Twin Study Research Papers - Academia.edu (original) (raw)

This team takes the position that what is commonly referred to as nonfamilial Alzheimer's disease (AD) is predominantly due to genetic factors. There are many genetic factors that already have been clearly associated with AD. In particular, there are genes with high penetrance that cause AD in individuals with onsets below 60 years of age, although these autosomal dominant genes account for less than 3% of the cases. The genes that cause AD in subjects over 60 years of age have penetrance that does not produce as clear of a line of heritability. However, population-based studies suggest that genetic factors may cause the majority of cases that begin after age 60 as well. There are several lines of evidence supporting this position:

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- Cognitive Science, Risk, Population based study, Social Environment
- by Michael Lyons
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- Cognitive Science, Publishing, Attention-Deficit/Hyperactivity Disorder, Twins

Evidence is accumulating that developmental abnormalities, poor neuromotor function, and increased problem behavior precede the manifestation of schizophrenia itself. Information on school performance and behavioral development was obtained for 49 pairs of twins in which at least one twin suffered from schizophrenia (20 monozygotic [MZ], 29 dizygotic [DZ]) and for 43 pairs of healthy control twins (25 MZ, 18 DZ). Cox regression was used to analyze the contribution of schizophrenia, zygosity, sex, and year of birth to the age at which developmental divergence occurred. In both MZ and DZ twin pairs with schizophrenia, divergence in school performance occurred about 7.5 years earlier than it did in control twins, at 12 years of age, preceding the onset of psychosis by 10 years. This suggests that the first prodromal signs of schizophrenia manifest themselves as cognitive symptoms at the onset of puberty. Underperformance at school might therefore be considered one of the first signs of an until then latent vulnerability for schizophrenia.

- by Clarine van Oel
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- Schizophrenia, Child Behavior, Child, Cox Regression

Twin studies are useful designs for decomposing genetic and environmental influences on traits, with the basic goal being to map variations in phenotypic similarity onto variances in genetic similarity. In this review, we provide an overview of twin studies that have been used to decompose the genetic and environmental influences behind body composition, eating style, and their covariation. Although many published reports have established that genes contribute anywhere from 50-90% of the variance in body mass index (BMI, kg/m 2 ), fewer studies have used laboratory-based body composition measures. Moreover, comparable studies of human eating style are even more scarce.

- by Kathleen Keller
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- Genetics, Adolescent, Child, Body Composition

We examined the association between endogenous sex hormones (estradiol, estrone, testosterone, and SHBG) and coronary heart disease (CHD) in white male twins. Stored plasma samples were available for 566 participants of the National Heart, Lung, and Blood Institute Twin Study, a longitudinal study of cardiovascular disease in male twins. Twenty-eight of these individuals were lost to follow-up, and outcome data were missing. Of the remaining 538 participants, 78 had CHD at baseline, and 154 subsequently developed CHD over 20 yr of follow-up. We observed no differences in mean unadjusted or age-and body mass index-adjusted log-transformed sex hormone concentrations for participants with and without CHD (all P > 0.08). Quartile and median split analyses revealed no significant association between any of the sex hormones and either prevalent or incident CHD. The discordant monozygotic twins showed no significant case-control group difference in estradiol, estrone, testosterone, and SHBG (all P > 0.3). The positive and negative concordant twin pairs had similar values for each of the sex hormones (all P > 0.3). We observed no relationship between endogenous sex hormone concentrations and prevalent or incident CHD in this sample of male twins. (J Clin Endocrinol

- by Harold Javitz and +1
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- Sex Hormones, Cardiovascular disease, Testosterone, Clinical

Despite many studies on the prevalence of breakfast eating, we know little about factors that determine breakfast eating patterns. Our aim was to find out to which extent breakfast eating frequency is influenced by genetic and environmental factors using twin and twinfamily models in a population sample of 16-year-old twins (n ¼ 5250) and their parents (n ¼ 4663). In common effects sex-limitation models, additive genetic effects explained 41% (95% CI: 21-63%) of the variance in breakfast eating in girls and 66% (95% CI: 47-79%) in boys, and common environmental effects 45% (95% CI: 23-62%) in girls and 14% (95% CI: 5-29%) in boys. Of twin-family models, phenotypic assortment models fitted the data best. Heritability estimates increased somewhat (72%, 95% CI: 46-98% in girls and 63%, 95% CI: 38-89%) in boys. Common family environment remained substantial in both sexes. Cultural transmission was nonsignificant. The relative influence of genetic and family factors on adolescent breakfast eating frequency differs by sex and is generation-specific.

