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.

Biometrical genetical techniques have been applied to the analysis of certain anthropometric characters measured in 134 pairs of adult twins. After allowing for assortative mating it appears that there is a family environment (E2) component for variation in height larger than previously reported. “Fatness” traits – weight, ponderal index, and skinfold thickness – all show higher heritabilities in males and substantial E2 components in females, and reasons for this are discussed. The same is true for cephalic index and forearm length but the reason for these differences is not so obvious. Head length shows a much higher heritability than head breadth. A larger sample of DZ opposite-sex pairs would allow more powerful discrimination, but the variety of patterns of variation revealed by the model-fitting approach used here justify its use over more traditional techniques.

A method for the estimation of genetic correlations based upon analysis of genetic marker phenotypes is presented. At a given marker locus, the probability of observing a pair of individuals with a specific combination of phenotypes can be expressed as a function of the gene frequencies at that locus and the genetic correlation (R) between that pair. The likelihood of obtaining a sample of n such pairs with their phenotypes at m marker loci can be expressed as a product of nm such functions. From the likelihood function, maximum likelihood estimates of R can be obtained, and hypotheses about R may be tested. A sample of Swedish twin families (61 dizygotic twin pairs, 268 husband-wife pairs, and 164 sib pairs) were analyzed by this method using information from 21 markers. It was found that for the twin pairs, R = 0.458, which was significantly different from the R calculated for sib pairs (R = 0.5 58) but not significantly different from the expected 0.5. For the husband-wife pairs, it was found that R = 0.086, which did differ significantly from the expected value of 0, indicating the presence of nonrandom mating in this population.

The inheritance of General Neurotic, Obsessive, and Impulsive Hysterical personality traits has been studied in a sample of 260 female and male adult same-sexed twins. At least one of the twins in each pair had been treated for neurotic or borderline disorders. The results showed that the General neurotic and the obsessive personality factor had a significantly but moderately high hereditary component. The same was also true for more than half of the 17 separate personality scales. No personality scales emerged as highly hereditary. Therefore, each item was analyzed separately, and items classified as either distinctly hereditary or environmental were placed in each of these two groups. The items of the two groups were separately factor-analyzed, and three hereditary and three environmental main factors emerged. The hereditary factors seemed to represent a basic core in the three personality factors of the total questionnaire, whereas the environmental factors could be explained as derivatives of early representation of the basic hereditary core, influenced by familial and cultural patterns.

As a part of a study of the dietary habits of monozygotic twins, a nutrition survey including a 24-hour dietary recall interview and a three-day dietary diary was conducted on the members of 15 male and 13 female pairs of identical Caucasian twins ranging from 25 to 61 years of age. In this study, overall mean caloric, protein, fat, and carbohydrate intake of males was significantly greater than females. Members of male twin pairs also tended to be more similar than female twins in mean intake of all major nutrient groups. In general, the observed differences in overall intake between males and females were maintained even after adjustment of body weight. Male twins were characterized by greater similarity in intake of sodium and potassium; however, there was no consistent difference in the degree of correlation in intakes of members of male and female twins for either iron, calcium or phosphorus.

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.

Longitudinal study of 514 pairs of US white male twin veterans was made possible by compiling data from military records, mailed questionnaires, and physical examinations. Height and weight were used to calculate the Quetelet index for twins at different ages covering induction into the Armed Forces (generally 17–24 years), 25 years, 40–50 years at questionnaire response, and 42–56 at physical examination. Cross-sectional analyses of Quetelet Index at these 4 points in time showed consistent evidence of significant genetic variance. Additional twin analyses of change in Quetelet Index both from induction and from age 25 to approximately age 48 were done. Both analyses showed significant genetic variance for change in Quetelet Index during this time span of 25 or more years for most subjects. Heritability estimates were 0.6 for change in Quetelet Index from induction (mean age 20) to examination (mean age 48) and 0.7 for change in Quetelet Index from age 25 to examination. While studies of younger twins have shown significant genetic influence on growth (height and weight), this study of adults demonstrates significant genetic influence on change in weight even after maturation.

Fifteen hundred palmar prints of normal persons and patients suffering from various diseases were studied and classified as to dermal ridge configurations at the base of digit IV, after the exclusion of main line C courses with endings at one of the palmar marginal regions. This classification scheme, based on the three classical reduction forms of line C (O,X(8),x), resulted in the differentiation of three main groups, A, B, and C. Within these main groups, pattern types with strong similarity were combined to subgroups allowing an easier documentation. Twin and family data were used to test whether the described pattern types, besides the classical abortive states O,X and x of line C (called special and intermediate forms), are heritable and for which of them a remarkable reduction tendency could be established. The results allow us to confirm the main grouping and the assumed reduction tendency of the special forms of group A.

A high heritability estimate was obtained for serum arylesterase activity level in a series of 40 male twin pairs (23 MZ, 17 DZ) aged 33–39 years. The heritability estimated for Cholinesterase activity was strikingly lower. In unrelated individuals a positive correlation between arylesterase, Cholinesterase, and various lipoprotein parameters appeared to support the suggestion that these enzymes interact with various lipoproteins at the molecular level. The nature of these associations is not known. It is, however, tempting to speculate that some of the hereditary influence on lipoprotein levels in man may be mediated through the genetic control of one or both of these enzymes.

Stillbirth rates of MZ and DZ twins in Japan gradually decreased during the period 1960–1967 and in 1974. The stillbirth rates of MZ and DZ twins were 0.270 and 0.224 in 1960, respectively, whereas the corresponding figures in 1974 were 0.135 and 0.099. The stillbirth rate was higher in MZ than DZ twins, in males than females, and in the second- than in the first-born. In both zygosities, the rates were higher in the first birth order than in the second, and then increased with birth order except in the sixth birth or more for MZ twins and for DZ twins born to mothers aged 35–39 years. The shorter the gestational age, the higher the stillbirth rate in both MZ and DZ twins, as expected. Mean gestational age was slightly shorter in MZ than in DZ twins. The decrease of the stillbirth rates can be explained partly by the reduction of twins with higher birth order, where high stillbirth rates of MZ and DZ twins are seen, and the reduction of older mothers.

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.

Neither the percentage in the previous use of contraceptive pills nor the distribution of the time interval between the discontinuation of the pill and conception has shown any significant difference in twins and control singletons. Twin conception within one month after discontinuation of the norethynodrel type pill, however, was significantly more frequent, whereas after the use of the linestrenol type, it was lower.

Twin data on bipolar and unipolar affective disorders are analyzed by multiple threshold models of inheritance. The two illness types are represented in the models on a continuum of genetic–environmental liability in which bipolar illness has a higher liability threshold than unipolar disorder. Autosomal single major locus model provides an acceptable fit to observed concordance rates in monozygotic twins. The multifactorial-polygenic model is rejected.

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.