Twin Similarity Research Articles

Machine-Vision-Based Enhanced Deep Genetic Algorithm for Robot Action Analysis.

The machine-driven products processed by CNC machine tools are divided into multiple parts, which are often transmitted to unqualified processing crystals due to the error of the part's posture, thus affecting the projection effect. Aiming at handling this problem, we, in this work, propose a method for locating and correcting machine capabilities based on bicycle visual recognition. The robot obtains coordinates and offset points through the vision system. In order to satisfy the requirement of fast sorting of the robot parts, a robot species method (BAS-GA) that supports machine vision and improved genetic algorithm rules is proposed. The rank method first preprocesses the part copies, then uses the Sift feature twin similarity notification algorithm to filter the part idols, and finally uses in-law deformation to place the target parts. Afterward, an particular model is built for the maintenance of the nurtural skill, and the mathematical mold is solved using the BAS-GA algorithmic program authority. The close trail of the robot is maintained to manifest the marijuana of the robot. Experimental inference have shown that the BAS-GA algorithm rule achieves the optimal conclusion similar to the pseudo-annealing algorithmic program plant. Meanwhile, the genetic algorithm rule is modified ant settlement algorithm program. Knife sharpening was also reduced by 7%, inferring that this process can effectively improve the robot's action success rate.

Twin Similarity for Neighborhood, Geographic Mobility, and Health Outcomes in Late Adulthood

Socioeconomic status (SES) is one of the most robust predictors of health. The source of SES-health associations is heavily debated; one approach is investigating neighborhood-level environmental characteristics. Challenges include selection effects and the possibility of reverse causation: people choose their neighborhoods. Longitudinal twin research can overcome these issues by assessing location choice over time as well as twin similarity; however, few existing twin studies have incorporated neighborhood-level data, and none of those focus on aging. Using longitudinal data from the Swedish Adoption/Twin Study of Aging, the current study examined the impact of location at various points in life. Location at birth and in 1993 were available for 972 participants. Birth years ranged from 1926 to 1948; mean age in 1993 was 54.55 (range = 35-67). Thirty-nine percent of the sample had moved to a different county between birth and midlife: individuals who moved had significantly higher parental SES and had achieved significantly higher education. Moreover, identical twin concordance for geographic mobility (77%) was significantly higher than fraternal twin concordance (65%), indicating a modest but significant genetic contribution. Geographic mobility did not impact identical twin similarity on a functional aging factor (corrected for age and education), but fraternal twins concordant for mobility were more similar than discordant twins, suggesting genetic contributions to mobility may also impact health. Ongoing retrieval of location information for twins born 1900-1925 and geocoding of location information available at 9 waves of data collection will allow for expanded investigation of the SES-health relationship at the neighborhood level.

Three Patterns of Inheritance of Quantitative Dermatoglyphic Traits: Kosovo Albanian Twin Study.

Dermatoglyphs are epidermal ridge configurations on the fingers, palms and soles that are formed during fetal development, and therefore only the intrauterine environment can have any influence on their formation. This study aims at investigating the genetic and environmental contribution in determining quantitative dermatoglyphic traits in 32 monozygotic (MZ) and 35 dizygotic (DZ) same-sex twins from the Albanian population of Kosovo. All genetic analyses were run in the statistical program Mx. After assumptions testing, based on the pattern of MZ-DZ correlations, univariate models were fitted to the data in order to estimate additive genetic (A), common (C) and individual (E) environmental influences for all variables. The exception was the atd-angle for which a model with nonadditive genetic (D) influences was tested, since DZ correlations were less than half of MZ correlations. Goodness of fit of the full ACE or ADE model was compared to the saturated model. The fit of nested models (AE, CE, DE or E) was compared to the full models (ACE or ADE). Our results indicate that additive genetic component strongly contributes to individual differences in finger ridge counts (49-81%), and weakly (0-50%) on the formation of the palmar ridge counts between the palmar triradii a, b, c, and d. The specific pattern found for the atd-angle implies the impact of a nonadditive genetic component, possibly the effect of a major gene. Further, more powered studies are needed to confirm this pattern, especially for resolving the issue of the huge difference in MZ and DZ twin similarity for the atd-angle palmar trait.