- by Anna Keski-Rahkonen and +1
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- Genetics, Psychology, Behavior Genetics, Adolescent
- by Sean Perrin
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- Genetics, Psychology, Cognitive Science, Anxiety Disorders

Human obesity is associated with greater-than-average energy intake, although relatively few studies have tested the heritability of food intake. The present study examined the genetic architecture of measured caloric intake during laboratory test meals in 36 monozygotic and 18 dizygotic twin pairs. A series of analyses tested the hypotheses that (1) there would be a genetic influence on total caloric intake,(2) there would be genes influencing total caloric intake above and beyond those influencing body composition,(3) ...

- by Michael Neale
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- Genetics, Psychology, Obesity, Behavior Genetics

Etiology of class-III malocclusion is generally believed to be genetic. A wide range of environmental factors have been suggested as contributing factors for the development of class-III malocclusion. Twin study is one of the most effective methods available for investigating genetically determined variables of malocclusion. Discordancy for class-III malocclusion is a frequent finding in dizygotic twins. However, class-III malocclusion discordancy in monozygotic twins is a rare finding. The purpose of this study of monozygotic twins is to assess the genetic and environmental components of variation within the cranio-dento-facial complex.

- by Hari Parkash
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- Genetics, Dentistry, Cephalometry, Twin Study

OBJECTIVE: This study investigated the etiology of autistic-like traits in the general population and the etiological overlap between the three aspects of the triad of impairments (social impairments, communication impairments, restricted... more

- by Tom Price
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- Genetics, Psychology, Communication Disorders, Medicine

Data from 4,029 male-male twin pairs who served in the United States military during the Vietnam era (1965Vietnam era ( -1975 were used to examine genetic and non-genetic factors that influence wartime exposure to traumatic events. Specific events examined were volunteering for service in Vietnam, actual service in Southeast Asia, a composite index of 18 combat experiences, and information from military records about being awarded combat decorations. Correlations within monozygotic (MZ) and dizygotic (DZ) twin pairs for volunteering for service in Vietnam were 0.40 and 0.22, respectively. For actually serving in Southeast Asia, the MZ correlation was 0.41 and the DZ correlation was 0.24. Analysis of twin pairs in which both siblings served in Southeast Asia (n = 820) demonstrated a correlation for self-reported combat experiences within MZ and DZ pairs of 0.53 and 0.30, respectively. Heritability estimates ranged from 35 to 47%. The family environment did not have a significant effect on any of the variables. Analyses of data from military records regarding being awarded a combat decoration provided very similar results to those found for self-reported combat experiences. 0 1993 Wiley-Liss, Inc.

- by Michael Lyons
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- Genetics, Psychopathology, United States, American

Objective: To answer two questions about the nature of the relationship between anorexia nervosa (AN) and dimensional temperament traits: Which traits are comorbid with AN? Which traits share transmitted liabilities with AN? Methods: A community sample of 1002 same-gender female twins was selected with respect to participation in two earlier waves of data collection. Measures of eating disorder diagnoses and features were ascertained through interview and continuous measures of temperament were ascertained from self-report measures. Results: Four temperaments were comorbid with AN, namely, higher levels of perfectionism (concern over mistakes, personal standards, doubt about actions), and higher need for organization. Comparison between the female co-twins of AN probands and controls (who had never had an eating disorder) showed that the former group reported higher levels of personal standards, organization, and reward dependence. The association between personal standards and reward dependence remained when controlling for the temperament of the proband or control in monozygotic twins. Conclusions: The evidence overall supports the suggestion that AN may represent the expression of a common underlying familial liability to a temperament style that reflects a striving for perfectionism, a need for order, and a sensitivity to praise and reward. The nature of the shared risk factors is likely to be, in part, genetic.

- by Tracey Wade
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- Personality, Australia, Adolescent, Linear models

Introduction. Cannabis use is strongly associated with the use and abuse/dependence of other illicit drugs. Gateway and common liabilities models have been employed to explain this relationship. We sought to examine this association using a combination of the discordant twin design and modeling methods.Method. We assess the relationship between early cannabis use and the subsequent use and abuse/dependence of other illicit drugs in a population-based sample of male and female twin pairs using four analyses: (i) analysis of the association between early cannabis use and other illicit drug use and abuse/dependence in the entire sample of twins, (ii) assessment of the influence of early cannabis use in twin 1 on twin 2's use or abuse/dependence of other illicit drugs, (iii) use of twin pairs discordant for early cannabis use in a discordant twin design and (iv) a model-fitting procedure.Results. We found: (i) a strong association between early cannabis use and use and abuse/depende...

