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AbstractNick Martin was a doctoral student of mine at the University of Birmingham in the mid 1970s. In this review, I discuss two of Nick's earliest and most seminal contributions to the field of behavior genetics. First, Martin and Eaves' (1977) extension of the model-fitting approach to multivariate data, which laid the theoretical groundwork for a generation of multivariate behavior genetic studies. Second, the Martin et al.'s (1978) manuscript on the power of the classical twin design, which showed that thousands of twin pairs would be required in order to reliably estimate components of variance, and has served as impetus for the formation of large-scale twin registries across the world. I discuss these contributions against the historical backdrop of a time when we and others were struggling with the challenge of figuring out how to incorporate gene-by-environment interaction, gene–environment correlation, mate selection and cultural transmission into more complex genetic models of human behavior.

Background:There continues to be significant investment in the detection of genotype × environment interaction (G × E) in psychiatric genetics. The implications of the method of assessment for the genetic analysis of psychiatric disorders are examined for simulated twin data on symptom scores and environmental covariates.Methods: Additive and independent genetic and environmental risks were simulated for 10,000 monozygotic (MZ) and 10,000 dizygotic (DZ) twin pairs and the 'subjects' administered typical simulated checklists of clinical symptoms and environmental factors. A variety of standard tests for G × E were applied to the simulated additive risk scores, sum scores derived from the checklists and transformed sum scores.Results:All analyses revealed no evidence for G × E for latent risk but marked evidence for G × E and other effects of modulation in the sum scores. These effects were all removed by transformation. An integrated genetic and psychometric model, accounting for both the causes of latent liability and a theory of measurement, was fitted to a sample of the simulated sum-score data and showed that there was no significant modulation of the parameters of the genetic model by environmental covariates (i.e., no G × E).Conclusions:Claims to detect G × E based on analytical methods that ignore the theory of measurement must be subjected to greater scrutiny prior to publication.

AbstractFew papers have attempted to address the potential of extended kinships of twins to resolve the complexities of biological and cultural inheritance in humans. Since the mid-80s these issues have largely been buried beneath the quest for specific genes that contribute to individual differences in complex traits and liability to disease. The articles of Keller et al. and Medland and Keller (current issue) revisit these issues. History will decide whether these new papers represent the final gasp of a paradigm superceded by the 'new' genetics or contain the timely seeds of new birth in the face of a 30-year genetic research program otherwise poised on the cusp of degeneration (Lakatos & Musgrave, 1970).

SummaryThe agreement between zygosity diagnosis in twins by reference to the twins' own answers to written questions and diagnosis based on an objective assessment of zygosity by blood-typing is studied for 178 pairs of same sex twins. Seventy-two female and 106 male twin pairs answered two questions concerning their similarity in appearance and frequency of confusion in childhood, and were blood-grouped using up to 23 different antisera. Of those pairs concordant for all markers and diagnosed as MZ, 98% agreed that they were confused in childhood and that they are alike in physical appearance. Only 55% of those pairs discordant for at least one marker and classified as DZ agreed that they were never confused and are not alike in appearance. The replies of the remaining 45% are spread over all the other combinations of responses. If all twins who agree that they were confused in childhood and are alike in appearance were to be classified as MZ on the basis of the questionnaire alone, and all others as DZ, the diagnoses of only seven pairs out of the total 178 (3–9%) would disagree with those based on blood group data.

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.

ObjectivesContemporary scholarly debate emphasizes the importance of spouse selection on population stratification, typically focusing on the traits of spouses themselves. In this study spouses and their parents were analyzed to resolve the effects of direct spousal assortment from family background assortment on three social traits that spouses correlate the highest: education, church attendance, and political affiliation.MethodsThe data set is comprised of a core of spousal pairs and their parents assessed by self‐report and a more extensive set of individuals on whom there are only ratings by relatives for educational attainment, church attendance, and political preference. Structural equation models were fitted to the observed polychoric correlations by diagonal weighted least squares.ResultsFor education and church attendance, assortment was based primarily on the traits of the spouses themselves, but models including independent assortment for the traits of parents‐in‐law gave a better fit. For political affiliation, assortment based on social background influenced by the traits of both parents gave a better fit.ConclusionsThe findings demonstrate that in humans, spousal similarity may reflect processes of selection and stratification that are more complex than commonly supposed in most models for family resemblance and social diversity.

Scholars have long focused on socio-psychological attachment, elite discourse, and rational action to explain the nature and structure of ideologies. Recently genetic and neurobiological influences have also emerged as predictors of ideological preferences. So far however, there has been little synthesis of these findings into the larger discourse on the structures and manifestations of ideology. The few studies which do include genetic sources of information imply that culture is merely a passenger on a genetic foundation. Here, we test this assumption and in doing so offer a foundation for merging social, psychological, rational, and biological theories of attitude formation and structure. Utilizing a genetically informative sample, we find striking differences between the genetic and environmental factor structures of inter-related attitudes that form ideologies. The structure imposed by social influences corresponds to recognized definitions of liberalism and conservatism on a left–right continuum; however, the genetic factor structure combines liberal attitudes toward sex and reproduction with conservative attitudes toward punishment, defense and immigration. That is, the structure imposed on social and political attitudes by the social environment is a cultural veneer laid on a potentially divergent underlying structure of genetic differences. Our findings should encourage a new understanding of ideology that encompasses genetic, individual, and cultural mechanisms that operates in both conflict and concert depending on local and temporal contexts.

Scholars have long focused on socio-psychological attachment, elite discourse, and rational action to explain the nature and structure of ideologies. Recently genetic and neurobiological influences have also emerged as predictors of ideological preferences. So far however, there has been little synthesis of these findings into the larger discourse on the structures and manifestations of ideology. The few studies which do include genetic sources of information imply that culture is merely a passenger on a genetic foundation. Here, we test this assumption and in doing so offer a foundation for merging social, psychological, rational, and biological theories of attitude formation and structure. Utilizing a genetically informative sample, we find striking differences between the genetic and environmental factor structures of inter-related attitudes that form ideologies. The structure imposed by social influences corresponds to recognized definitions of liberalism and conservatism on a left-right continuum; however, the genetic factor structure combines liberal attitudes toward sex and reproduction with conservative attitudes toward punishment, defense and immigration. That is, the structure imposed on social and political attitudes by the social environment is a cultural veneer laid on a potentially divergent underlying structure of genetic differences. Our findings should encourage a new understanding of ideology that encompasses genetic, individual, and cultural mechanisms that operates in both conflict and concert depending on local and temporal contexts. [Reprinted by permission of Sage Publications Ltd., copyright holder.]