It has been suggested that dizygotic twin pairs share two HLA haplotypes more often than ordinary siblings and thus might be genetically more alike. We tested this hypothesis in dizygotic twin pairs from the Danish Twin Registry. A total of 114 (60 female and 54 male) same-sexed healthy twin pairs aged 18–45 years participated. Dizygosity was established by means of DNA sequencing of nine polymorphic markers. HLA-A, B and Cw specificities were typed with serology, and if data were inconclusive, with DNA typing. If twin partners had the same HLA-types, they were assumed to share two haplotypes. If they had 1 HLA A, B and C antigen group in common they were assumed to share one haplotype and if they had no HLA types in common they were assumed to share zero haplotypes. Since HLA-types from parents were unavailable we could not test for identity-by-descent and thus had a risk of overestimating the number of twins sharing two haplotypes. A Chi-square test was used to compare observed numbers in each haplotype sharing group with the expected numbers. Twenty-nine (expected 28.5) twin pairs had two HLA-types in common, 52 (expected 57) had one HLA-type in common and 33 (expected 28.5) had zero HLA-types in common, p = 0.56. Our data show that DZ twins are not more similar than sibs from different pregnancies in general.

Gene–environment interaction is likely to be a common and important source of variation for complex behavioral traits. Often conceptualized as the genetic control of sensitivity to the environment, it can be incorporated in variance components twin analyses by partitioning genetic effects into a mean part, which is independent of the environment, and a part that is a linear function of the environment. The model allows for one or more environmental moderator variables (that possibly interact with each other) that may i) be continuous or binary ii) differ between twins within a pair iii) interact with residual environmental as well as genetic effects iv) have nonlinear moderating properties v) show scalar (different magnitudes) or qualitative (different genes) interactions vi) be correlated with genetic effects acting upon the trait, to allow for a test of gene–environment interaction in the presence of gene–envi-ronment correlation. Aspects and applications of a class of models are explored by simulation, in the context of both individual differences twin analysis and, in a companion paper (Purcell & Sham, 2002) sibpair quantitative trait locus linkage analysis. As well as elucidating environmental pathways, consideration of gene–environment interaction in quantitative and molecular studies will potentially direct and enhance gene-mapping efforts.

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.

In this work we present a new method for genetic analysis of twin data which is based on generalized estimating equations and allows for analysis of various response types (e.g., continuous, binary, counts) combined with estimation of residual correlations. The new approach allows for control of covariates of any kind (e.g., continuous, counts) by modeling the dependence of mean and variance on background variables. The proposed method was applied to identify the covariates that have a significant influence on elderly people’s functional abilities, and find the estimates for the correlation coefficients of residuals for MZ and DZ twins in a sample of 2401 Danish twin 75 years of age or older. The bootstrap method was used to obtain standard errors for correlation coefficients. It was shown, that the chosen covariates have similar effects on MZ and DZ twins, and that the residual correlation in MZ twins is significantly higher than in DZ twins, which indicates that genetic factors play an etiological role in the determination of physical status of elderly people, controlled for 10 background variables.

Gene–environment interaction (G × E) is likely to be a common and important source of variation for complex behavioral traits. Gene–environment interaction, or genetic control of sensitivity to the environment, can be incorporated into variance components twin and sib-pair analyses by partitioning genetic effects into a mean part, which is independent of the environment, and a part that is a linear function of the environment. An approach described in a companion paper (Purcell, 2002) is applied to sib-pair variance components linkage analysis in two ways: allowing for quantitative trait locus by environment interaction and utilizing information on any residual interactions detected prior to analysis. As well as elucidating environmental pathways, consideration of G × E in quantitative and molecular studies will potentially direct and enhance gene-mapping efforts.

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.

In studies on the Finnish Twin Cohort, we investigate genetic and environmental determinants of common, complex diseases, and their behavioral risk factors in Finland, a genetically unique and culturally homogenous population. We have formed the following databases: 1) Like-sexed twin pairs (13,888 pairs of known zygosity) form the older Twin Cohort. They have participated since 1975 in mail surveys, in clinical examinations for subsamples, and have been followed-up for morbidity using national medical registers; 2) The older Twin Cohort was expanded in 1996 to include opposite-sex pairs born 1938–1957 (c. 8000 pairs); 3) Two, new longitudinal studies of adolescent twins and their families, form a complementary, ongoing study base described in more detail in an accompanying article. Genetic and environmental effects vary over the life-span, and only longitudinal studies in genetically informative data sets permits the evaluation of such effects. Finally, the inclusion of DNA-based genetic information in a phenotypically rich family data base will offer a unique resource for research in genetic epidemiology and behavioral medicine.

While heritability studies show that most of the variance in adult personality can be attributed to genetic or so-called nonshared environmental influence, this does not mean that shared events lack importance for the development of later personality differences. We studied the relationship between Big Five personality differences in monozygotic (MZ) twins at age 29, and life stressors at age 6 to 15, using prospective data from 26 MZ pairs studied from birth onwards. A positive significant correlation was found between stressors in childhood and early adolescence, and intrapair personality differences in Agreeableness, Openness, Conscientiousness, and five-factor profiles. We note that the effects of shared events are labeled “nonshared” environment when the effect is to make siblings more different. Case examples illustrate the relationship between stress and personality differences, and provide hypotheses for further studies in larger samples.

