Several studies report apparent sibling contrast effects in analyses of twin resemblance. In the presence of genetic differences, contrast effects reduce the dizygotic (DZ) twin correlation relative to that in monozygotic (MZ) twins and produce higher DZ than MZ variance. Explanations of contrast effects are typically cast in terms of direct social interaction between twins or an artifact of the process of rating children by their parents. We outline a model for sibling imitation and contrast effects that depends on social interaction between parents and children. In addition to predicting the observed pattern of twin variances and covariances, the parental mediation of child imitation and contrast effects leads to differences in the variance of parents of MZ and DZ twins and differences between the correlations of parents with their MZ and DZ children.

All etiological studies of complex human traits focus on analyzing the causes of variation. Given this complexity, there is a premium on studying those processes that mediate between gene products and cellular or organismal phenotypes. Studies of levels of gene expression could offer insight into these processes and are likely to be especially useful to the extent that the major sources of their variation are known in normal tissues. The classical study of monozygotic (MZ) and dizygotic (DZ) twins was employed to partition the genetic and environmental influences in gene expression for over 6500 human genes measured using microarrays from lymphoblastoid cell lines. Our results indicate that mean expression levels are correlated about .3 in monozygotic (MZ) and .0 in dizygotic (DZ) twins suggesting an overall epistatic regulation of gene expression. Furthermore, the functions of several of the genes whose expression was most affected by environmental effects, after correction for measurement error, were consistent with their known role in mediating sensitivity to environmental influences.

Genome-wide linkage analysis using multiple traits and statistical software packages is a tedious process which requires a significant amount of manual file manipulation. Different linkage analysis programs require different input file formats, making the task of analyzing data with multiple methods even more time-consuming. We have developed a software tool, AUTOGSCAN, that automates file formatting, the running of statistical analyses, and the summarizing of resulting statistics for whole genome scans with a push of a button, using several independent, and often idiosyncratic, statistical software packages such as MERLIN, SOLAR and GENEHUNTER. We also describe a program, ANALYZE, designed to run qualitative linkage analysis with several different statistical strategies and programs to efficiently screen for linkage and linkage disequilibrium for a given discrete trait. The ANALYZE program can also be used by AUTOGSCAN in a genome-wide sense.

The choice of where to live would appear to be determined by a combination of economic constraints and personal preferences. We have tested how far this choice is affected by the continuing effects of the environment shared within families, and genetic variation between people, using data from twin studies conducted in Australia. The addresses provided by study participants were categorized as urban, suburban and nonurban, and data were analyzed in three adult age groups. There were significant effects of both shared environment and genes, and the balance between them was affected by both sex and age. Shared environment accounted for some 50% of variation in the youngest group, but only about 10% in the oldest. As shared environmental effects decreased, additive genetic effects increased. These results have implications for internal migration of people within countries and, over the long term, for gene flow within and between populations. They may also be pertinent to the different prevalences of certain psychiatric diseases between city and country locations. Comparisons between countries with different demography are needed to confirm and further characterize these effects.

This study reports on genetic and environmental influences on the frequency of orgasm in women during sexual intercourse, during other sexual contact with a partner, and during masturbation. Participants were drawn from the Australian Twin Registry, and recruited from a large, partly longitudinal twin-family study. Three thousand and eighty women responded to the anonymous self-report questionnaire, including 667 complete monozygotic (MZ) pairs and 377 complete dizygotic (DZ) same-sex pairs, 366 women from complete DZ opposite-sex pairs, and 626 women whose co-twins did not participate. Significant twin correlations were found for both MZ and DZ twin pairs for all three items of interest. Age effects were statistically significant for some items. Models incorporating additive genetic, shared and nonshared environmental influences provided the best fit for Items 1 and 3, while a model with additive and nonadditive genetic influences along with nonshared envir-onment fitted the data from Item 2. While an independent pathway model fits the data most par-simoniously, a common pathway model incorporating additive genetic (A), shared environment (C), and unique environment (E) effects cannot be ruled out. Overall, genetic influences account for approximately 31% of the variance of frequency of orgasm during sexual intercourse, 37% of the variance of frequency of orgasm during sexual contact other than during intercourse, and 51% of the variance of frequency of orgasm during masturbation. Following Baker (1996), we speculate that this additive genetic variance might arise from frequency-dependent selection for a variety of female sexual strategies.

