The collection of papers in this special edition of Twin Research and Human Genetics represents a major land-mark at the intersection of behavioral genetics and political science. This issue is the fruit of 20 political scientists attending the Behavioral Genetics Association Methods Workshop in Boulder and a hands-on training practicum at the Virginia Institute for Psychiatric and Behavioral Genetics, and includes results from the first wave of political science twin surveys.

Using data from the Minnesota Twins Political Survey, this paper tests for the presence of unequal environments (EEA) by zygosity in political attitudes. Equal environment measures used include shared bedroom, friends, classes, and dressing alike. Results show an EEA violation at p < .05 in 5% of the models tested. The violations' impact on heritability estimates and their confidence levels appear random in magnitude and direction. No reasonable post hoc explanation emerged for understanding the presence of the violation in some items but not others. This article establishes reasonable priors for the absence of EEA violations in political phenotypes based on the tested environmental components. The findings place the burden on critics to present theoretical work on the specific mechanisms of EEA violations based on which additional empirical assessments could (and should) be conducted.

Interest in politics is important for a host of political behaviors and beliefs. Yet little is known about where political interest comes from. Most studies exploring the source of political interest focus on parental influences, economic status, and opportunity. Here, we investigate an alternative source: genetic transmission. Using two twin samples, one drawn from Denmark and the other from USA, we find that there is a high degree of heritability in political interest. Furthermore, we show that interest in politics and political efficacy share the same underlying, latent genetic factor. These findings add to the growing body of literature that documents political behaviors and attitudes as not simply the result of socialization, but also as part of an individual's genetically informed disposition.

Recent studies have shown that trusting attitudes and behavior are biologically influenced. Focusing on the classic trust game, it has been demonstrated that oxytocin increases trust and that humans are endowed with genetic variation that influences their behavior in the game. Moreover, several studies have shown that a large share of the variation in survey responses to trust items is accounted for by an additive genetic component. Against this backdrop, this article makes two important contributions. First, utilizing a unique sample of more than 2,000 complete Swedish twin pairs, we provide further evidence of the heritability of social trust. Our estimates of the additive genetic component in social trust were consistent across the sexes – .33 for males and .39 for females – and are similar to the results reported in earlier studies. Secondly, we show that social trust is phenotypically related to three psychological traits – extraversion, personal control, and intelligence – and that genetic factors account for most of these correlations. Jointly, these psychological factors share around 30% of the genetic influence on social trust both for males and females. Future studies should further explore the possible causal pathways between genes and trust using panel data on both psychological traits and social trust.

Political sophistication is a concept that encompasses political reasoning, the coherence of people's issue attitudes, and their knowledge of political processes. To what extent is political sophistication affected by genes and environments? Do these distinct but related measures of sophistication share a common genetic structure? We analyze survey data collected from participants in the Minnesota Twin Registry to estimate influences of genes and environments on variables used to measure political sophistication. Additive genetic factors explain 48–76% of the variation in educational attainment, political interest, and political knowledge, while dominance genetics influence 28% of the variance of ideological consistency. Multivariate analyses show that, although these measures share common genetic and unique environmental factors to a modest extent, much of the variance is explained by specific genetic and unique environmental factors. Ideological consistency appears to be mostly distinct from the other measures, as it is strongly accounted for by unique environmental influences.

Traditional theories of political participation have emphasized its social and normative components; however, recent evidence suggests that political participation has a strong additive genetic component. Accordingly, it is necessary to reevaluate the traditional empirical findings and begin to integrate the two approaches. To do so, a variety of analyses were conducted that explore the modes of genetic and environmental transmission. I find that the empirical estimates of the relationships between the phenotypic level traits are highly consistent with what has been found in the traditional literature, the vast majority of the variance in political participation that is accounted for by the predictor variables is being accounted for at the genetic level, and not at the environmental level. Thus, the current findings suggest that the empirical results found in prior studies are quite likely very accurate. However, the interpretation of those empirical findings, as well as the subsequent theoretical implications, require serious revision.