Understanding the Effects of the COVID-19 Pandemic on Youth Psychopathology: Genotype–Environment Interplay

BackgroundAdversity has consistently been found to predict poor mental health outcomes in youth. Perhaps the most omnipresent form of adversity in the past several decades has been the coronavirus pandemic of 2020, a global health crisis linked to elevated rates of numerous forms of youth psychopathology. The ongoing nature of the pandemic renders it critical to identify the mechanisms underlying its effects on mental health. MethodsThe current study examines pandemic-related disruption across multiple domains (e.g., home life, finances) as an etiologic moderator of several common forms of youth psychopathology. Participants were 637 adolescent twin pairs from the Twin Study of Behavioral and Emotional Development in Children (TBED-C). Mothers reported on disruption experienced by the family, using the Epidemic-Pandemic Impacts Inventory. ResultsA series of biometric genotype-by-environment interaction models revealed that disruption augmented the nonshared environmental contributions to emotional distress and conduct problems but had little effect on the etiology of attention-deficit hyperactivity problems. ConclusionsOur results indicate that identical and fraternal twin similarity in both emotional symptoms and conduct problems decreased with greater disruption, such that children in the same family became less alike, and did so regardless of their degree of genetic resemblance. Put differently, each twin sibling appeared to have their own idiosyncratic experience of pandemic-related disruptions, with downstream consequences for their mental health.

Frontoparietal pattern similarity analyses of cognitive control in monozygotic twins

The ability to flexibly adapt thoughts and actions in a goal-directed manner appears to rely on cognitive control mechanisms that are strongly impacted by individual differences. A powerful research strategy for investigating the nature of individual variation is to study monozygotic (identical) twins. Evidence of twin effects have been observed in prior behavioral and neuroimaging studies, yet within the domain of cognitive control, it remains to be demonstrated that the neural underpinnings of such effects are specific and reliable. Here, we utilize a multi-task, within-subjects event-related neuroimaging design with functional magnetic resonance imaging to investigate twin effects through multivariate pattern similarity analyses. We focus on fronto-parietal brain regions exhibiting consistently increased activation associated with cognitive control demands across four task domains: selective attention, context processing, multi-tasking, and working memory. Healthy young adult monozygotic twin pairs exhibited increased similarity of within- and cross-task activation patterns in these fronto-parietal regions, relative to unrelated pairs. Twin activation pattern similarity effects were clearest under high control demands, were not present in a set of task-unrelated parcels or due to anatomic similarity, and were primarily observed during the within-trial timepoints in which the control demands peaked. Together, these results indicate that twin similarity in the neural representation of cognitive control may be domain-general but also functionally and temporally specific in relation to the level of control demand. The findings suggest a genetic and/or environmental basis for individual variation in cognitive control function, and highlight the potential of twin-based neuroimaging designs for exploring heritability questions within this domain.

Correlates and genetics of self-reported sleep and awake bruxism in a nationwide twin cohort.

Sleep bruxism (SB) and awake bruxism (AB) have been considered different entities, although co-occurrence between them has been shown. While genetic factors have a marked influence on phenotypic variance in liability to SB, this remains unclear for AB. To examine the degree of co-occurrence of SB and AB, and whether they have common correlates and also twin similarity of SB and AB bruxism traits by zygosity and sex. A questionnaire was mailed to all twins born 1945-1957 in Finland in 2012 (n=11766). Age and sex adjusted logistic regression models were used. Twin similarity was assessed using polychoric correlations, and crosstwin-crosstrait correlations were computed. The response rate was 72% (n=8410). Any SB was reported by 14.8% and≥3 nights weekly by 5.0%. Percentages for any AB were 18.4% and 6.3%, respectively. There was substantial co-occurrence (29.5%) between SB and AB, and several shared correlates were found. For SB, the polychoric intra-class correlation was 0.366 in monozygotic (MZ) and 0.200 in dizygotic (DZ) pairs, without gender difference. A twofold crosstwin-crosstrait correlation was observed in MZ twins compared to DZ twins. The risk factor profiles of SB and AB were largely but not entirely similar. The higher correlation in MZ than in DZ pairs suggests the influence of genetic factors on both SB and AB. The higher crosstwin-crosstrait correlation in MZ than in DZ pairs suggests some degree of genetic influences shared by SB and AB.