- by Michael Neale
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- Psychology, Risk, Causality, Psychological Medicine

Background. Prior research on the nature of the vulnerability of neuroticism to psychopathology suggests biases in information processing towards emotional rather than neutral information. It is unclear to what extent this relationship... more

- by FrÜhling Rijsdijk
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- Genetics, Psychology, Cognition, Attention
- by Michael Neale
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- Genetics, Psychology, Bipolar Disorder, Behavior Genetics

Three different measures of the Big Five personality dimensions were developed from the battery of questionnaires used in the National Merit Twin Study: one from trait self-rating scales, one from personality inventory items, and one from an adjective check list. Behavior-genetic models were fit to what the three measures had in common, and to the variance distinctive to each. The

- by John Loehlin and +1
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- Psychology, Cognitive Science, Personality, Behavior Genetics

Epidemiological studies continue to provide insights into the prevalence of back pain and have identified many individual, psychosocial, and occupational risk factors for its onset. Psychological factors have an important role in the transition from acute to chronic pain and related disability. Recent advances show that there is a significant genetic effect on severe low back pain in the community. Data emerging from candidate gene studies show an association between lumbar disc disease and mutations of genes encoding the a-2 and a-3 subunits of collagen IX. Summary Back pain is among the most common conditions for which patients seek medical care. Interventions based on behavioral and cognitive principles and exercise programs are effective in improving disability in chronic back pain. Although progress has been made in understanding the role of genetic mutations in disorders such as lumbar disc disease, further investigation of the interaction between genetic and environmental factors such as physical stress is needed.

- by Nisha Manek
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- Genetics, Rheumatology, Developing Countries, Low back pain

Background. Familial factors, which are partly genetic, influence risk for phobias. Prior family and twin studies, however, were based on a single lifetime assessment, which may be only moderately reliable. Methods. We obtained, 8 years apart, two assessments of lifetime history of five unreasonable fears and phobias (agoraphobia and social, situational, animal and blood-injury phobia) from face-toface and telephone interviews from 1708 individual female twins from a population-based registry. We also obtained, 1 month apart, test-retest reliability on 192 twins. We fitted, using the program Mx, a measurement model that estimates the role of genetic and environmental risk factors correcting for measurement error. Results. Short-term reliability of the five phobias was modest (mean κ l 0n46), but higher than longterm stability (mean κ l 0n30). Unreliability occurred both for subject recall of unreasonable fears and for interviewer assessment of which fears constituted phobias. Examining fears and phobias together, in a multiple threshold model, results suggested that twin resemblance was due solely to genetic factors, with estimated total heritabilities, corrected for unreliability, of : any 43 %, agoraphobia 67 %, animal 47 %, blood\injury 59 %, situational 46 % and social 51 %. With the exception of animal phobia, similar results were obtained analysing phobias alone. Conclusions. Lifetime histories of unreasonable fears and phobias assessed at personal interview have substantial unreliability. Correcting for unreliability, the liability to fears and their associated phobias is moderately heritable. Individual-specific environmental experiences play an important role in the development of phobias, while familial-environmental factors appear to be of little aetiological significance.

- by Laura Thornton
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- Genetics, Psychology, Fear, Psychological Medicine

Background: The Dutch project" Calibration 2000" aims at harmonization of laboratory results via calibration by development of commutable, matrix-based, secondary reference materials. An alternative approach to the NCCLS EP14... more

- by Christa Cobbaert
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- Algorithms, Clinical Chemistry, Medical Biotechnology, Clinical

Recently, the nature of personality disorders and their relationship with normal personality traits has received extensive attention. The five-factor model (FFM) of personality, consisting of the personality traits neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness, is one of the proposed models to conceptualize personality disorders as maladaptive variants of continuously distributed personality traits.

- by Catherine Derom and +1
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- Genetics, Personality, Borderline Personality Disorder, International Cooperation

The present study examined the patterns and sources of 10-year stability and change of adult personality assessed by the 5 domains and 30 facets of the Revised NEO Personality Inventory. Phenotypic and biometric analyses were performed on data from 126 identical and 61 fraternal twins from the Bielefeld Longitudinal Study of Adult Twins (BiLSAT). Consistent with previous research, LGM analyses revealed significant mean-level changes in domains and facets suggesting maturation of personality. There were also substantial individual differences in the change trajectories of both domain and facet scales. Correlations between age and trait changes were modest and there were no significant associations between change and gender. Biometric extensions of growth curve models showed that 10-year stability and change of personality were influenced by both genetic as well as environmental factors. Regarding the etiology of change, the analyses uncovered a more complex picture than originally stated, as findings suggest noticeable differences between traits with respect to the magnitude of genetic and environmental effects.

