Twin–singleton differences in intelligence?

Negative association between anterior insula activation and resilience during sustained attention: an fMRI twin study

BACKGROUND

While previous studies have suggested that higher levels of cognitive performance may be related to greater wellbeing and resilience, little is known about the associations between neural circuits engaged by cognitive tasks and wellbeing and resilience, and whether genetics or environment contribute to these associations.

METHODS

The current study consisted of 253 monozygotic and dizygotic adult twins, including a subsample of 187 early-life trauma-exposed twins, with functional Magnetic Resonance Imaging data from the TWIN-E study. Wellbeing was measured using the COMPAS-W Wellbeing Scale while resilience was defined as a higher level of positive adaptation (higher levels of wellbeing) in the presence of trauma exposure. We probed both sustained attention and working memory processes using a Continuous Performance Task in the scanner.

RESULTS

We found significant negative associations between resilience and activation in the bilateral anterior insula engaged during sustained attention. Multivariate twin modelling showed that the association between resilience and the left and right insula activation was mostly driven by common genetic factors, accounting for 71% and 87% of the total phenotypic correlation between these variables, respectively. There were no significant associations between wellbeing/resilience and neural activity engaged during working memory updating.

CONCLUSIONS

The findings suggest that greater resilience to trauma is associated with less activation of the anterior insula during a condition requiring sustained attention but not working memory updating. This possibly suggests a pattern of 'neural efficiency' (i.e. more efficient and/or attenuated activity) in people who may be more resilient to trauma.

Early Life Socioeconomic Differences in Associations between Childhood Sleep and Academic Performance

Poor sleep can negatively impact children's academic performance. However, it is unknown whether early-life socioeconomic status (SES) moderates later sleep and academics. We tested associations between actigraphy-based sleep duration and midpoint time, and parent-reported sleep problems with objective and subjective measures of academic performance. We also examined whether relations varied by early and concurrent SES. Children (n=707; 52% female; M _age=8.44 years; 28.7% Hispanic/Latinx; 29.7% at/below poverty line) were assessed at 12 months for SES and eight years for SES, sleep, and academics. There were no main effects of sleep on academics. More sleep problems predicted lower Applied Problems performance for low SES children (b=-.73, p<.05) and better performance for high SES children (b=.69, p<.05). For high SES children, greater sleep problems (b=-.11, p<.05) and longer sleep duration (b=-.11, p<.05) predicted lower academic achievement. However, most associations were consistent across SES, illustrating the complex interplay between sleep, academic outcomes, and SES.

School Readiness and Achievement in Early Elementary School: Moderation by Students' Temperament

The goals of this study were to examine the longitudinal relations between school readiness and reading and math achievement and to test if these relations were moderated by temperament. The sample included socio-economically and ethnically diverse twins (N=551). Parents reported on school readiness when children were five years old. Teachers reported on temperament (effortful control, anger, and shyness) three years later. Standardized measures of reading and math were obtained when children were eight years old. Effortful control and shyness moderated the effect of school readiness on reading. Prediction of reading from school readiness was strongest when students were high in effortful control and low in shyness. Effortful control and shyness predicted math beyond school readiness. There were no relations involving anger. Findings demonstrate that temperament can potentiate the relations between school readiness and reading and highlight the importance of promoting school readiness and effortful control, while decreasing shyness.

Residential green space and child intelligence and behavior across urban, suburban, and rural areas in Belgium: A longitudinal birth cohort study of twins

BACKGROUND

Exposure to green space has beneficial effects on several cognitive and behavioral aspects. However, to our knowledge, no study addressed intelligence as outcome. We investigated whether the level of urbanicity can modify the association of residential green space with intelligence and behavior in children.