On the Nature of Monozygotic Twin Concordance and Discordance for Autistic Trait Severity: A Quantitative Analysis

The characterizing features of autism spectrum disorder (ASD) are continuously distributed in nature; however, prior twin studies have not systematically incorporated this knowledge into estimations of concordance and discordance. We conducted a quantitative analysis of twin–twin similarity for autistic trait severity in three existing data sets involving 366 pairs of uniformly-phenotyped monozygotic (MZ) twins with and without ASD. Probandwise concordance for ASD was 96%; however, MZ trait correlations differed markedly for pairs with ASD trait burden below versus above the threshold for clinical diagnosis, with R2s on the order of 0.6 versus 0.1, respectively. Categorical MZ twin discordance for ASD diagnosis is rare and more appropriately operationalized by standardized quantification of twin–twin differences. Here we provide new evidence that although ASD itself is highly heritable, variation-in-severity of symptomatology above the diagnostic threshold is substantially influenced, in contrast, by non-shared environmental factors which may identify novel targets of early ASD amelioration.

Identifying subtypes of cannabis users based on simultaneous polysubstance use

BackgroundCannabis use patterns vary considerably, with many users reporting simultaneous and non-simultaneous use (co-use) of other substances. Despite this, little research has examined the extent to which subtypes of cannabis users may be identified based on their simultaneous and co-use behaviors. MethodsThe sample consisted of adult Australian twins and siblings who reported lifetime cannabis use (n = 2590). A latent class analysis was conducted to determine subtypes of cannabis users based on five indicators of substance co-use and simultaneous use. Adolescent correlates (age of substance initiation and conduct disorder) and adult correlates (substance use/disorder and depression) of class membership were assessed. Twin similarity for class membership was also examined. ResultsFour subtypes of users were identified: 1) alcohol co-users, 2) simultaneous alcohol users, 3) simultaneous tobacco users, and 4) simultaneous alcohol, tobacco, and drug users. Compared to co-users of alcohol, simultaneous alcohol users were at increased risk for alcohol problems. Patterns of use that involved simultaneous tobacco and cannabis use (i.e., simultaneous tobacco users and simultaneous alcohol, tobacco, and drug users) were associated with the most problematic outcomes, including substance use and disorder. There was evidence for genetic influences (12–58%) on cannabis use patterns, with higher concordance for latent class membership among monozygotic compared to dizygotic twins (χ2 (1) = 7.19, p = 0.007). ConclusionsThe current study identified four classes of cannabis users at varying degrees of risk. Results suggest that simultaneous tobacco and cannabis use may be especially associated with deleterious outcomes.

Does sibling and twin similarity in cognitive ability differ by parents’ education?