- by Wiebke Bleidorn
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- Marketing, Genetics, Psychology, Cognitive Science

Quantitative studies of retinal vascular caliber using new computer-assisted retinal imaging systems have allowed physicians and researchers to understand the influence of systemic, environmental, and genetic factors on retinal vascular caliber. Retinal vascular caliber changes reflect cumulative response to aging, cardiovascular risk factors, inflammation, nitric oxide--dependent endothelial dysfunction, and other processes. Recent epidemiological studies have shown that changes in retinal arteriolar and venular caliber size may reflect the differential effects of a range of systemic, environmental, and genetic risk factors. Narrower retinal arteriolar caliber and smaller arteriovenous ratio are associated with older age; higher levels of past, current, and future blood pressure and obesity; and predict the incidence of diabetes and coronary heart disease. Wider retinal venular caliber, in contrast, is associated with younger age; impaired fasting glucose and diabetes; dyslipidemia; obesity; systemic marker of inflammation, endothelial dysfunction, and cigarette smoking; and predicts the risk of stroke and coronary heart disease. New data from family and twin studies indicate a significant genetic contribution to retinal vascular caliber, an area that is under investigation. Elucidating the complete range of systemic, environmental, and genetic factors linked with retinal vascular caliber changes may provide critical insight into the etiology, pathogenesis, and natural history of early vascular disease not only in the eye but elsewhere in the body. (Surv Ophthalmol 54:74--95, 2009. Ó 2009 Elsevier Inc. All rights reserved.) Key words. arteriovenous ratio generalized arteriolar narrowing genetics hypertension pathophysiology retinal imaging retinal arteriolar caliber retinal vascular caliber retinal venular caliber retinal venular dilation 74

- by Cong Sun
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- Genetics, Natural History, Blood Pressure, Nitric oxide

Twin studies have been a valuable source of information about the genetic basis of complex traits. To maximize the potential of twin studies, large, worldwide registers of data on twins and their relatives have been established. Here, we provide an overview of the current resources for twin research. These can be used to obtain insights into the genetic epidemiology of complex traits and diseases, to study the interaction of genotype with sex, age and lifestyle factors, and to study the causes of co-morbidity between traits and diseases. Because of their design, these registers offer unique opportunities for selected sampling for quantitative trait loci linkage and association studies.

- by Andreas Busjahn
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- Genetics, Genetic Epidemiology, Association study, Opportunity for Selection

Previous twin studies attempting to assess the origins of psychopathic personality traits have mainly focused on an overt behavioral conceptualization of the syndrome as defined by a history of chronic antisocial behaviors. This investigation instead focused on a personality-based approach which emphasizes maladaptive personality traits as central to the syndrome. Psychopathic traits were indexed by the Psychopathic Personality Inventory (PPI),

- by Daniel Blonigen and +1
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- Genetics, Psychology, Cognitive Science, Personality and Individual Differences

The purpose of this study was to validate and cross-validate the Beunen-Malina-Freitas method for non-invasive prediction of adult height in girls. A sample of 420 girls aged 10-15 years from the Madeira Growth Study were measured at yearly intervals and then 8 years later. Anthropometric dimensions (lengths, breadths, circumferences, and skinfolds) were measured; skeletal age was assessed using the Tanner-Whitehouse 3 method and menarcheal status (present or absent) was recorded. Adult height was measured and predicted using stepwise, forward, and maximum R 2 regression techniques. Multiple correlations, mean differences, standard errors of prediction, and error boundaries were calculated. A sample of the Leuven Longitudinal Twin Study was used to cross-validate the regressions. Age-specific coefficients of determination (R 2 ) between predicted and measured adult height varied between 0.57 and 0.96, while standard errors of prediction varied between 1.1 and 3.9 cm. The cross-validation confirmed the validity of the Beunen-Malina-Freitas method in girls aged 12-15 years, but at lower ages the cross-validation was less consistent. We conclude that the Beunen-Malina-Freitas method is valid for the prediction of adult height in girls aged 12-15 years. It is applicable to European populations or populations of European ancestry.

- by Albrecht Claessens
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- Anthropometry, Adolescent, Child, Regression Analysis

Age-related macular degeneration (AMD) is the leading cause of vision loss and blindness among older adults in the USA and throughout the developed world. Etiological research suggests that AMD is a complex disease, caused by the actions and interactions of multiple genes and environmental factors. Familial aggregation studies, twin studies, and segregation analyses have provided strong evidence for the heritability

- by Susan Santangelo
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- Genetics, Macular Degeneration, Association study, Genetics of complex disease

Background. Twin research has consistently shown substantial genetic influence on individual differences in cognitive ability; however, much less is known about the genetic and environmental aetiologies of school achievement.

- by Frank Spinath
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- Psychology, Intelligence, Individuality, British

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.

- by Thomas Bouchard
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- Genetics, Cognitive Science, Personality, Personality Assessment

This study examined whether universality of the 5-factor model (FFM) of personality operationalized by the Revised NEO Personality Inventory is due to genetic influences that are invariant across diverse nations. Factor analyses were conducted on matrices of phenotypic, genetic, and environmental correlations estimated in a sample of 1,209 monozygotic and 701 dizygotic twin pairs from Canada, Germany, and Japan. Five genetic and environmental factors were extracted for each sample. High congruence coefficients were observed when phenotypic, genetic, and environmental factors were compared in each sample as well as when each factor was compared across samples. These results suggest that the FFM has a solid biological basis and may represent a common heritage of the human species.