Paternal influence on twinning was investigated through a study of all the state and church records of Scotland for the period 1800–2000 (nine generations) in relation to one Scottish patronymic — in total 50,000 births. All recorded twins born with the chosen patronymic were identified and their whole paternal ascent and descent on the male line were charted for twins. There were established three pedigrees A, B and C manifesting clear paternal twinning hereditary transmission. Detailed familial reproduction patterns were traced for pedigree A, including phenotypic identification of twin zygosity in relation to seven same sexed pairs of twins in the pedigree and one same sexed pair out of dizygotic triplets. It is the most comprehensive description to date of such a kind of twin familial trait. The data presented show (i) the unique feature of clear direct paternal influence on twinning in three families; (ii) paternal factor(s) determination in both DZ and MZ twinning; and (iii) a definite association of twinning tendency with a partial male infertility, which corresponds to the prediction of the Infertility/Twinning Paternally Dependent syndrome hypothesis. The hypothesis of a founder effect explaining the similarity of A, B, C families and the possible localisation of the paternally dependent twinning factor on the Y-chromosome are currently under molecular investigation.

The 2001 honoring of Russian twin researcher, Dr. Inna V. Ravich-Shcherbo, at the Tenth International Congress on Twin Studies (ISTS), in London, brought timely recognition to an international colleague. It also marked an occasion for reflecting on the course of twin studies in Russia. Historical trends and current accomplishments are examined with an eye toward future developments. Next, the distinguished careers of Russian monozygotic twins, Drs. Alexander and Andrew Fingelkurts, exemplify twin research findings on intelligence and occupational choice, and illuminate the status of twin studies and other scientific research in their country, are told. Their life histories are followed by the initially tragic, but ultimately heartwarming, story of young twins, Max and Andy, whose physical disabilities were overcome through the efforts of an empathic war veteran. The recent Moscow Summer School, the first in a three-time lecture series, encouraged crucial academic exchange among scientific investigators and students from around the world. Final thoughts are that much can be learned and much can be accomplished, given that we continue to nurture the twin-related resources available to us.

From early pregnancy into childhood, higher multiples have much higher rates of mortality, whether from spontaneous abortion, the ‘vanishing twin’ syndrome, fetal or infant death. Many parents must cope with the death of one baby whilst the siblings remain critically ill or later become disabled and yet there grief is often underestimated. Little is known about the long term feelings of parents who choose to have a multifetal pregnancy reduction (MFPR). Most say they made the right decision but also that there was insufficient respect for their loss. They are often anxious about what, if anything, to tell the survivors and how they might react. Long term follow-up studies of the children as well as the parents are needed. Meanwhile parents who chose to have a MFPR must be given more information and ongoing support.

Twin researchers from Russia have announced new opportunities for research collaboration at the Psychological Institute of the Russian Academy of Education, in Moscow. Members of the laboratory are also establishing Moscow's first Psychological Twins counseling room to facilitate practical activities, such as parental advisement and teacher training. Reference is also made to the work of twin researcher, Bronson Price, who spent several years at the Medico-Genetical Research Institute in Moscow. His papers are available at the library of the American Philosophical Society, in Philadelphia. Brief reviews of twin studies involving facial asymmetry and parental favoritism, social relationships and disputed memories are also presented. The article concludes with a look at several twin-related human interest items.

Family, twin and adoption studies have provided evidence for familial and genetic influences on individual differences in disease risk and in human behavior. Attempts to identify individual genes accounting for these differences have not been outstandingly successful to date, and at best, known genes account for only a fraction of the familiality of most traits or diseases. More detailed knowledge of the dynamics of gene action and of specific environmental conditions are needed. Twin and twin-family studies with multiple measurements of risk factors and morbidity over time can permit a much more detailed assessment of the developmental dynamics of disease risk and the unfolding of behavioral risk factors.

The Yoruba are an important ethnic group mainly occupying Southwestern Nigeria. Mainly for genetic reasons, this very large tribe happens to present the highest dizygotic twinning rate in the world (4.4 % of all maternities). The high perinatal mortality rate associated with such pregnancies has contributed to the integration of a special twin belief system within the African traditional religion of this tribe. The latter is based on the concept of a supreme deity called Olodumare or Olorun, assisted by a series of secondary gods (Orisha) while Yoruba religion also involves immortality and reincarnation of the soul based on the animistic cult of ancestors. Twins are therefore given special names and believed to detain special preternatural powers. In keeping with their refined artistic tradition, the Yoruba have produced numerous wooden statuettes called Ibejis that represent the souls of deceased newborn twins and are involved in elaborate rituals. Among Yoruba traditional beliefs and lore some twin-related themes are represented which are also found in other parts of the world. Basic features of the original Yoruba beliefs have found their way into the religious traditions of descendants of African slaves imported in the West Indies and in South America.

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 German Observational Study of Adult Twins (GOSAT) is the largest population-based observational twin study in Germany to date. Embedded in the Bielefeld Longitudinal Study of Adult Twins (BiLSAT), it addresses the etiology of personality, temperament and cognitive ability in a sample of 300 monozygotic (MZ) and dizygotic (DZ) adult twin pairs between 18 and 70 years of age. A major aim of the GOSAT lies in the utilization of different modes of measurement, (i.e., peer reports and observational data), in addition to self-reports which have been used predominantly in past behavioral genetic research on personality and temperament in adults. Participants completed a full day assessment at the University of Bielefeld including videotaped social interactions and presentations, psychometric intelligence tests and computerized elementary cognitive tasks as well as objective measures and unobtrusive behavior counts. The research design of the GOSAT was devised to reduce the potential impact of systematic rater bias on estimates of genetic and environmental influences to a minimum. In combination with extensive self-and peer report data on key personality and personality related dimensions available from the BiLSAT, the GOSAT provides a rich dataset, which currently includes DNA samples from 80% of its participants.