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The Interplay of Genes and Environment across Multiple Studies (IGEMS) is a consortium of 18 twin studies from 5 different countries (Sweden, Denmark, Finland, United States, and Australia) established to explore the nature of gene-environment (GE) interplay in functioning across the adult lifespan. Fifteen of the studies are longitudinal, with follow-up as long as 59 years after baseline. The combined data from over 76,000 participants aged 14-103 at intake (including over 10,000 monozygotic and over 17,000 dizygotic twin pairs) support two primary research emphases: (1) investigation of models of GE interplay of early life adversity, and social factors at micro and macro environmental levels and with diverse outcomes, including mortality, physical functioning and psychological functioning; and (2) improved understanding of risk and protective factors for dementia by incorporating unmeasured and measured genetic factors with a wide range of exposures measured in young adulthood, midlife and later life.

The Louisville Twin Study (LTS) is nationally recognized as one of the largest and most comprehensive studies of child development related to multiple birth status. The LTS is unique because of the extensive longitudinal face-to-face assessments, the frequency of data collection, the inclusion of data on additional family members (i.e., parents, siblings, grandparents; and later, twins' own spouses and children), and the variety of data collection methods used. Data preservation efforts began in 2008 and are largely complete, although efforts are ongoing to obtain funding to convert the electronic data to a newer format. A pilot study was completed in the summer of 2018 to bring the twins, who are now middle-aged, back for testing. A grant is currently under review to extend the pilot study to include all former participants who are now ≥40 years of age. Opportunities for collaboration are welcome.

The Interplay of Genes and Environment across Multiple Studies (IGEMS) group is a consortium of eight longitudinal twin studies established to explore the nature of social context effects and gene-environment interplay in late-life functioning. The resulting analysis of the combined data from over 17,500 participants aged 25-102 at baseline (including nearly 2,600 monogygotic and 4,300 dizygotic twin pairs and over 1,700 family members) aims to understand why early life adversity, and social factors such as isolation and loneliness, are associated with diverse outcomes including mortality, physical functioning (health, functional ability), and psychological functioning (well-being, cognition), particularly in later life.

Reaction time (RT) data from the Minnesota Twin Study of Adult Development and Aging were used to investigate genetic and environmental influences on intraindividual variability in RT. The sample was 738 individuals (aged 27 to 95 years), including 185 monozygotic (MZ) and 131 dizygotic (DZ) twin pairs. Two RT factors were created: decision time and movement time. Age-corrected heritability estimates were 40% for mean RT; 21% for intraindividual variability in movement time; and 0% for intraindividual variability in decision time. Multivariate analysis indicated that all genetic variance in the RT measures was shared among measures; environmental variance was specific to each RT measure.

A twin study of infant attachment security at age 24 months was conducted on archival data for a sample of 99 MZ pairs and 108 DZ pairs from the Louisville Twin Study. MZ concordance for attachment was 62.6%, which was significantly greater than the DZ concordance of 44.4%. Concordances were transformed into polychoric correlations, and LISREL was used to conduct a quantitative genetic analysis of the data. Results indicated that 25% of the variability in attachment was attributable to genetic factors, and the remaining 75% was attributable to non-shared environmental effects. No evidence was found for a contribution from shared environmental influences to attachment security. Possible concerns about the validity of twin methodology are addressed and various interpretations of the results are presented.

A sex-limitation model was applied to the Multidimensional Personality Questionnaire (MPQ) data from the Minnesota Twin Family Registry. The sample included 626 monozygotic (MZ) twin pairs, 517 same-sex dizygotic (DZ) twin pairs, and 114 opposite-sex DZ twin pairs ranging in age from 27 to 64 years (mean age = 37.8). In addition, whenever possible, family members of the twins were also assessed: 1,690 spouses, 495 parents, 322 siblings, and 535 offspring. The goals of the present investigation were to analyze the covariance matrices to determine whether (a) there was evidence for sex limitation of heritability of the MPQ scales and (b) there was evidence for nonadditive genetic influences on these scales. Significant sex differences in heritability were found for 3 of the scales: Alienation, Control, and Absorption. In addition, evidence for dominance was found for all scales except Traditionalism and Absorption.

Cross-sectional reports suggest heritability of cognitive ability increases throughout adulthood. To investigate this hypothesis, quantitative genetic analyses were conducted on four measures of cognitive ability (verbal, spatial, perceptual speed, memory). Data from Minnesota and Swedish twin studies of aging were compared. Heritability estimates and the factor structure of cognitive abilities could be equated across younger twins (age, 27-50) and middle-aged twins (age, 50-65) from both studies, suggesting stability of heritability during adulthood. The heritability of 81% for a general cognitive factor confirmed earlier findings of high heritability in younger and middle-aged samples. Older Swedish twins (age, 65-85) demonstrated significantly lower heritability estimates for cognitive abilities (54%) and a significantly different factor structure of cognitive ability.

Twin analyses were conducted to determine the relative influence of genetic and environmental factors on functional aging. As part of the ongoing Minnesota Twin Study of Adult Development and Aging (MTSADA), measures of 30 demographic, cognitive, physiological, personality, and behavioral variables were available from 140 monozygotic twin pairs and 97 dizygotic twin pairs ranging in age from 27 to 88 years. Functional age was based on a general linear regression model with chronological age as the dependent variable. Stepwise regression determined the subset of variables by MTSADA providing the best prediction of chronological age. Factor analysis of these 12 variables resulted in three factors: physiological measures, cognitive abilities, and processing speed. When entered into, a regression equation, the three factors accounted for 66% of the variance in chronological age. Analysis of twin similarity for components of functional age suggested the relative influence of genetic and environmental factors varies greatly for different components of functional aging. In addition, the genetic and shared environmental influences on the three components were common to all three, while the nonshared environmental influences were specific to each component.