- by Frank Spinath and +1
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- Marketing, Genetics, Psychology, Cognitive Science

Background: Individuals with early warning signs of life-long psychopathy, callous-unemotional traits (CU) and high levels of antisocial behaviour (AB) can be identified in childhood. We report here the first twin study of high levels of psychopathic tendencies in young children. Methods: At the end of the first school year, teachers provided ratings of CU and AB for 3687 twin pairs from the Twins Early Development Study (TEDS). For the analyses of extreme CU, we selected same-sex twin pairs where at least one twin scored 1.3 or more standard deviations above the mean on the CU scale (612 probands, 459 twin pairs). For the analysis of extreme AB, we selected same-sex twin pairs where at least one twin scored 1.3 or more standard deviations above the mean on AB scale (444 probands, 364 twin pairs). Furthermore, the extreme AB sample was divided into those who were also extreme on CU (children with psychopathic tendencies; 234 probands, 187 twin pairs) and those who did not score in the extreme for CU (children without psychopathic tendencies; 210 probands, 177 twin pairs). Results: DeFries-Fulker extremes analysis indicated that exhibiting high levels of CU is under strong genetic influence. Furthermore, separating children with AB into those with high and low levels of CU showed striking results: AB in children with high levels of CU is under extremely strong genetic influence and no influence of shared environment, whereas AB in children with low levels of CU shows moderate genetic and shared environmental influence. Conclusions: The remarkably high heritability for CU, and for AB children with CU, suggests that molecular genetic research on antisocial behaviour should focus on the CU core of psychopathy. Our findings also raise questions for public policy on interventions for antisocial behaviour.

- by Essi Viding
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- Genetics, Psychology, Cognitive Science, Anti Social Behaviour

Two issues that complicate behavioral genetic analyses are the interaction and correlation between genetic and environmental influences. In this report, the effects of genotype-environment interaction and correlation on behavioral genetic studies (twin and adoption studies) are examined. The analysis suggests that genotype-environment interaction may bias twin study estimates of genetic and environmental influence, but need not affect adoption studies. On the other hand, genotype-environment correlation may affect both twin and adoption study estimates of genetic and environmental influence, the direction of the effect depending on the sign of the correlation. Finally, new tests of genotypeenvironment interaction and correlation, using adoption data, are proposed.

- by John Loehlin
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- Marketing, Genetics, Psychology, Cognitive Science

No twin study has previously investigated the whole range of personality disorders (PDs) recorded by interviews. Based on twin and patient registries, 92 monozygotic (MZ) and 129 dizygotic (DZ) twin pairs were interviewed with the Structured Clinical Interview for DSM-III-R Personality Disorders (SCID-II). Observed prevalence rates from a normal population study of more than 2,000 individuals were used in combination with data from the present study to generate statistics assumed to be valid for a normal twin population, and these statistics were used for structural equation modeling. The best-fitting models had a heritability of .60 for PDs generally, .37 for the eccentric (A) cluster, .60 for the emotional (B) cluster, and .62 for the fearful (C) cluster. Among the specific PDs, the heritability appeared to be .79 for narcissistic, .78 for obsessivecompulsive, .69 for borderline, .67 for histrionic, .61 for schizotypal, .57 for dependent, .54 for self-defeating, .29 for schizoid, .28 for paranoid, and .28 for avoidant PDs. The best-fitting models never included shared-infamilies environmental effects. However, a model with only shared familial and unique environmental effects could not be ruled out for dependent PD. Shared familial environmental effects may also influence the development of any PD and borderline PD. Passiveaggressive PD did not seem to be affected by genes or family environment at all. The low occurrence of antisocial PD in the twin sample precluded any model for this disorder. PDs seem to be more strongly influenced by genetic effects than almost any axis I disorder, and more than most broad personality dimensions. However, we observed a large variation in heritability among the different PDs, probably partly because of a moderate sample size and low prevalence of the specific disorders.

- by Ingunn Skre
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- Personality Disorders, Obsessive-Compulsive Disorder, Norway, Personality Disorder

Background: Fear conditioning is a traditional model for the acquisition of fears and phobias. Studies of the genetic architecture of fear conditioning may inform genefinding strategies for anxiety disorders. The objective of this study was to determine the genetic and environmental sources of individual differences in fear conditioning by means of a twin sample.