METHODS AND FINDINGS

This study includes 620 children and is part of the East Flanders Prospective Twin Survey (EFPTS), a registry of multiple births in the province of East Flanders, Belgium. Intelligence was assessed with the Wechsler Intelligence Scale for Children-Revised (WISC-R) in 620 children (310 twin pairs) between 7 and 15 years old. From a subset of 442 children, behavior was determined based on the Achenbach Child Behavior Checklist (CBCL). Prenatal and childhood residential addresses were geocoded and used to assign green space indicators. Mixed modeling was performed to investigate green space in association with intelligence and behavior while adjusting for potential confounding factors including sex, age, parental education, neighborhood household income, year of assessment, and zygosity and chorionicity. We found that residential green space in association with both intelligence and behavior in children was modified by the degree of urbanicity (p < 0.001). In children living in an urban environment, multivariable adjusted mixed modeling analysis revealed that an IQR increment of residential green space (3,000-m radius) was associated with a 2.6 points (95% CI 1.4-3.9; p < 0.001) higher total intelligence quotient (IQ) and 2.0 points (95% CI -3.5 to -0.4; p = 0.017) lower externalizing behavioral score. In children residing in a rural or suburban environment, no association was found. A limitation of this study is that no information was available on school location and the potential for unmeasured confounding (e.g., time spend outdoors).

CONCLUSIONS

Our results indicate that residential green space may be beneficial for the intellectual and the behavioral development of children living in urban areas. These findings are relevant for policy makers and urban planners to create an optimal environment for children to develop their full potential.

Is there something special about twin families? A comparison of parenting styles in twin and non-twin families

Twin comparisons offer a powerful quasi-experimental design to study the impact of the family of origin on children's life chances. Yet, there are concerns about the generalizability of results obtained from twin studies because twin families are structurally different and twins have a genetic resemblance. We examine these concerns by comparing mothers' reports on their parenting styles for twin and non-twin children between twin and non-twin families, as well as within twin families. We use two German studies for our comparisons: TwinLife and pairfam. Our results demonstrate that twins receive more differential treatment and more emotional warmth than non-twins; however, these differences are largely accounted for by age differences between children. Overall, our results indicate that results on parenting obtained from twin studies can be generalized to non-twin families.

The Nigerian Twin and Sibling Registry: An Update

Here we provide an update of the 2013 report on the Nigerian Twin and Sibling Registry (NTSR). The major aim of the NTSR is to understand genetic and environmental influences and their interplay in psychological and mental health development in Nigerian children and adolescents. Africans have the highest twin birth rates among all human populations, and Nigeria is the most populous country in Africa. Due to its combination of large population and high twin birth rates, Nigeria has one of the largest twin populations in the world. In this article, we provide current updates on the NTSR samples recruited, recruitment procedures, zygosity assessment and findings emerging from the NTSR.

Close relationships and depression: A developmental cascade approach

Previous research has shown that problematic parent-child, peer, and romantic partner relationships are associated with an increased likelihood for major depressive disorder (MDD). Less research has evaluated the developmental unfolding of how these interpersonal relationship features are both an antecedent versus a consequence of MDD symptoms from adolescence through young adulthood. These gaps were evaluated using a large community sample (N = 1,127; 54% female, 96% white) via a developmental cascade model. Results showed support for significant antecedent effects, as greater parent-child relationship problems at ages 11 and 17 predicted rank-order increases in MDD symptoms at ages 14 and 20. Supporting a developmental cascade of problematic social relationships, greater parent-child relationship problems at ages 11 and 14 also predicted greater subsequent rank-order increases in antisocial peer affiliation at ages 14 and 17. Greater affiliation to antisocial peers at age 20 predicted greater rank-order increases in romantic relationship problems at age 24, which in turn predicted greater MDD symptoms at age 29. Cross-effects were generally small (βs ≤ .16), illustrating other factors may be relevant to the development or consequences of MDD. Nonetheless, findings support the importance of efforts to strengthen social support networks to offset risk as well as potentially treat depression.

Neurocognitive function of 10-year-old multiples born less than 28 weeks of gestational age

INTRODUCTION

Few studies have examined the relationship between birth plurality and neurocognitive function among children born extremely preterm.

