For the geneticist, or for anyone else, to make a speech in Jerusalem means a cultural effort that is extraordinary. Like every other researcher, the geneticist must locate his scientific investigations in the context of a truth which embraces nature and the history of man. Those of us who are concerned with twin studies, however, cannot simply leave our concrete and specific field to deal with theoretical themes; rather, we must extract from our research these concepts that bridge the gap between the particular truths we seek and that total truth, which man desires in order to resolve the mystery which surrounds his existence.

I would like to submit to the distinguished personalities who are honoring our Congress with their presence, as well as to my very dear colleagues, the idea that the best project for creating a link between our field of interest and the other interests of modern culture would be to survey the state of our knowledge about the personality of the human twin.

Therefore, I will not talk about the application of the method of twin studies, but about the identical twin, the monozygotic (MZ) twin individual. That is, I will not talk about those dizygotic (DZ) twins like Esau and Jacob, who were different. DZ twins are, of course, real twins, and about twice as frequent as the MZ ones. But I will refer only to the twin who has another twin identical to himself, and who represents the classical model of the phenomenon of twins.

Analysis of the multiple correlations in body height within a sample of 117 monozygotic twin pairs and their spouses confirmed the existence of a high degree of assortative mating. However, the data also revealed an underlying asymmetry in the mate selection process. With respect to height, male twins were more selective than were members of female pairs, and among male twin pairs, those who were intermediate in height appeared to make the greatest contribution to the observed pattern of nonrandom mating. Because of their magnitude and potential asymmetry, the effects of phenotypic correlations between the spouses of related individuals must be correctly specified in any rigorous quantitative genetic analysis that extends beyond the nuclear family unit.

The determination of twin zygosity by bloodtyping is reconsidered, and the model for the individual case is reformulated. The crucial diagnostic question may be phrased as follows: Given the particular array of bloodgroup phenotypes that the twins display and are concordant for, how might this array have been obtained by a pair of dizygotic twins, and how might the array have been obtained by a monozygotic pair? The solution yields a differential probability value that is uniquely tailored to the actual phenotype array shown. The procedure offers a coherent and more direct method for arriving at the needed probability figures, and it is recommended to supersede previous methods. Some similarities and differences between the methods are discussed.

For 173 pairs of like-sex adult twins, self-assessment of zygosity was verified by laboratory diagnosis. Seventeen percent of twins who were very likely monozygous (MZ) believed themselves dizygous (DZ), frequently citing two placentas at their delivery as “evidence.” We suggest that twins be asked what leads them to their assessment of their own zygosity. For 93% of Caucasian and 89% of American Black like-sex twins in our sample, DZ twins could be differentiated based on six polymorphic markers retrievable from frozen sera. MZ twins who believe themselves DZ can be considered “environmentally DZ, genetically MZ” twins, and might be used to study genetic and environmental influences on the treatment of twins and on twins' choices of social characteristics.

A total of 22 monozygotic (MZ) and 41 dizygotic (DZ) twin pairs were given 72 trials on a stabilometer balance task over six days to study the extent of the genetic contribution to learning and performance of a gross motor skill. The expectations that interindividual differences would be less for the MZ than for the DZ twins and that intraindividual variability would not be different between the two groups were supported. Intraclass correlations were used to provide estimates for the proportions of total phenotypic variance accounted for by heritability (h2), systematic environmental variance (E2), and nonsystematic environmental effects (e2). Heritability was found to be low during the early stages of learning, before it increased to stabilize at approximately 65% for the remaining practice. E2 was highest during these early stages (24%), then declined quickly to stabilize at half that level. Error variance (e2) constituted the remaining variance. Learning profiles of the twin pairs were also analyzed, with a greater intrapair resemblance being found for the MZ twins. The present findings indicate that, for gross motor skills, there is considerable potential for influencing both the levels of performance (and learning) and the differences between individuals by judicious use of systematic environmental effects.

Twin concordance rates are usually reported without reference to the number of parents affected, apparently because the simple demonstration that monozygotic (MZ) twins are more concordant than are dizygotic (DZ) twins is the goal of most twin studies. Depending on the underlying mechanism, however, twin concordance rates can vary widely when conditioned on the number of parents affected. For the generalized single-locus model it is shown that conditional concordance rates in monozygotic twins, along with an estimate of the disorder's prevalence in the population, uniquely specify the underlying parameters of this important model. Knowledge of the exact parameter set is essential for competent genetic counseling.

Systolic blood pressure, standardized for age and sex, was measured in 76 MZ twin pairs and their 341 children. Maximum likelihood estimation of genetical and environmental parameters from the independent parental and offspring ANOVAs indicated a complete absence of both maternal and shared environmental effects, together with a heritability of 63%. These results are shown to be reasonably consistent with those from previous studies.

There are several dynamic influences on anterior fontanelle development in infants; among them, brain growth, dural attachments, suture development, and osteogenesis. It thus seems reasonable to hypothesize that variation in anterior fontanelle development between infants, related and unrelated, might have a significant genetic component. Anterior fontanelle size was quantitated by the method of Popich and Smith for 94 monozygotic (MZ) and 187 dizygotic (DZ) four-month-old twin pairs. The general model for estimating genetic variance from quantitative twin data was applied to MZ and DZ twins and then separately by chorion type. Since there were significant mean differences between blacks and whites, races were analyzed separately. The within-pair mean square estimates of genetic variance (ĜWT) were highly significant for both blacks (P < 0.02) and whites (P < 0.002). Comparisons of means, total variances, and among-pair mean squares within races revealed no heterogeneity. There were also no significant chorion effects. Since the anterior fontanelle closes at around 1–1½ years of age, it was evaluated at age one in 95 MZ and 194 DZ twin pairs as a qualitative trait – ie, open vs closed, concordance vs discordance. There were no significant differences in proband concordance rates between MZ and DZ twin pairs for either blacks (P > 0.5) or whites (P > 0.10). Again, there were no significant chorion effects. These data suggest that anterior fontanelle developmental variation has a significant genetic variance component at four months of age but not at one year. This finding may be related to the rapid brain growth witnessed between birth and eight to nine months of age.

A sample of 119 pairs of twins, together with their 87 siblings and 100 pairs of parents, were examined with respect to susceptibility to the motion sickness syndrome. Children were found to exhibit resistance to the syndrome below two years of age, and females were more susceptible than males. The incidence appeared to be highest in families with both parents affected. Concordance rates were much higher in MZ (100%) than in DZ pairs (27.6%). Genetic effects are likely, but cultural factors should not be ignored.

The co-twin control model using paired-comparison analysis provides an extremely powerful and cost-effective experimental design for clinical research. This paper suggests the possibility of using sequential analysis to further increase the efficiency and specificity of cotwin control studies.