Attitudes towards foreign policy have typically been explained by ideological and demographic factors. We approach this study from a different perspective and ex amine the extent to which foreign policy preferences correspond to genetic variation. Using data from the Minnesota Twin Family Study, we show that a moderate share of individual differences in the degree to which one's foreign policy preferences are hawkish or dovish can be attributed to genetic variation. We also show, based on a bivariate twin model, that foreign policy preferences share a common genetic source of variation with political ideology. This result presents the possibility that ideology may be the causal pathway through which genes affect foreign policy preferences.

This paper examines the association between individuals' beliefs that the world is a dangerous place and their support for a variety of national security policies. We find that the source of the covariance between perceived danger and support for aggressive national security policies is primarily due to a common genetic factor. Latent genetic factors that influence individuals' perception of danger also appear to influence their positions on policies purported to alleviate such danger. Covariation between individuals' experiences and genes suggests that priming messages alone do not drive the covariation between feelings of danger and acceptance of policy changes.

The preponderance of research on the study of ethnocentrism has primarily attributed such attitudes to learned behavior. The research here advances the argument that both socialization and genetic inheritance contribute to the development of ethnocentric attitudes and behavior. This analysis employs the Minnesota Twins Political Survey data consisting of 596 complete twin pairs. Using the classical twin design, we employed structural equation modeling to model the covariance of twins in regards to additive genetic effects, shared environmental effects, and unique environmental effects (i.e., the classic ACE model). The findings reveal that genetic inheritance is significant in explaining the variance in genetic attitudes. Specifically, genetic inheritance explains 18% of the variance, with the overwhelming 82% being explained by the unique environment.

We compare a recent Danish twin survey on political attitudes and behaviors to a nationally representative survey covering similar topics. We find very similar means and variances for most of our constructed scales of political attitudes and behaviors in the two surveys, although even small differences tend to be statistically significant due to sample size. This suggests that the twin study can be used to make inferences on the heritability of several political traits in the Danish population.

Cervical cancer is the third most common cancer in women worldwide. Persistent infection with an oncogenic human papillomavirus (HPV) is necessary, but not sufficient, for its development. Over many years, only a small proportion of women with chronic HPV infection progress to develop disease. The role of host genes and environmental factors in the pathogenesis of, or predisposition to, cervical cancer is still unclear. We conducted a systematic review of published literature in MEDLINE–PubMed to identify studies of cervical cancer susceptibility that used a twin study design. We used standard MeSH terms (controlled vocabulary) as well as specific free-text terms and combinations of terms related to cervical cancer, with no restriction on publication date. We performed a full text review to ensure the identified articles met our inclusion criteria and, if so, extracted information on demographics, sample size, study definitions, and key statistical findings. Of the 285 articles identified, three utilized a classical twin design and reported results specific to cervical cancer. The studies were based on cancer registry data from Scandinavia, with sample sizes ranging from 312 to 710 twin pairs. The findings from one study were consistent with a genetic mechanism for the causation of carcinoma in situ. Future research studies using the strength of the classic twin design, together with incorporation of HPV DNA status, are indicated to determine whether there is a potential role for genetic factors in the development of cervical cancer or high-grade cervical dysplasia from chronic oncogenic HPV infection.

Although it is well established that experience seeking behavior (ES) is positively related to cognitive functioning, the mechanisms underlying this association are not clearly understood. In a large sample of adult twins and siblings (N = 864, age range 23–75), we studied the causes of covariation between ES and general cognitive ability and we studied whether ES moderates the genetic and environmental causes of variation in general cognitive ability. Results demonstrate a phenotypic correlation of .17 (p <.001) between general cognitive ability and ES, with a common genetic and common environmental background. Moreover, the extent to which genetic and environmental factors are shared between general cognitive ability and ES is increased in individuals with either lower or higher levels of ES. In addition, the extent to which genetic and environmental factors influence individual differences in general cognitive ability in adults partly depended on ES. Standardized influences of additive genetic factors on general cognitive ability ranged from 13% to 99%, with lower estimates in higher levels of ES, while standardized estimates of environmental factors ranged from almost 1% to 87%, with higher estimates in higher levels of ES. Hence, ES and cognitive ability are not only associated through common genetic and environmental factors, but also via moderating effects of genetic and environmental influences on cognitive ability by ES. These findings have implications for future studies on the association between ES and general cognitive ability, and for future research on the genetics of cognitive ability.