STUDY DESIGN

We compared rates of Z-scores ≤-2 on 18 tests of neurocognitive function and academic achievement at age 10 years in 245 children arising from twin pregnancies, 55 from triplet pregnancies, and 6 from a septuplet pregnancy to that of 568 singletons, all of whom were born before the 28th week of gestation.

RESULTS

In total, 874 children were evaluated at the age of 10 years. After adjusting for confounders, children of multifetal pregnancies performed significantly better on one of six subtests of executive function than their singleton peers. Performance was similar on all other assessments of intelligence, language, academic achievement, processing speed, visual perception, and fine motor skills.

CONCLUSION

We found no evidence that children born of multifetal pregnancies had worse scores than their singleton peers on assessments of neurocognitive and academic function.

Intrauterine environment and cognitive development in young twins

Intrauterine factors important for cognitive development, such as birth weight, chorionicity and umbilical cord characteristics were investigated. A total of 663 twin pairs completed the Wechsler Intelligence Scale for Children-Revised and scores were available for Performance, Verbal and Total Intelligence Quotient (IQ). The intrauterine factors examined were birth weight, placental weight and morphology, cord knots, cord length and cord insertion. IQ scores for the varying levels of the intrauterine markers adjusting for gender and gestational age were calculated. The heritability of IQ and the association between IQ and intrauterine environment were examined. Twins with lower birth weight and cord knots had lower IQ scores. The aetiology of IQ is largely distinct from that of birth weight and cord knots, and non-shared environment may influence the observed relationships.

Twins Reunited: Scientific and Personal Perspectives/Twin Research Studies: Multiple Birth Effects on IQ and Body Size; Life Style, Muscles, and Metabolism; Monochorionic Dizygotic Twin with Blood Chimerism; Amniocentesis for Twins/Twins in the Media: Identical Doctors; Freedom Fighter for Twins; Twin Scholarships; Auguste and Jean-Felix Piccard; Twins Born Apart

A reunion of 38-year-old female monozygotic twins took place in Daegu, South Korea, on January 14, 2014. Scientific and personal perspectives on this extraordinary event are provided. A review of timely twin research follows, covering the effects of multiple births on IQ and body size, lifestyle and physical fitness associations, a rare case of a dizygotic twin with blood chimerism and definitional issues surrounding amniocentesis-related loss in multiple birth pregnancies. Interesting and informative mention of twins in the media includes twin doctors, a twin freedom fighter, the availability of college scholarships for twins, a new book about the Piccard family (two of whose members were twins), and co-twins born before and after the new year. A follow-up to a previous mention of identical twin biatheletes is also provided.

Twin–Singleton Differences in Cognitive Abilities in a Sample of Africans in Nigeria

Recent studies comparing cognitive abilities between contemporary twins and singletons in developed countries have suggested that twin deficits in cognitive abilities no longer exist. We examined cognitive abilities in a sample of twins and singletons born recently in Nigeria to determine whether recent findings can be replicated in developing countries. Our sample consisted of 413 pairs of twins and 280 singletons collected from over 45 public schools in Abuja and its neighboring states in Nigeria. The ages of twins and singletons ranged from 9 to 20 years with a mean (SD) of 14.6 years (2.2 years) for twins and 16.1 years (1.8 years) for singletons. Zygosity of the same-sex twins was determined by analysis of 16 deoxyribonucleic acid markers. We asked participants to complete a questionnaire booklet that included Standard Progressive Matrices-Plus Version (SPM+), Mill-Hill Vocabulary Scale (MHV), Family Assets Questionnaire, and demographic questions. The data were corrected for sex and age and then analyzed using maximum likelihood model-fitting analysis. Although twins and singletons were comparable in family social class indicators, singletons did better than twins across all the tests (d = 0.10 to 0.35). The average of d for SPM+ total [0.32; equivalent to 4.8 Intelligence Quotient (IQ) points] and d for MHV (0.24; equivalent to 3.6 IQ points) was 0.28 (equivalent to 4.2 IQ points), similar to the twin–singleton gap found in old cohorts in developed countries. We speculate that malnutrition, poor health, and educational systems in Nigeria may explain the persistence of twin deficits in cognitive abilities found in our sample.