Stratification scholars predominantly investigate how differences among children from different families emerge and tend to neglect differences among children from the same family. I study sibling similarity in cognitive ability and examine whether their similarity varies by parents’ education. Although economic approaches and their extensions argue that disadvantaged parents reinforce differences while advantaged parents compensate for differences, I argue that parents may also make equal investments and thus accept differences among their children. I refer to the literature on stratified parenting that demonstrates that parents are engaged differently in child-rearing and their children’s skill formation processes. Because advantaged parents foster children’s talents more individually compared with disadvantaged parents, I propose that sibling similarity is lower in advantaged than in disadvantaged families. Previous studies based on sibling correlations provide conflicting evidence. To account for observable and unobservable differences among siblings, I extend the established sibling correlation approach and study dizygotic and monozygotic twins in addition to siblings. The analyses draw on novel data from a population register-based study of twin families. I find that young adult siblings and twins are less alike in cognitive ability in highly educated families than in less educated families. Hence, my results support the hypothesis concerning equal investments and indicate that stratified parenting has a long-lasting influence on children’s cognitive ability.
 Zusammenfassung
 In der Stratifizierungsliteratur wird überwiegend zur Frage geforscht, wie sich Unterschiede zwischen Kindern aus unterschiedlichen Familien erklären lassen, wobei Unterschiede, die sich zwischen Kindern aus einer Familie ergeben, unberücksichtigt bleiben. Diese Studie untersucht die Ähnlichkeit von Geschwistern in Bezug auf ihre kognitiven Fähigkeiten und fragt, ob diese in Abhängigkeit des Bildungshintergrunds variiert. Die ökonomische Literatur und Erweiterungen davon vertreten die Ansicht, dass benachteiligte Eltern Unterschiede zwischen Geschwistern verstärken, wohingegen besser gestellte Eltern Unterschiede zwischen Geschwistern eher ausgleichen. Ich dagegen argumentiere, dass Eltern auch gleiche Investitionen in ihre Kinder machen können und Unterschiede zwischen ihren Kindern akzeptieren. Ausgangspunkt hierfür ist die Literatur zu stratifizierten elterlichen Verhaltensweisen und Erziehungspraktiken, die belegt, dass Eltern ihre Kinder unterschiedlich erziehen und unterschiedlich in die Entwicklungsprozesse der Kinder eingebunden sind. Weil besser gestellte Eltern die individuellen Talente ihrer Kinder stärker fördern als benachteiligte Eltern, wird angenommen, dass sich Geschwister aus besser gestellten Familien unähnlicher sind als Geschwister von sozial benachteiligten Eltern. Bisherige Forschungsarbeiten, die die Ähnlichkeit von Geschwistern untersuchen, liefern uneinheitliche Befunde. Um beobachtbare und nicht beobachtbare Unterschiede zwischen Geschwistern berücksichtigen zu können, erweitere ich den herkömmlichen Ansatz, in dem Geschwister verglichen werden, um ein- und zweieiige Zwillinge. Die empirischen Analysen basieren auf den Daten der TwinLife Studie, einer repräsentativen Studie von Zwillingen und ihren Familien. Meine Ergebnisse zeigen, dass sich Geschwister und Zwillinge im jungen Erwachsenenalter aus bildungsnahen Familien unähnlicher sind in Bezug auf ihre kognitiven Kompetenzen im Vergleich zu Geschwistern und Zwillingen aus bildungsfernen Familien. Meine Ergebnisse unterstützen damit die Hypothese bezüglich gleicher Investitionen und deuten darauf hin, dass stratifizierte Erziehungsweisen der Eltern einen langanhaltenden Einfluss auf die Realisierung von kognitiven Fähigkeiten der Kinder haben.

Heritability of Type 2 Diabetes in the Washington State Twin Registry.

Approximately 12% of U.S. adults have type 2 diabetes (T2D). Diagnosed T2D is caused by a combination of genetic and environmental factors including age and lifestyle. In adults 45 years and older, the Discordant Twin (DISCOTWIN) consortium of twin registries from Europe and Australia showed a moderate-to-high contribution of genetic factors of T2D with a pooled heritability of 72%. The purpose of this study was to investigate the contributions of genetic and environmental factors of T2D in twins 45 years and older in a U.S. twin cohort (Washington State Twin Registry, WSTR) and compare the estimates to the DISCOTWIN consortium. We also compared these estimates with twins under the age of 45. Data were obtained from 2692 monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs over 45 and 4217 twin pairs under 45 who responded to the question 'Has a doctor ever diagnosed you with (type 2) diabetes?' Twin similarity was analyzed using both tetrachoric correlations and structural equation modeling. Overall, 9.4% of MZ and 14.7% of DZ twins over the age of 45 were discordant for T2D in the WSTR, compared to 5.1% of MZ and 8% of DZ twins in the DISCOTWIN consortium. Unlike the DISCOTWIN consortium in which heritability was 72%, heritability was only 52% in the WSTR. In twins under the age of 45, heritability did not contribute to the variance in T2D. In a U.S. sample of adult twins, environmental factors appear to be increasingly important in the development of T2D.