Previous longevity studies of related individuals such as twins or siblings based on the major gene model have shown that the frequency and the relative risk of mortality of a beneficial allele in the population could be estimated. If, in addition to survival data for related individuals, the genetic markers data are available, one could try to locate the longevity allele in the genome. In the case where the phenotypic trait is life span or age at onset of disease, a two-step procedure can be used. First, the parameters of bivariate survival functions must be estimated from bivariate survival data for twins without markers. The second step is focused on determining the position of longevity genes between respective markers. To calculate the joint distribution of inheritance vector and genetic markers, the hidden Markov chain technique is used. This approach is illustrated with a simulation example for one longevity gene.

Monozygotic (MZ) twins may be discordant for complex traits due to differential environmental exposure in utero, epigenetic variability in imprinting, X chromosome inactivation, or stochastic effects. Occasionally MZ twins may be discordant for chromosomal and single gene disorders due to somatic mosaicism. For complex traits, which are due to the interactive effects of multiple genes and environmental factors, the affected twin of a discordant MZ pair offers the possibility for identifying somatic mutations in candidate genes. DNA sequencing of candidate genes in discordant MZ twins can identify those rare etiologic mutational events responsible for the different phenotypes since the confounding effects of common single nucleotide polymorphisms are eliminated, as DNA sequences should be identical in MZ pairs. In this report we describe the extensive DNA sequencing of 18 candidate genes in a sample of MZ and dizygotic (DZ) twins with nonsyndromic cleft lip with or without cleft palate. We were unable to identify any somatic differences in approximately 34 Kb of DNA sequenced in 13 MZ pairs, for a total of approximately 900 Kb of sequence comparisons, supporting the hypothesis that nonetiologic posttwinning mutations are rare. While no etiologic variants were identified in this study, sequence comparisons of discordant MZ twins can serve as a tool for identifying etiologic mutations in clefting and other complex traits.

The aim of the study was to assess the heritability as well as the personality correlates of sociopolitical attitudes in a Polish sample. Attitudes were examined on two separate dimensions: (1) moral conservatism and liberalism; and (2) free market economy and state interventionism. The studied sample comprised three groups of participants: (1) 242 twin pairs aged 18 to 25 years (119 monozygotic [MZ] and 123 dizygotic [DZ] same-sex twins reared together); (2) 358 females and males (‘nontwins’) of various professions aged 35 to 64 years; and (3) 197 male and female technical university students aged 19 to 25 years (‘nontwins’). We detected substantial genetic factor input exclusively in the variability of results on the conservatism–liberalism scale (.28). Results on both scales were explained to a large extent by the impact of shared environment, and to a lesser extent by the nonshared environment. Only in the student group was a significant negative correlation found between attitudes toward economy and neuroticism, extraversion, agreeableness and conscientiousness. Openness to experience correlated negatively with attitudes towards issues of morality in the adult female and male group (the remaining correlations were statistically insignificant).

Cause-specific mortality data on Danish monozygotic and dizygotic twins are used to analyze heritability estimates of susceptibility to coronary heart disease (CHD) after controlling for smoking and Body Mass Index (BMI). The sample includes 1209 like-sexed twin pairs born between 1890 and 1920, where both individuals were still alive in 1966. The participants completed a questionnaire in 1966 which included questions on smoking, height and weight. The analysis was conducted with both sexes pooled due to the relatively small number of twin pairs. Follow-up was conducted from January 1, 1966 to December 31, 1993. The correlated gammafrailty model with observed covariates was used for the genetic analysis of frailty to account for censoring and truncation in the lifetime data. During the follow-up, 1437 deaths occurred, including 435 deaths due to CHD. Proportions of variance of frailty attributable to genetic and environmental factors were analyzed using the structural equation model approach. Different standard biometric models are fitted to the data to evaluate the magnitude and nature of genetic and environmental factors on mortality. Using the best-fitting model without covariates, heritability of frailty to CHD was found to be 0.45 (0.11). This result changes only slightly to 0.55 (0.13) in a DE model after controlling for smoking and BMI. This analysis underlines the existence of a substantial genetic influence on individual frailty associated with mortality caused by CHD.