Twin Olympians: the hard science and the lives behind it

A look at scientific studies of sports performance is informative in view of the 2012 Olympic games in London. The athletic life histories of selected twin competitors supplement the scientific data. Reviews of recent twin studies concerning cardiometabolic risk factors, postnatal brain maturation, parental responses to rearing triplets, and the birth of an unusual chimaeric twin pair are presented. General interest items include the separate careers of twin soccer players, an unexpected identical triplet birth, the interesting life of twin Geoffrey D. Langlands, and the Lone Twin Boat Project.

Twin-singleton differences in intelligence: a register-based birth cohort study of Norwegian males

The aim was to determine the difference in intelligence between singletons and twins in young adulthood. Data from the Medical Birth Register of Norway were linked with register data from the Norwegian National Conscript Service. The study base consisted of data on the 445,463 males who were born alive in either single or twin births in Norway during 1967-1984 and who were examined at the time of the mandatory military conscription (age 18-20). Within this study base, there were data on 1,653 sibships of full brothers that included at least one man born in single birth and at least one man born in twin birth (4,307 persons, including 2,378 twins and 1,929 singletons). The intelligence scores of the singletons were 11% (95% confidence interval [CI]: 9-14%) of a standard deviation higher than those of the twins, after adjustment for birth year, birth order, parental ages at delivery, parental education levels, and other factors. The adjusted within-family difference was also 11% (95 % CI: 6-16%) of a standard deviation, indicating that unmeasured factors shared by siblings (e.g., maternal body height) have not influenced the estimate in important ways. When gestational age at birth was added to the model, the estimate for the difference in intelligence score was approximately the same. Including birth weight in the model strongly reduced the estimate. In conclusion, twins born in Norway during 1967-1984 had slightly lower intelligence in early adulthood compared with the singletons.

A Comparison of Educational Achievement in a National Sample of Dutch Female Twins and Their Matched Singleton Controls

The aim of this study was to compare the educational achievement of female twins (n = 577) and their matched singleton controls (n = 447), who were selected from participants of a national test of educational achievement in the years 1993 to 1998. To assure the representativeness of the selected groups we also compared the achievement scores of the twins and the controls to those of the total Dutch female population tested in the same period. We analyzed the results of the following educational achievement scales: Language, Mathematics and Information Processing. The results indicated that the singleton classmates performed significantly better than the twins on all three scales. However, the twins performed equally well as compared to the Dutch female population. We believe that our singleton control group was not as properly selected as we intended, a selection bias operative at the level of the schools may have confounded the comparison. We therefore conclude on the basis of a comparison with the performance of the total Dutch female population that there are no differences in educational achievement between female twins and singletons.

The Personalities of Twins: Just Ordinary Folks

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

The Cognitive Cost of Being a Twin: Two Whole-Population Surveys

Do twins have a lower mean IQ score than singletons? Previous studies have not examined whole populations and are likely to be biased. Twin data from two whole-population surveys of IQ at age 11 years were examined: the Scottish Mental Surveys of 1932 and 1947. Additional variables from childhood were examined as possible mediating effects. There were 1080 twins from the 1932 survey, and 949 from the 1947 survey. In both surveys twins scored lower on the Moray House Test of verbal reasoning, equivalent to a deficit of about 5 IQ points. In the Scottish Mental Survey of 1947 the whole-population group of twins was compared in detail with a representative population sample. The same mental ability difference of about 5 IQ points was found, and was not accounted for by father's occupation, overcrowding in the childhood home, childhood height, school attendance or the number of people in the family.