There has been almost no overlap between behavior genetics and consumer behavior research, despite each field's importance in understanding society. In particular, both have neglected to study genetic influences on consumer adoption and usage of new technologies — even technologies as important as the mobile phone, now used by 5.8 out of 7.0 billion people on earth. To start filling this gap, we analyzed self-reported mobile phone use, intelligence, and personality traits in two samples of Australian teenaged twins (mean ages 14.2 and 15.6 years), totaling 1,036 individuals. ACE modeling using Mx software showed substantial heritabilities for how often teens make voice calls (.60 and .34 in samples 1 and 2, respectively) and for how often they send text messages (.53 and. 50). Shared family environment – including neighborhood, social class, parental education, and parental income (i.e., the generosity of calling plans that parents can afford for their teens) — had much weaker effects. Multivariate modeling based on cross-twin, cross-trait correlations showed negative genetic correlations between talking/texting frequency and intelligence (around –.17), and positive genetic correlations between talking/texting frequency and extraversion (about .20 to .40). Our results have implications for assessing the risks of mobile phone use such as radiofrequency field (RF) exposure and driving accidents, for studying adoption and use of other emerging technologies, for understanding the genetic architecture of the cognitive and personality traits that predict consumer behavior, and for challenging the common assumption that consumer behavior is shaped entirely by culture, media, and family environment.

The main aim of this study was to examine twin specific risk factors that influence educational achievement in primary school. We included prenatal factors that are not unique to twins, except for zygosity, but show a higher prevalence in twins than in singletons. In addition, educational achievement was compared between twins and their nontwin siblings in a within-family design. Data were obtained from parents and teachers of approximately 10,000 twins and their nontwin siblings registered with the Netherlands Twin Register. Teachers rated the proficiency of the children on arithmetic, language, reading, and physical education, and reported a national educational achievement test score (CITO). Structural equation modeling showed that gestational age, birth weight, and sex were significant predictors of educational achievement, even after correction for socioeconomic status. Mode of delivery and zygosity did not have an effect, while parental age only influenced arithmetic. Mode of conception, incubator time, and birth complications negatively affected achievement in physical education. The comparison of educational achievement of twins and singletons showed significantly lower ratings on arithmetic, reading, and language in twins, compared to their older siblings, but not compared to their younger siblings. Low gestational age and low birth weight were the most important risk factors for lower educational achievement of twins in primary school. It seems that the differences observed between twins and their nontwin siblings in educational achievement can largely be explained by birth order within the family.

The augmented multiple regression model for the analysis of data from selected twin pairs was extended to facilitate analyses of data from twin pairs and nontwin siblings. Fitting this extended model to data from both selected twin pairs and siblings yields direct estimates of heritability (h2) and the difference between environmental influences shared by members of twin pairs and those of sib or twin–sib pairs (i.e., c2(t) – c2 (s)). When this model was fitted to reading performance data from 293 monozygotic and 436 dizygotic pairs selected for reading difficulties, and 291 of their nontwin siblings, h2 = .48 ± .22, p = .03, and c2 (t) – c2 (s) = .22 ± .12, p = .06. Although the test for differential shared environmental influences is only marginally significant, the results of this analysis suggest that environmental influences on reading performance that are shared by members of twin pairs (.36) may be substantially greater than those for less contemporaneous twin–sibling pairs (.14).