- by Peter Annas
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- Genetics, Fear, Family, Multivariate Analysis

The adult form of attention deficit/hyperactivity disorder (aADHD) has a prevalence of up to 5% and is the most severe long-term outcome of this common neurodevelopmental disorder. Family studies in clinical samples suggest an increased familial liability for aADHD compared with childhood ADHD (cADHD), whereas twin studies based on self-rated symptoms in adult population samples show moderate heritability estimates of 30-40%. However, using multiple sources of information, the heritability of clinically diagnosed aADHD and cADHD is very similar. Results of candidate gene as well as genome-wide molecular genetic studies in aADHD samples implicate some of the same genes involved in ADHD in children, although in some cases different alleles and different genes may be responsible for adult versus childhood ADHD. Linkage studies have been successful in identifying loci for aADHD and led to the identification of LPHN3 and CDH13 as novel genes associated with ADHD across the lifespan. In addition, studies of rare genetic variants have identified probable causative mutations for aADHD. Use of endophenotypes based on neuropsychology and neuroimaging, as well as next-generation genome analysis and improved statistical and bioinformatic analysis methods hold the promise of identifying additional genetic variants involved in disease etiology. Large, international collaborations have paved the way for well-powered studies. Progress in identifying aADHD risk genes may provide us with tools for the prediction of disease progression in the clinic and better treatment, and ultimately may help to prevent persistence of ADHD into adulthood.

The concept of schizophrenia (SZ) (nee dementia praecox) has been widely used in medicine for the last 100 years. However, major controversies concerning the construct have yet to be resolved. The traditional categorical nosology of functional psychoses is challenged by observations that SZ, schizoaffective disorder (SAD), major depression (MDD) and bipolar disorder (BPD) share clinical presentations, endophenotypes and several genes. The present overview presents various theoretical frameworks for categorical and dimensional models, and specifically their applicability to the fields of epidemiology, genetic epidemiology, genetics and endophenotype studies. It should be noted that clinical dimensions, candidate genes and endophenotypes have not been found to be specific to any one type of functional psychosis. Clinical syndromes, including depression, anxiety, and substance use disorders, co-occur with SZ, SAD, MDD and BPD at appreciable rates. Genetic linkage studies have primarily focused on the phenotype of functional psychoses (SZ, SAD, MDD, and BPD) susceptibility. To date, however, relatively limited work has been conducted to identify the genetic variants associated with symptom dimensions. While the potential advantages of an endophenotype based approach are widely appreciated in the investigation of the genetics of functional psychoses, there is no consensus for achieving this goal. Overlapping endophenotype processes include physiological or electrophysiological anomalies, psychological or neurocognitive deficits and biochemical alterations. Specific challenges need to be addressed in the future if we hope to move forward in our goal to reach meaningful and applicable clinical results. We conclude that we need: (a) a new concept for functional psychoses in order to develop a new classification for research purposes, (b) new and improved clinical assessment tools, (c) to target persons with functional psychoses; and (d) to conduct molecular genetic studies using a number of candidate genes and endophenotypes with measured symptom dimensions and patterns.

- by Irving Gottesman
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- Genetics, Bipolar Disorder, Working Memory, Learning and Memory

Hardiness Personality Big five theory of personality Twin study a

- by Philip Vernon
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- Genetics, Psychology, Cognitive Science, Personality

Twin studies are a crucial source of information about genetic and environmental influences on general intelligence. A brief summary of key events and controversies in the history of twin studies of intelligence is followed by a review of the contributions of twin studies to the field thus far. We then discuss new twin research designs, analytic methods, findings and their implications. Topics include evidence of genetic and environmental influences on general intelligence from studies of twins and non-twins, twins reared apart and together, virtual twins, the significance of shared environmental factors, the effects of prenatal environments, the impact of parenting practices, epigenetic processes (e.g., DNA methylation), the heritability of relevant endophenotypes, and the search for specific genes underlying intelligence. Links between twin studies and other research areas, both within and outside behavioral genetics, are explored.

- by Wendy Johnson
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- Genetics, Behavior Genetics, Twin Study

The present study is the first behavioral genetic investigation of the Dark Triad traits of personality, consisting of Machiavellianism, narcissism, and psychopathy, and the variable of mental toughness, reflecting individual differences in the ability to cope when under pressure. The purpose of this investigation was to explore a potential explanation for the success of individuals exhibiting the Dark Triad traits in workplace and social settings. Participants were adult twins who completed the MACH-IV, the Narcissistic Personality Inventory, and the Self-Report Psychopathy Scale assessing Machiavellianism, narcissism, and psychopathy, respectively, as well as the MT48, measuring mental toughness. Correlational analyses of the data revealed significant positive phenotypic associations between mental toughness and narcissism. Psychopathy and Machiavellianism, however, both showed some significant negative phenotypic correlations with mental toughness. Bivariate behavioral genetic analyses of the data were conducted to assess the extent to which these significant phenotypic correlations were attributable to common genetic and/or common environmental factors. Results indicate that correlations between narcissism and mental toughness were attributable primarily to common non-shared environmental factors, correlations between Machiavellianism and mental toughness were influenced by both common genetic and common non-shared environmental factors, and the correlations between psychopathy and mental toughness were attributable entirely to correlated genetic factors. Implications of these findings in the context of etiology and organizational adaptation are discussed.