Comparing Blood Pressure of Twins and Their Singleton Siblings: Being a Twin Does Not Affect Adult Blood Pressure

The hypothesis was tested that monozygotic (MZ) and dizygotic (DZ) twins, with their lower average birth weight, have higher adult blood pressure than their singleton brothers or sisters. From the Netherlands Twin Registry, 261 twin families were recruited from a young adult and an older adult cohort with mean ages of 26.2 and 50.4 respectively. These families yielded 204 MZ twins with 71 singleton siblings and 271 DZ twins with 103 of their singleton siblings. Anti-hypertensive medication use of these 649 participants was assessed twice with a two-year interval. Resting blood pressure was measured thrice during a standardized laboratory protocol. In spite of a significant difference in birth weight (1036 gram), no differences were found in anti-hypertensive medication use at both time points between twins and singletons nor between their resting laboratory diastolic or systolic blood pressure. These results applied to each gender and to both age cohorts. Limiting the analyses to matched twin-sibling pairs of the same families and taking current weight and height into account did not change the results; no evidence was found for a twin-singleton difference. It was concluded that estimates of genetic and environmental contributions to blood pressure deriving from twin studies do not appear to be biased and may be generalized to singletons. Our results suggest that the lower birth weight in twins does not reflect the intrauterine disadvantage described by the Barker hypothesis.

Twin-singleton differences in neonatal brain structure

Twin studies suggest that global and regional brain volumes are highly heritable. However, estimates of heritability vary across development. Given that all twin studies are open to the potential criticism of non-generalizability due to differences in intrauterine environment between twins and singletons, these age effects may reflect the influence of perinatal environmental factors, which are unique to twins and which may be especially evident early in life. To address this question, we compared brain volumes and the relationship of brain volumes to gestational age in 136 singletons (67 male, 69 female) and 154 twins (75 male, 79 female; 82 DZ, 72 MZ) who had received high resolution MRI scans of the brain in the first month of life. Intracranial volume, total white matter, and ventricle volumes did not differ between twins and singletons. However, cerebrospinal fluid and frontal white matter volume was greater in twins compared to singletons. While gray matter volumes at MRI did not differ between groups, the slope of the relationship between total and cortical gray matter and gestational age at the MRI scan was steeper in MZ twins compared to DZ twins. Post-hoc analyses suggested that gray matter development is delayed in MZ twins in utero and that they experience 'catch-up' growth in the first month of life. These differences should be taken into account when interpreting and designing studies in the early postnatal period.

The effect of intrauterine growth on verbal IQ scores in childhood: a study of monozygotic twins

OBJECTIVE

Given the adverse neurobiological effects of suboptimal nutrition on the developing brain, it is of social and medical importance to determine if the global prevalence of poor intrauterine growth causes lasting cognitive deficits. We examined whether suboptimal intrauterine growth relates to impaired cognitive outcome by comparing birth weight and cognition in monozygotic twins and considered whether children within-pair differences in birth weight were related to within-pair differences in IQ scores.

METHODS

A total of 71 monozygotic twin pairs (aged 7 years 11 months to 17 years 3 months) participated. The Wechsler Intelligence Scale for Children, Third Edition, was administered, and verbal IQ (VIQ) and performance IQ (PIQ) scores were calculated. Regression was used to relate within-pair differences in birth weight to within-pair differences in IQ scores.

RESULTS

VIQ but not PIQ score was affected by prenatal growth restriction. The results suggest that the mean advantage for heavier twins relative to their lighter co-twins can be as much as half an SD in VIQ points. In pairs with minimal discordance, heavier twins had lower VIQ scores than their lighter co-twins.

CONCLUSIONS

Our study results suggest that lower birth weight in monozygotic twins can also have a negative long-term impact on cognition both in infants who are small at birth and also those with birth weights across the spectrum. Studying monozygotic twins enabled us to examine the effect of reduced intrauterine growth on cognition independently of confounding factors, including parental IQ and education and infant gender, age, genetic characteristics, and gestation.

Effects of gestational age and birth weight on brain volumes in healthy 9 year-old children

OBJECTIVE

To assess the effects of gestational age and birth weight on brain volumes in a population-based sample of normal developing children at the age of 9 years.