Familial hemiplegic migraine (FHM) is a rare autosomal dominant subtype of migraine with aura. It is divided into three subtypes FHM1, FHM2 and FHM3, which are caused by mutations in the CACNA1A, ATP1A2 and SCN1A genes respectively. As part of a regular diagnostic service, we investigated 168 patients with FHM symptoms. Samples were tested for mutations contained within the CACNA1A gene. Some tested samples (4.43%) showed an FHM1 mutation, with five of the mutations found in exon 5, one mutation in exon 16 and one in exon 17. Four polymorphisms were also detected, one of which occurred in a large percentage of samples (14.88%). The exon 16 2094G>A polymorphism, however, has been found to occur in healthy Caucasian control populations up to a frequency of 16% and is not considered to be significantly associated with FHM. A finding of significance, found in a single patient, was the detection of a novel mutation in exon 5 that results in a P225H change. The affected individual was an 8-year-old female. The exact phenotypic effect of this mutation is unknown, and further studies are needed to understand the pathophysiology of this mutation in FHM1. New information will allow for diagnostic procedures to be constantly updated, thus improving accuracy of diagnosis. It is possible that new information will also aid the development of new therapeutic agents for the treatment of FHM.

Mutations in the thyroglobulin (TG) gene, which has an estimated incidence of approximately 1 in 100,000 new-borns, cause autosomal recessive congenital hypothyroidism. The mutational spectrum of the TG gene and the phenotype–genotype correlations have not yet fully been established. We report a compound heterozygous mutation in the TG gene in a Chinese twin family with congenital goiter and hypothyroidism. We also describe the gene mutation associated with the genotype–phenotype of these children with congenital goiter and hypothyroidism. The whole coding sequence of the TG gene was analyzed by direct sequence, and the identified changes in the sequence were tested for benign polymorphism by denaturing high-performance liquid chromatography screening of the mutation and sequencing 200 chromosomes from normal controls. Analysis of the TG gene of the affected twin revealed a compound heterozygous mutation, including a novel missense mutation G2687A, which is predicted to result in a glutamine to arginine substitution at codon 877, and a known nonsense mutation C7006T, predicted to result in an arginine to stop codon at codon 2317. Analysis of 200 normal chromosomes did not identify the same change in healthy subjects. This is the first report of a TG gene mutation in the Chinese Han population. Our study provides further evidence that mutations in the TG gene cause congenital goiter and hypothyroidism, demonstrates genetic heterogeneity of the mutation, and increases our understanding of phenotype–genotype correlations in congenital hypothyroidism.

Mechanisms underlying effects of physical factors on twin births are unclear. The present work studied an association between monthly and yearly multiple birth rates (MBRs) in the Novosibirsk region, south-west Siberia, in relation to solar activity (sunspot numbers) and geomagnetic activity (Ap index) from 1957 to 2008. The difference was verified by comparing the yearly MBR observed in 3-year peaks (M = 7.36, SD = 0.33 per 1,000 births) and 3-year troughs (M = 8.10, SD = 0.31 per 1,000 births, p < .001) of an 11-year solar cycle. An inverse correlation (r = –.60, p < .001) was found between sunspot numbers and MBR for a lag of 1 year. Cross-spectral analysis of a 52-year time series established a common signal with the period of 10.5 years, as well as high coherence (K(2) = 0.87). The multiple regression analysis revealed a significant interaction of solar and geomagnetic effects upon the frequency of twin maternities. The results show that elevated solar activity within the 11-year cycle coinciding with the time of conception inhibits multiple births in a manner depending on geomagnetic activity. It is hypothesized that the likely mechanism underlying the association is early fetal loss induced by solar radiation/flux and its terrestrial mediators.

The First International Twins Conference held in the Czech Republic (CR) took place in Prague on October 20–21, 2011. The conference, ‘Life With Twins’, was arranged under the auspices of the Parliament's Commission for Family and Equal Opportunities. The setting was an auditorium in one of the Parliament's magnificent old buildings. Mrs. Klara Vitkova Rulikovà, mother of 18-year-old opposite-sex twins, chairperson of the Czech multiple birth association (CAKDAV) and member of the International Council of Multiple Birth Organizations (ICOMBO), was instrumental in organizing the meeting. Mrs. Vitkova Ruliková has also authored three books on twins and parenting, and was responsible for writing the Epilogue for my 2005 book, Indivisible by Two: Lives of Extraordinary Twins (Ned litelnà Dvěma: životy Výjime ných Dvojǎt), which was translated into Czech by Triton Press in 2009. She is currently working in the Ministry of Labour and Social Affairs. She is shown in Figure 1 addressing the conference.