- by Michael Onley and +1
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- Cognitive Science, Personality Disorders, Adolescent, Twins

This study used 239 twin pairs from a volunteer community sample to investigate how anxious and avoidant attachment are related to personality disorder (PD). Factor analysis showed that self-reported anxious attachment and 11 PD scales from the Dimensional Assessment of Personality Problems loaded onto one factor (emotional dysregulation), and avoidant attachment and four PD scales loaded onto a second factor (inhibitedness). Biometric models indicated that 40% of the variance in anxious attachment was heritable, and 63% of its association with corresponding PD dimensions was attributable to common genetic effects. Avoidant attachment was influenced by the shared environment instead of genes. Correlations between avoidance and corresponding PD dimensions were attributable to experiences in the nonshared environment that influenced both variables.

- by Phillip Shaver
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- Psychology, Personality Disorder, European, Twin Study
- by Andrea Burri
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- Genetics, Twins, Sexual Medicine, Sexual Behavior

The equal environments assumption suggests that the environment does not contribute more strongly to the similarity of monozygotic (MZ) compared to dizygotic (DZ) twins. It was tested in 525 MZ, 200 samesex DZ, and 68 opposite-sex DZ twin pairs. Almost 80% of the twins were female, and their age varied widely around a mean of 32 years. Similarity of twin environments was assessed by a self-report questionnaire on childhood experiences, and personality resemblance was measured using the self-report and peer-report versions of the NEO-Five-Factor Inventory. MZ twins reported more similar experiences than DZ twins and were also more alike in personality. Across twin pairs, however, treatment similarity was unrelated to personality resemblance, except in the combined group of MZ and DZ twins. These results are consistent with the equal-environments assumption. # Quantitative behavior-genetics accounts for higher resemblance between monozygotic (MZ) compared to dizygotic (DZ) twins in terms of genetic influence on the phenotype. According to the basic twin model, additive as well as interactive effects of genes correlate perfectly for MZ twins but 0.50 (additive gene effects) or 0.25 (effects of gene dominance) for DZ twins, whereas 0191-8869/02/$ -see front matter #

- by Frank Spinath
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- Genetics, Psychology, Cognitive Science, Behavior Genetics

To determine the degree of genetic and environmental influences on assessments of aggression and irritability in male subjects, the "Motor Aggression" subscales of the Buss-Durkee Hostility Inventory (BDHI) were mailed to 1208 male twins in the Vietnam Era Twin Registry. Data from monozygotic 182 and 118 dizygotic twin pairs were available and were analyzed using model-fitting procedures. Three of the four BDHI subscales demonstrated significant heritability of a nonadditive nature: 40% for Indirect Assault, 37% for Irritability, and 28% for Verbal Assault. Additive genetic variance accounted for 47% of the individual differences in Direct Assault. Nonshared, but not shared, environmental influences contributed to explaining the variance in the model, with estimates ranging from 53% (Direct Assault) to 72% (Verbal Assault). Because some of these BDHI scales have been shown to correlate with indices of central serotonin function, it is possible that impulsive aggression, as reflected by these scales, is heritable in men.

- by Alesha Seroczynski and +1
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- Genetics, Biological Sciences, Biological Psychiatry, Biological

Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are both highly heritable neurodevelopmental disorders. Evidence indicates both disorders co-occur with a high frequency, in 20-50% of children with ADHD meeting criteria for ASD and in 30-80% of ASD children meeting criteria for ADHD. This review will provide an overview on all available studies [family based, twin, candidate gene, linkage, and genome wide association (GWA) studies] shedding light on the role of shared genetic underpinnings of ADHD and ASD. It is concluded that family and twin studies do provide support for the hypothesis that ADHD and ASD originate from partly similar familial/genetic factors. Only a few candidate gene studies, linkage studies and GWA studies have specifically addressed this co-occurrence, pinpointing to some promising pleiotropic genes, loci and single nucleotide polymorphisms (SNPs), but the research field is in urgent need for better designed and powered studies to tackle this complex issue. We propose that future studies examining shared familial etiological factors for ADHD and ASD use a family-based design in which the same phenotypic (ADHD and ASD), candidate endophenotypic, and environmental measurements are obtained from all family members. Multivariate multi-level models are probably best suited for the statistical analysis.