STUDY DESIGN

A total of 192 children from twin births were included in the analyses. Data on gestational age and birth weight were reported shortly after birth. Total brain, cerebellum, cerebrum, gray and white matter, and lateral ventricle volumes were assessed with structural magnetic resonance imaging. The Wechsler Intelligence Scale for Children-III was administered to assess general cognitive abilities. Structural equation modeling was used to analyze the effects of gestational age and birth weight on brain volumes.

RESULTS

Shorter gestational age was associated with a relatively smaller cerebellar volume (P = .002). This effect was independent of IQ scores. Lower birth weight was associated with lower IQ score (P = .03). Birth weight was not associated with brain volumes.

CONCLUSION

The effect of gestational age on cerebellar volume is not limited to children with very premature birth or very low birth weight, but is also present in children born >32 weeks of gestation and with birth weight >1500 g.

Does parental education have a moderating effect on the genetic and environmental influences of general cognitive ability in early adulthood?

Hereditary influences account for a substantial proportion of the variance in many cognitive abilities. However, there is increasing recognition that the relative importance of genetic and environmental influences may vary across different socioeconomic levels. The overall goal of the present study was to examine whether parental education has a moderating effect on genetic and environmental influences of general cognitive ability in early adulthood (age 19.6 +/- 1.5). Participants were 5,955 male twins from the Vietnam Era Twin (VET) Registry. Significant effects of parental education on mean level of general cognitive ability scores were found, but a model without moderating effects of parental education on genetic or environmental influences on cognitive scores proved to be the best fitting model. Some, but not all, previous studies have found significant moderating effects; however, no consistent pattern emerged that could account for between-study differences regarding moderating effects on genetic and environmental influences.

Are there differences in brain morphometry between twins and unrelated singletons? A pediatric MRI study

Twins provide a unique capacity to explore relative genetic and environmental contributions to brain development, but results are applicable to non-twin populations only to the extent that twin and singleton brains are alike. A reason to suspect differences is that as a group twins are more likely than singletons to experience adverse prenatal and perinatal events that may affect brain development. We sought to assess whether this increased risk leads to differences in child or adolescent brain anatomy in twins who do not experience behavioral or neurological sequelae during the perinatal period. Brain MRI scans of 185 healthy pediatric twins (mean age = 11.0, SD = 3.6) were compared to scans of 167 age- and sex-matched unrelated singletons on brain structures measured, which included gray and white matter lobar volumes, ventricular volume, and area of the corpus callosum. There were no significant differences between groups for any structure, despite sufficient power for low type II (i.e. false negative) error. The implications of these results are twofold: (1) within this age range and for these measures, it is appropriate to include healthy twins in studies of typical brain development, and (2) findings regarding heritability of brain structures obtained from twin studies can be generalized to non-twin populations.

Twin-singleton differences in intelligence: a meta-analysis

Since the emergence of twin studies in the 1920s, time and again the question of possible twin-singleton differences in intelligence has been posed. This study addressed the issue through a meta-analysis of published studies on this theme. Twins on the average seem to have lower IQs than singletons. The best estimate for this group difference is 4.2 IQ points (less than one-third of a standard deviation), with a great divide between study outcomes of less vs more recent birth cohorts (5.1 vs 0.5 IQ points, respectively). The evidence is based on studies from six countries (including population-based ones of entire birth cohorts), a massive database (comparisons of more than 30,000 twins with nearly 1.6 million singletons), a variety of intelligence tests, and birth cohorts spanning most of the 20th century, but, for the most part, on an age range limited to children and adolescents. The effect shows considerable between-study heterogeneity but appears robust (fail-safe N calculations), not due to influential individual studies (sensitivity analysis) or publication bias, was present since the very first published studies (cumulative meta-analysis), and appears generalizable across sex, zygosity, and other intelligence domains beyond the verbal. There are insufficient data as to whether the effect persists over the lifespan, exists as well within families, or has ceased in recent birth cohorts of highly developed countries (Denmark and The Netherlands). Likely causes of the effect appear to be prenatal and perinatal factors (reduced fetal growth and shorter gestation for twins).

