestation was significantly different as compared to singleton preterm infants of the same gestation; and ii) to determine and evaluate any differences between monochorionic (MC) and dichorionic (DC) twins. Twins 24-30 weeks gestation at birth born between 01/01/97-30/06/99 were identified and prospectively followed to 18-24 months corrected age (c.a.). They were matched with a singleton infant of the same gender and within 1 week of the same gestation. Obstetrical, neonatal and neurodevelopmental data were gathered and analyzed. The primary outcome was death or the presence of a severe neurodevelopmental deficit at 18-24 months corrected age. Of the 56 sets of twins identified, 52 sets were followed prospectively with 101 infants available for matching. In this cohort, twin pregnancies had a lower incidence of pregnancy-induced hypertension and premature rupture of membranes than singletons (p < 0.05). The two groups were comparable in neonatal characteristics. The incidence of death or severe disability was 29.7% in twins vs. 22.8% in singletons (p = 0.337, Fisher's exact test). The major area of defect was in the cognitive category for both groups, 9.9% vs. 7.9% respectively. MC twins made up 35.6%; DC twins 64.4%. Twin to twin transfusion syndrome (TTTS) occurred in 6.9%. Discordant growth occurred more frequently in MC pregnancies (p = 0.016). MC twins tended to be more premature, lower in birth weight, and experience neonatal morbidity in the form of patent ductus arteriosus and sepsis (p < 0.05) as compared to DC twins. However, the primary outcome of death or severe neurodevelopmental deficit at 18-24 months c.a. was not significantly different between the two groups, 38.9% (MC) vs. 24.6% (DC), (p = 0.173, Fisher's exact test). Neurodevelopmental morbidity or mortality in twins with TTTS was 42%. Mortality and severe neurodevelopmental morbidity were not signif cantly higher in twins as compared to singletons in this cohort. However, the trend is slightly higher in twins, which may have clinical significance. Though not statistically significant, the incidence of 38.9% in adverse outcome wth MC twins may be clinically significant. With the number of twins steadily increasing, further monitor ng is required to determine future directions in intervention and research. Early recognition of monochorionicity remains essential to optimize care and neurodevelopment for these infants.