ABSTRACTS
The Importance of Twin Studies for Individual Differences Research
Nancy L. Segal
California State University, Fullerton, CA
Twin research designs and methods are valuable tools for examining genetic and environmental influences on behavioral and medical characteristics. A review of the biological bases of twinning and descriptions of 10 research designs are presented. Findings from a selected sampling of twin studies of learning disabilities, personality and temperament, attitudes, psychopathology, and social behavior are summarized. The findings are discussed with special reference to the activities of mental health practitioners and counselors.
Segal, N. L. (1990). The importance of twin studies for individual differences research. Journal of Counseling and Development, 68, 612-622
Twins in the Classroom: School Policy Issues and Recommendations
Nancy L. Segal
California State University, Fullerton, CA
Jean M. Russell
Veterans Administration Hospital, Minneapolis, MN
Decisions concerning the assignment of young twin children to the same or separate classrooms have proven difficult for many educators, counselors, and parents. As part of an ongoing study of cooperation and competition, 63 mothers of young twins and triplets were questioned with respect to satisfaction regarding school policies, reasons to support and reject separate classrooms for twins, and related issues. Forty-eight percent of the parents who were aware of the school's policy did not endorse a general practice of separating twins. More parents of monozygotic (MZ) twins (35%) than dizygotic (DZ) twins (13%) favored common placement in the early grades. The individual needs of each twin pair need to be considered by both educational and psychological consultants and parents in formulating school placement decisions. Additional implications for consultation are considered, and future research directions are proposed.
Segal, N. L., & Russell, J. M. (1992). Twins in the classroom: School policy issues and recommendations. Journal of Educational and Psychological Consultation, 3(1), 69-84.
Implications of Twin Research for Legal Issues Involving Young Twins
Nancy L. Segal
California State University, Fullerton, CA
Twin studies have generated numerous findings that have had substantial impact upon research conducted in psychological and medical science disciplines. Twin research has, however, been underutilized as an information base relevant to decision making in the legal domain. Current empirical knowledge of twins and the various legal issues for which they offer implications are reviewed. Selected case studies of young twins, illustrative of a meaningful relationship between twin research and the legal field, are presented. A research program to further address issues raised by the collaborative efforts of twin investigators and legal experts is proposed
Segal, N. L. (1993). Implications of twin research for legal issues involving young twins. Law and Human Behavior, 17(1).
Twin, Sibling, and Adoption Methods
Tests of Evolutionary Hypotheses
Nancy L. Segal
California State University, Fullerton, CA
Twin, sibling, and adoption studies have long been used by behavioral geneticists to identify genetic and environmental influences underlying human behavioral and physical variation. The full potential of these methodologies for unraveling the blend of biological, cultural, and experiential factors affecting human development has been insufficiently appreciated. The application of twin, sibling, and adoption designs for examining hypotheses generated by evolutionary theory is described. Potential contributions from a closer association between these disciplines are underlined.
Segal, N. L. (1993). Twin, sibling, and adoption methods. American Psychologist, 48(9), 943-956
Twin Analysis of Odor Identification
and Perception
Nancy L. Segal, Tari D. Topolski, Steven M. Wilson, Kathleen W. Brown, and Linda Araki
Department of Psychology and Twins Studies Center, California
State University Fullerton, CA and
Department of Psychology, California State University, San Bernardino, CA.
Findings from the twin analysis using the University of Pennsylvania Smell Identification Test (UPSIT) and a phenyl ethyl alcohol (PEA) threshold detection test are presented. A genetic influence on odor identification was suggested for males, but not for females, consistent with previous twin research on physical measures. In addition, females scored significantly higher on the UPSIT than males. A curvilinear age trend for odor identification was detected for males, but not for females. In contrast, neither genetic, age, nor gender effects were suggested for PEA sensitivity. Performance on the olfactory tests was unrelated to scores on standard measures of intelligence.
Segal, N. L., Topolski, T. D., Wilson, S. M., Brown, K. W., & Araki, L. (1994). Twin analysis of odor identification and perception. Physiology and Behavior, 57(3), 605-609.
Behavioral Aspects of Intergenerational Human Cloning: What Twins Tell Us
Nancy L. Segal
California State University, Fullerton, CA
Discussion of behavioral aspects of intergenerational cloning would benefit substantially from reference to the rich psychological literature on experiential aspects of twinning. Researchers representing diverse theoretical and methodological approaches have undertaken analyses of the unique social features of monozygotic (MZ or identical) twins, and experiential differences between MZ and dizygotic (DZ or fraternal) twins. Discussion of human cloning would also benefit from reference to the wealth of twin, family, and adoption studies of human behavioral variation. These investigations of genetic and environmental influences on individual differences in intellectual abilities, personality traits, and physical measures offer insights into average expected resemblance between relatives.