Relationships between multiple births and autism spectrum disorders, cerebral palsy, and intellectual disabilities: autism and developmental disabilities monitoring (ADDM) network-2002 surveillance year

Since the 1970s, the prevalence of multiple births (MBs) in the United States has increased significantly. This has been attributed, in large part, to iatrogenic MBs resulting from infertility treatments that include ovulation stimulation. A past study has indicated that children from MBs have an increased prevalence of cerebral palsy (CP). Other studies also have suggested an association between MBs and intellectual disabilities (ID) and autism spectrum disorders (ASDs); however, results have been inconsistent. From the Autism and Developmental Disabilities Monitoring (ADDM) Network, a surveillance project among several US populations, we obtained MB estimates among children born in 1994 and classified by 8 years of age as having: an ASD (n=1,626 total children from 11 sites; 50 born as part of an MB); CP (n=302 total children from 3 sites; 25 born as part of an MB); or ID (n=1,195 total children from 3 sites; 45 born as part of an MB). All three MB estimates were notably higher than age-adjusted expected estimates of naturally conceived MBs derived from 1971 US natality data. However, when MB estimates from the ADDM Network were compared with expected MB estimates derived from 1994 natality data for the states corresponding to the relevant ADDM Network sites, we observed no association with ASDs (observed/expected=1.08 [0.78-1.38]), a moderate, but not statistically significant association with ID (observed/expected=1.34 [0.95-1.73]), and a strong association with CP (observed/expected=2.96 [1.80-4.12]). Further investigation of specific types of MBs (natural vs. iatrogenic) is warranted.

A note on twin-singleton differences in asthma

Twins constitute a valuable resource for genetic studies of asthma. However, critics argue that twins are 'special' in terms of prenatal environment and upbringing and therefore nonrepresentative. In respect to asthma a small range of studies report differential morbidity in twins compared with singletons. We review some of the possible explanations for these findings and conclude that results from twin studies of asthma can be extrapolated to the general population.

Gene-environment interaction in adults' IQ scores: measures of past and present environment

Gene-environment interaction was studied in a sample of young (mean age 26 years, N = 385) and older (mean age 49 years, N = 370) adult males and females. Full scale IQ scores (FSIQ) were analyzed using biometric models in which additive genetic (A), common environmental (C), and unique environmental (E) effects were allowed to depend on environmental measures. Moderators under study were parental and partner educational level, as well as urbanization level and mean real estate price of the participants’ residential area. Mean effects were observed for parental education, partner education and urbanization level. On average, FSIQ scores were roughly 5 points higher in participants with highly educated parents, compared to participants whose parents were less well educated. In older participants, IQ scores were about 2 points higher when their partners were highly educated. In younger males, higher urbanization levels were associated with slightly higher FSIQ scores. Our analyses also showed increased common environmental variation in older males whose parents were more highly educated, and increased unique environmental effects in older males living in more affluent areas. Contrary to studies in children, however, the variance attributable to additive genetic effects was stable across all levels of the moderators under study. Most results were replicated for VIQ and PIQ.

RESPONSE INTERFERENCE AND WORKING MEMORY IN 12-YEAR-OLD CHILDREN

A group of 69 12-year-old children performed three well-known response interference tasks: the Stroop task, the Eriksen flanker task, and the Simon task. Individual differences in accuracy and speed correlated across the tasks. However, there was no correlation between the interference effects on these three tasks. Stroop interference, but not the Simon or flanker effect, was correlated with working memory capacity, as obtained from the WISC-R. These results may help clarify the nature of ADHD, which is characterized by problems with response interference.