It has been reported that the ability to solve syllogisms is highly g-loaded. In the present study, using a self-administered shortened version of a syllogism-solving test, the BAROCO Short, we examined whether robust findings generated by previous research regarding IQ scores were also applicable to BAROCO Short scores. Five syllogism-solving problems were included in a questionnaire as part of a postal survey conducted by the Keio Twin Research Center. Data were collected from 487 pairs of twins (1021 individuals) who were Japanese junior high or high school students (ages 13–18) and from 536 mothers and 431 fathers. Four findings related to IQ were replicated: 1) The mean level increased gradually during adolescence, stayed unchanged from the 30s to the early 50s, and subsequently declined after the late 50s. 2) The scores for both children and parents were predicted by the socioeconomic status of the family. 3) The genetic effect increased, although the shared environmental effect decreased during progression from adolescence to adulthood. 4) Children's scores were genetically correlated with school achievement. These findings further substantiate the close association between syllogistic reasoning ability and g.► Performance on five syllogism-solving problems included in a simple self-administered questionnaire using postal survey replicates typical characteristics for g. ► The mean level and genetic and environmental structures for syllogism solving change systematically according to age. ► Scores for syllogism solving are predicted by SES of the family. ► Scores for syllogism solving are genetically correlated with school achievements.

- by Kai Hiraishi
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- Genetics, Psychology, Cognitive Science, Intelligence

To clarify the role of genetic and environmental risk factors in alcohol use disorders (AUDs), we performed a meta-analysis of twin and adoption studies and explored the impact of sex, assessment method (interview v. hospital/population records), and study design (twin v. adoption study) on heritability estimates. The literature was searched for all unique twin and adoption studies of AUD and identified 12 twin and five adoption studies. The data were then reconstructed and analyzed using ordinal data full information maximum likelihood in the OpenMx program. Heterogeneity was tested with likelihood ratio tests by equating the parameters across studies. There was no evidence for heterogeneity by study design, sex or assessment method. The best-fit estimate of the heritability of AUD was 0.49 [95% confidence interval (CI) 0.43-0.53], and the proportion of shared environmental variance was 0.10 (95% CI 0.03-0.16). Estimates of unique environmental proportions of variance differed sign...

- by Michael Neale
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- Genetics, Psychology, Psychological Medicine, Meta Analysis

Objective: Twin methodology has been used to delineate etiological factors in many medical disorders and behavioral traits including eating disorders. Although twin studies are powerful tools, their methodology can be arcane and their implications easily misinterpreted. Method: The goals of this study are to (a) review the theoretical rationale for twin studies; (b) provide a framework for their interpretation and evaluation; (c) review extant twin studies on eating disorders; and (d) explore the implications for understanding etiological issues in eating disorders. Discussion: On the basis of this review, it is not possible to draw firm conclusions regarding the precise contribution of genetic and environmental factors to anorexia nervosa. Twin studies confirm that bulimia nervosa is familial and reveal significant contributions of additive genetic effects and of unique environmental factors in liability to bulimia nervosa. The magnitude of the contribution of shared environment is less clear, but in the studies with the greatest statistical power, it appears to be less prominent than additive genetic factors.

- by Tracey Wade
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- Eating Disorders, Anorexia Nervosa, Social Environment, Risk factors

and sharing with colleagues.

- by Constantine Sedikides
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- Psychology, Cognitive Science, Personality, Behavior Genetics

We conducted a longitudinal-biometric study examining stability and change in personality from ages 17 to 24 in a community sample of male and female twins. Using Tellegen's (in press) Multidimensional Personality Questionnaire (MPQ), facets of Negative Emotionality (NEM) declined substantially at the mean and individual levels, whereas facets of Constraint (CON) increased over time. Furthermore, individuals in late adolescence who were lowest on NEM and highest on CON remained the most stable over time, whereas those exhibiting the inverse profile (higher NEM, lower CON) changed the most in a direction towards growth and maturity. Analyses of gender differences yielded greater mean-level increases over time for women as compared to men on facets of CON and greater mean-level increases for men than women on facets of Agentic Positive Emotionality (PEM). Biometric analyses revealed rank-order stability in personality to be largely genetic, with rank-order change mediated by both the nonshared environment (and error) as well as genes. Findings correspond with prior evidence of a normative trend toward growth and maturity in personality during emerging adulthood.

- by Daniel Blonigen
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- Psychology, Cognitive Science, Personality, Personality Assessment

Analyzing data from Loehlin and Nichols's (1976) classic twin study, we computed measures of Masculine Instrumentality (M), Feminine Expressiveness (F), and Gender Diagnosticity (GD). Quantitative genetic modeling analyses of within-sex individual differences in M, F, and GD indicated that: (1) Additive genetic factors contribute significantly to individual differences in M, F, and GD. (2) The environmental effects on M, F, and GD tend to be nonshared. (3) The genetic and environmental components of individual differences in M, F, and GD tend not to show gender differences. Finally, (4) the estimated within-sex heritability of GD (.53) is significantly greater than the estimated within-sex heritabilities of either M (.36) or F (.38). Psychologists have tried to measure within-sex gender-related individual differences for over half a century. From 1936 to the early 1970s, such individual differences were conceptualized as falling along a bipolar

- by Scott Hershberger
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- Genetics, Psychology, Cognitive Science, Personality