Paranoidna psihoza u patrijarhalnoj kulturi - jedan razoren um Lazara Komarčića

In the novel Jedan razoren um (A Devastated Mind) (1893) Lazar Komarčić, by using himself of the motive of duplicate, enters a Serbian prose the description of paranoid psychosis. Due to twin's similarity of the two protagonists Steva and Velja, it comes out first Velja's shaky feeling of identity which activates defence mechanisms characteristic for paranoid psychosis: splitting of the self and projected identification. Komarčić despite the sudden disease outbreak with the protagonist, managed to display consistently a paranoid progression: from suspicion to the mystery of pursuit, paranoid delusion which is obviously in contradiction with the facts. The psychotic cleavage of hero is considered in the context of mental disease representation in patriarchal milieu, and in this context Velja's paranoid psychosis is manifested as the solution in the situation in which the rejection of values imposed by the closed ones would be equalized with the break of the important cultural prohibition. On the other hand, Steva is to be found between two simultaneous but contradictional requests which the culture throws it out in front of him: from one side, there is the necessity of entrepreneurial activity which would enable friend's treatment in the beginning, and from another side, the necessity of silence, which means the protection of Velja's reputation as long as possible. Culture, that, lakanovski said, symbolic order of the great other, is the assigned space in which the rule of silence was written on something which disturbed it. Komančić's novel testifies on the pattern masculinity in culture change: from heroic masculinity saturated with national feeling, which is personified by the generations of grandfathers and fathers toward reflexive, melancholic intellectual of civic type, which is personified by Steva and Velja, the second one without necessary mental force to resist the cultural pressure, and first of all the one which acts from inside, form hero's being itself shaped by the culture.

The Impact of Variation in Twin Relatedness on Estimates of Heritability and Environmental Influences.

By taking advantage of the natural variation in genetic relatedness among identical (monozygotic: MZ) and fraternal (dizygotic: DZ) twins, twin studies are able to estimate genetic and environmental contributions to complex human behaviors. Recently concerns have been raised about the accuracy of twin studies in light of findings of genetic and epigenetic changes in twins. One of the concerns raised is that MZ twins are not 100% genetically and epigenetically similar because they show variations in their genomes and epigenomes leading to inaccurate estimates of heritability. This article presents findings from a simulation study that examined the degree of bias in estimates of heritability and environmentality when the genetic and epigenetic similarity of MZ twins differs from 1.00 and when the genetic and epigenetic similarity of DZ twins differs from 0.50. The findings suggest that in the standard biometric model when MZ or DZ twin similarity differs from 1.00 or 0.50, respectively, the variance that should be attributed to genetic influences is instead attributed to nonshared environmental influences, thus deflating the estimates of genetic influences and inflating the estimates of nonshared environmental influences. Although estimates of genetic and nonshared environmental influences from the standard biometric model were found to deviate from "true" values, the bias was usually smaller than 10% points indicating that the interpretations of findings from previous twin studies are mostly correct.

Attitudes to Gun Control in an American Twin Sample: Sex Differences in the Causes of Variation.

The genetic and social causes of individual differences in attitudes to gun control are estimated in a sample of senior male and female twin pairs in the United States. Genetic and environmental parameters were estimated by weighted least squares applied to polychoric correlations for monozygotic (MZ) and dizygotic (DZ) twins of both sexes. The analysis suggests twin similarity for attitudes to gun control in men is entirely genetic while that in women is purely social. Although the volunteer sample is small, the analysis illustrates how the well-tested concepts and methods of genetic epidemiology may be a fertile resource for deepening our scientific understanding of biological and social pathways that affect individual risk to gun violence.

One-year developmental stability and covariance among oddball, novelty, go/no-go, and flanker event-related potentials in adolescence: A monozygotic twin study.