Surprising lack of sex differences in normal cognitive aging in twins

Sex differences in the etiology of normal cognitive functioning in aging remain largely unexplored. We conducted an investigation of genetic and environmental contributions to sex differences in level of cognitive performance and rate of decline in the Swedish Adoption/Twin Study of Aging (SATSA) (Finkel & Pedersen, 2004) data set. Behavioral genetic parameterizations of a latent growth curve model were fit to longitudinal data on 11 cognitive measures. Seven hundred and ninety-eight non-demented individuals had cognitive data across four waves of measurement covering 13 years. Participants ranged in age from 44 to 88 at first testing wave; 60% were female. Results indicated sex differences in mean performance for five cognitive measures and in rates of decline for Information and Card Rotations. Only Synonyms demonstrated sex differences in genetic and environmental contributions to mean performance: heritability was higher in men than women. Despite differential longevity and susceptibility to disease, there are no consistent indications that men and women show different patterns of cognitive aging.

Twinsburg, Ohio; Twinsburg Research Institute; Twin Study Summaries; The Outside World

Recent plans to establish a Twinsburg Research Institute in Twinsburg, Ohio, home of the annual Twins Days Festival, are reviewed. Included among the topics under discussion were the development of an Ohio and/or national United States twin registry and the appropriate location for such a facility. Next, recent findings on chromosomal anomalies and other adverse consequences of artificial reproduction are summarized — some results are surprising and unexplained. This information is followed by further examination of epigenetic twin studies, supplementing a review in a previous issue. The last section of this article presents insightful material about twin leaders, scholars, athletes, designers and parents.

Genetic and environmental influences on human psychological differences

Psychological researchers typically distinguish five major domains of individual differences in human behavior: cognitive abilities, personality, social attitudes, psychological interests, and psychopathology (Lubinski, 2000). In this article we: discuss a number of methodological errors commonly found in research on human individual differences; introduce a broad framework for interpreting findings from contemporary behavioral genetic studies; briefly outline the basic quantitative methods used in human behavioral genetic research; review the major criticisms of behavior genetic designs, with particular emphasis on the twin and adoption methods; describe the major or dominant theoretical scheme in each domain; and review behavioral genetic findings in all five domains. We conclude that there is now strong evidence that virtually all individual psychological differences, when reliably measured, are moderately to substantially heritable.

Biometrical genetics

Biometrical genetics is the science concerned with the inheritance of quantitative traits. In this review we discuss how the analytical methods of biometrical genetics are based upon simple Mendelian principles. We demonstrate how the phenotypic covariance between related individuals provides information on the relative importance of genetic and environmental factors influencing that trait, and how factors such as assortative mating, gene-environment correlation and genotype-environment interaction complicate such interpretations. Twin and adoption studies are discussed as well as their assumptions and limitations. Structural equation modeling (SEM) is introduced and we illustrate how this approach may be applied to genetic problems. In particular, we show how SEM can be used to address complicated issues such as analyzing the causes of correlation between traits or determining the direction of causation (DOC) between variables.

Twin-singleton differences in brain structure using structural equation modelling

Twin studies are important to investigate genetic influences on variation in human brain morphology in health and disease. However, the twin method has been criticized for its alleged non-generalizability due to differences in the intrauterine and family environment of twins, compared with singletons. To test whether twin-singleton differences complicate interpretation of genetic contributions on variation in brain volume, brains from 112 pairs of twins and 34 of their siblings with a mean (standard deviation) age of 30.7 (9.6) years were scanned using MRI. The influence of birth order, zygosity and twin-sibling differences on brain volume measures was analysed using maximum-likelihood model fitting. Variances were homogeneous across birth order, zygosity and twin-singleton status. Irrespective of zygosity, intracranial volume was smaller in second-born twins compared with first-born twins and compared with siblings. Grey matter volume was smaller in second-born twins compared with first-born twins. White matter was smaller in twins compared with siblings. Differences in grey and white matter between these groups were no longer significant after correction for intracranial volume. Total brain, and lateral and third ventricle volumes were comparable in twins and singletons. In conclusion, second-born twins have a smaller intracranial volume than their first-born co-twins and siblings. This suggests aberrant early brain development in second-born twins, which is consistent with the suboptimal pre- and perinatal environment related to birth order in twins. Since other brain volume measures were comparable between the groups, twin studies can provide reliable estimates of heritabilities in brain volume measures and these can be generalized to the singleton population.