Segal, N. L. (1997). Behavioral aspects of intergenerational human cloning: What twins tell us. Jurimetrics, 38, 57-67
Same-Age Unrelated Siblings: A Unique Test of Within-Family
Environmental Influences on IQ Similarity
Nancy L. Segal
California State University, Fullerton, CA
Pairs of unrelated siblings of the same age, reared together from infancy (UST-SA), uniquely replicate the rearing situations of dizygotic (DZ) twins. These dyads offer a new behavioral-genetic design for examining genetic and environmental influences on behavior. An IQ intraclass correlation of .17, based on 21 UST-SAs, is substantially lower than the correlations of .86, .60, and .50 reported for monozygotic (MZ) twins, DZ twins, and siblings, respectively. This finding supports an explanatory model of intelligence that includes genetic factors. The very modest IQ similarity between UST-SAs, despite their common rearing, suggest that the shared environment has a very small effect on intellectual development and supports the position that individuals respond to environments in ways consistent with their genetic predispositions. The results also challenge some critics' views that the behavioral resemblance of MZ twins is primarily a function of shared experience
Segal, N. L. (1997). Same-age unrelated siblings: A unique test of within-family environmental influences on IQ similarity. Journal of Educational Psychology, 89(2), 381-390.
Decrease in Grief Intensity for Deceased Twin and Non-Twin Relatives: An Evolutionary Perspective
Nancy L. Segal
California State University, Fullerton, CA
Sarah L. Ream
California State University, Fullerton, CA
Based on evolutionary reasoning it was hypothesized that: (1) dizygotic (DZ) twins would show greater reduction in grief intensity than monozygotic (MZ) twins for their deceased co-twins, (2) female twins would show less reduction in grief intensity than male twins for their deceased co-twins, and (3) twins would show less reduction in grief intensity for deceased co-twins than for other deceased relatives. Using a 7-point scale, 175 bereaved twins assessed their grief intensities as recalled 1-2 months after the loss and currently. Decrease in grief intensity was significantly greater for DZ twins than for MZ twins, as expected, although significant gender differences were not found. Participants also evidenced significantly less decrease in grief for deceased co-twins than for other deceased relatives. The implications of these findings for an evolutionary theory of bereavment are discussed.
Segal, N. L., & Ream, S. L. (1998). Decrease in grief intensity for deceased twin and non-twin relatives: an evolutionary perspective. Personality and Individual Differences, 25, 317-325.
Behavioral Genetics and Evolutionary Psychology:
Unified Perspective on Personality Research
Nancy L. Segal
California State University, Fullerton, CA
Kevin B. MacDonald
California State University, Long Beach, CA
Behavioral geneticists and evolutionary psychologists have generally pursued human behavioral analyses with little theoretical or methodological exchange. However, significant benefits might accrue from increased communication between these disciplines. The primary goals of this article are (1) to identify meaningful junctures between behavioral genetic and evolutionary psychology, (2) to describe behavioral genetics and research designs and their applications to evolutionary analyses, and (3) to reassess current personality research in light of behavioral genetic and evolutionary concepts and techniques. The five-factor model of personality is conceptualized as subsuming variation in normative species-typical systems with adaptive functions in the human environment of evolutionary adaptation. Considered as universal evolved mechanisms, personality systems are often seen in dynamic conflict within individuals and as highly compartmentalized in their functioning between settings. However, genetically influenced individual differences in personality may also be understood within an evolutionary framework. Studies in the 0.40-0.50 range with evidence of substantial nonadditive genetic variation and nonshared environmental influences. Evidence indicates that evolutionary theory (e.g., inclusive fitness theory) predicts patterns of social interaction (e.g., cooperation and bereavement) in relatives. Furthermore, variation in personality may constitute a range of viable strategies matching the opportunities available in the complex niche environment of human societies. Within this wide range of viable strategies, personality variation functions as a resource environment for individuals in the sense that personality variation is evaluated according to the interests of the evaluator (e.g., friendships, coalitions, or mate choice).
Segal, N. L, & MacDonald, K. B. (1998). Behavioral genetics and evolutionary psychology: Unified perspective on personality research. Human Biology, 70(2), 159-184.
Cooperation and Competition Between Twins: Findings from a Prisoner's Dilemma Game
Nancy L. Segal
California State University, Fullerton, CA
Scott L. Hershberger
California State University, Long Beach, CA
Attention to factors influencing cooperation and competition during human social interaction has increased within recent years. This study tested the hypothesis that higher levels of cooperation would be associated with increased genetic relatedness between interactants, and explored questions concerning the expression of cooperative behavior over time. A Prisoner's Dilemma game, in which participants elect to display cooperative, competitive, or exploitative behaviors relative to a partner, was administered to 59 monozygotic and 37 dizygotic twin pairs, between 10.92 and 82.67 years of age. Results from multivariate analysis of variance procedures, cross-lag sequential analyses, and hierarchical linear modeling supported associations between zygosity, and frequency and continuity of cooperation. Mechanisms by which zygosity may affect cooperation were explored by examining relationships between response combinations, and twins' IQ similarity and social closeness. The findings are considered with reference to an evolutionary perspective on behavior that offers a theoretical basis for considering how the relative genetic relatedness of social partners affects their social-interactional processes and outcomes. This report is the first in a series of studies designed to address mechanisms underlying differences in cooperation among pairs who vary in average genetic commonality.
Segal, N. L., Hershberger, S. L. (1999). Cooperation and competition between twins: Findings from a Prisoner's Dilemma game. Evolution and Human Behavior, 20, 29-51.