ERP measures may index genetic risk for psychopathology before disorder onset in adolescence, but little is known about their developmental rank-order stability during this period of significant brain maturation. We studied ERP stability in 48 pairs of identical twins (age 14-16 years) tested 1 year apart. Trial-averaged voltage waveforms were extracted from electroencephalographic recordings from oddball/novelty, go/no-go, and flanker tasks, and 16 amplitude measures were examined. Members of twin pairs were highly similar, whether based on ERP amplitude measures (intraclass correlation [ICC] median = .64, range = .44-.86) or three factor scores (all ICCs ≥ .69) derived from them. Stability was high overall, with 69% of the 16 individual measures generating stability coefficients exceeding .70 and all factor scores showing stability above .75. Measures from 10 difference waveforms calculated from paired conditions within tasks were also examined, and were associated with lower twin similarity (ICC median = .52, .38-.64) and developmental stability (only 30% exceeding .70). In a supplemental analysis, we found significant developmental stability for error-related negativity (range = .45-.55) and positivity (.56-.70) measures when average waveforms were based on one or more trials, and that these values were equivalent to those derived from averages using the current field recommendation, which requires six or more trials. Overall, we conclude that the studied brain measures are largely stable over 1 year of mid- to late adolescence, likely reflecting familial etiologic influences on brain functions pertaining to cognitive control and salience recognition.

Genetic and Environmental Contributions to Behavioral Stability and Change in Children 6-36months of Age Using Louisville Twin Study Data.

The Infant Behavior Record (IBR) from the Bayley Scales of Infant Development has been used to study behavioral development since the 1960s. Matheny (1983) examined behavioral development at 6, 12, 18, and 24months from the Louisville Twin Study (LTS). The extracted temperament scales included Task Orientation, Affect-Extraversion, and Activity. He concluded that monozygotic twins were more similar than same-sex dizygotic twins on these dimensions. Since this seminal work was published, a larger LTS sample and more advanced analytical methods are available. In the current analyses, Choleksy decomposition was applied to behavioral data (n=1231) from twins 6-36months. Different patterns of genetic continuity vs genetic innovations were identified for each IBR scale. Single common genetic and shared environmental factors explained cross-age twin similarity in the Activity scale. Multiple shared environmental factors and a single genetic factor coming on line at age 18months contributed to Affect-Extraversion. A single shared environmental factor and multiple genetic factors explained cross-age twin similarity in Task Orientation.

High twin resemblance for sensitivity to hypoxia.

Physiological responses to hypoxia vary between individuals, and genetic factors are conceivably involved. Using a monozygotic twin design, we investigated the role of genetic factors in physiological responses to acute hypoxia. Thirteen pairs of monozygotic twin brothers participated in two experimental sessions in a normobaric hypoxic facility with a 2-wk interval. In one session, fraction of inspired O2 (FiO2) was gradually reduced to 10.7% (approximately 5300 m altitude) over 5 h. During the next 3 h at 10.7%, FiO2 subjects performed a 20-min submaximal exercise bout (EXSUB, 1.2 W·kg) and a maximal incremental exercise test (EXMAX). An identical control experiment was done in normoxia. Cardiorespiratory measurements were continuously performed, and 8-h urine output was collected. Compared with normoxia, hypoxia decreased (P < 0.05) arterial O2 saturation (%SpO2) at rest (-22%) and during exercise (-28%). Furthermore, V˙O2max (-39%), HRmax (HR, -8%), maximal pulmonary ventilation (V˙Emax, -11%), and urinary norepinephrine excretion (-31%) were reduced (P < 0.05) whereas HR at rest (25%) and during EXSUB (16%) and V˙E at rest (38%) and during EXSUB (70%) were increased (P < 0.05). However, hypoxia-induced changes (Δ) were not randomly distributed between subjects. Between-pair variance was substantially larger than within-pair variance (P < 0.05) for Δ%SpO2 at rest (approximately threefold) and during exercise (approximately fourfold), ΔV˙O2max (approximately fourfold), ΔHR during exercise (approximately seven- to eightfold), hypoxic ventilatory response (approximately sixfold), and Δ urinary norepinephrine output (approximately threefold). Incidence of acute mountain sickness (AMS) also yielded significant twin similarity (P < 0.05). AMS subjects showed approximately 50% greater drop in urinary norepinephrine and lower hypoxic ventilator response than AMS individuals. Our data suggest that genetic factors regulate cardiorespiratory responses, exercise tolerance, and pathogenesis of AMS symptoms in acute severe hypoxia. Hypoxia-induced sympathetic downregulation was associated with AMS.