Several large studies have demonstrated that the liability to smoke cigarettes is strongly genetically influenced. However, the role of genetic and environmental risk factors in the use of other common forms of tobacco use has yet to be studied. Data on the regular use of cigarettes, cigars, pipes, dip (moist snuff), and chewing tobacco from 2634 male twins were analyzed with ACE structural equation models. Twin similarity for regular cigarette and dip use was largely genetic in origin. However, twin resemblance for chewing tobacco was just about equally the result of genes and shared environment, and twin similarity for use of pipes and cigars was entirely the result of shared environmental factors. Thus, the genetic influences on the liability for regular tobacco use appear to vary based on tobacco type. The causes for the use of different forms of tobacco are complex and worthy of further study.

The purpose of this study is to compare obstetric and birthweight data of twin children from the two largest databases in Japan to estimate the difference between them in sample collection. The first group consisted of 1131 twin-pair school applicants, and the second group consisted of members' children from several maternal associations devoted to twins and included 951 pairs. All data were gathered by questionnaire. The mean birth years of the twins in these two databases were 1979 and 1995 respectively. The percentage of mothers treated with ovulation-stimulating drugs or in-vitro fertilization was markedly higher in the maternal associations group. Gestational age was around 1 week less in the maternal associations group, whereas birthweight according to gestational weeks and intrapair relative birth weight difference as a percentage according to zygosity showed little difference between both groups. We conclude that the obstetric and birthweight feature data from both groups should be considered to construct twin growth charts based on the methods of sample selection.

The purpose of this study was to clarify the genetic contribution to stuttering and tics in childhood using the largest databases of Japanese twins. The subjects were 1896 pairs of twin children consisting of 1849 males and 1943 females with a mean age of 11.6 years (3 years to 15 years). All data were gathered by questionnaire. The prevalence of stuttering was 6.7% in males and 3.6% in females (p < .0001). The prevalence of tics was 6.8% in males and 4.1% in females (p = .0021). Concordance rates and polychoric correlations were all higher in monozygotic pairs than in dizygotic pairs irrespective of sex combination. Structural equation modeling showed that the proportion of total phenotypic variance attributable to genetic influences was 80% in males and 85% in females for stuttering, and 28% in males and 29% in females for tics. Moreover, co-occurrence between stuttering and tics was observed in 0.8% of males (tetrachoric correlation: r = .18) and 0.5% of females (r = .31), which was attributed partly (nearly 10% of total genetic variance of each trait) to the common genetic factors, with genetic correlation of r = .32.

The present study investigates the twinning rate trends in South Korea for the years 1981 to 2002 utilizing the birth record data from the South Korea National Statistical Office. The twinning rates between 1981 and 1991 remained nearly constant and were slightly less than 10 twin individuals, that is, approximately five pairs per thousand births. Since the early 1990s, however, the twinning rate has increased sharply and reached 19.30 twin individuals, that is, around 10 pairs per thousand births in the year 2002. Application of the Weinberg method to birth data for the years 2000 to 2002 revealed that the dizygotic twin rate in South Korea increased almost threefold between 1981 to 1991 and 2002. In the 1980s the effect of maternal age on twinning rates appeared to be minimal. In the 1990s, however, increases in twinning rates occurred more markedly among older mothers than among younger mothers. We speculate that the rapid rise in twinning rates in South Korea in the 1990s is probably attributable to the spread of Assisted Reproductive Technologies among older mothers who seek treatments for infertility. The present study also examined whether residing in industrial areas is associated with multiple births in the South Korean population. The results did not support the recent finding of higher twinning incidence in industrialized regions.

Research comparing the auditory systems of opposite-sex co-twins, homosexual females and bisexual females is reviewed. Suggestions for how the findings may apply to other populations are provided. Next, recent twin studies of disputed memories and male sexual behaviors are summarized, as are recent events of scientific interest to twin researchers. Finally, a tribute to the late Russian twin researcher Dr. Inna Vladimirovna Ravich-Scherbo is presented.