A Twin Study on Humor Appreciation

The present study focused on genetic and environmental influences on appreciation of structure and content of humor. Monozygotic (n = 135) and dizygotic (n = 60) adult twin pairs rated their trait-like humor appreciation using the 3 WD humor test (Ruch, 1992) which assesses three basic humor stimuli (incongruity-resolution humor; nonsense humor; sexual humor), and two basic components of responses to humor (funniness; aversiveness). Additionally, two indices were derived from these scales, namely structure preference and liking of sexual content (i.e., controlled for humor structure). Intraclass correlations and behavior genetic model-fitting analyses indicated a moderate genetic effect for funniness ratings of liking the sexual content. The remaining funniness scales seemed entirely influenced by environmental effects. Aversiveness scales mainly showed environmental effects represented in reduced CE models, although twin similarity coefficients showed hints of genetic influences as well, which needs to be unraveled in future research. The results demonstrated clearly that funniness ratings should be separated for structure and content, to obtain detailed information about heritability of humor appreciation. Future research should validate these promising initial findings by utilizing larger samples.

How Does the Inclusion of Twins Conceived via Fertility Treatments Influence the Results of Twin Studies?

Rates of twinning have risen dramatically over the last 30 years, from 1 in 53 births in 1980 to 1 in 30 births in 2009 (Martin et al. (January 2012). Three decades of twin births in the United States, 1980-2009. Atlanta, GA: Centers for Disease Control and Prevention, National Center for Health Statistics). This increase is largely attributable to increases in the use of fertility treatments (i.e., ovulation induction and in vitro fertilization) combined with delays in parenthood. Although this increase means that more twins are available for recruitment into twin studies, it also has potential consequences for the heritability estimates obtained in these studies. This study sought to evaluate this possibility, making use of the ongoing Michigan Twins Project (N = 7,261 families with twins aged 3-17 years), an arm of the Michigan State University Twin Registry. Results revealed that, on average, twins conceived via fertility treatments had lower rates of behavior problems than those conceived naturally, although these behavioral differences could be explained largely by demographic and socio-economic differences across the two types of twin families. Twin similarity did not meaningfully differ across fertility treatment status. We thus conclude that estimates of genetic and environmental influences obtained from twin studies over the last 10-15 years are more or less unaffected by the inclusion of twins conceived via fertility treatments in their samples.

Growing old but not growing apart: twin similarity in the latter half of the lifespan.

While a substantial amount of behavioral genetic research has helped to characterize developmental trends in twin similarity in early life, relatively little is known about changes in twin similarity with age in adulthood. We investigated age moderation of twin similarity for a composite measure of cognitive ability, depression symptomatology and hand grip strength in a cross-sectional sample of 2,332 like-sex pairs of Danish twins age 46-96 years. All three outcomes were strongly correlated with age, indicating that the three phenotypes analyzed are not developmentally static. Nonetheless, in moderated regression analysis we found no evidence of declining twin similarity for any of the three outcomes in either zygosity group. Moreover, biometric analysis of the twin data revealed minimal differences in heritability estimates across the age range sampled. While small sample size limits our ability to draw firm conclusions at very advanced ages, these findings call into question the hypothesis that the cumulative impact of life experiences diminishes twin similarity at least through age 80. We hypothesize that twins are able to maintain similarity over extended periods of time because in part they are able to construct similar environments that reinforce that similarity.

Statistical Methodology for Estimating Twin Similarity for Count Data

Statistical Methodology for Estimating Twin Similarity for Count Data