A formal model is presented for the analysis of longitudinal twin data, based on the underlying analysis-of-variance model for repeated measures. The model is developed in terms of the expected values for the variance components representing twin concordance, and the derivation is provided for computing within-pair (intraclass) correlations, and for estimating the percent of variance explained by each component. The procedures are illustrated with physical growth data extending from birth to six years, and concordance estimates are obtained for average size and for the pattern of spurts and lags in growth. A test of significance is also described for comparing monozygotic twins with dizygotic twins. The procedures are particularly useful for assessing chronogenetic influences on development, especially whether the episodes of acceleration and lag occur in parallel for genetically matched twins. The model may be employed with psychological data also.

A method is presented for partitioning the variance associated with human smoking behavior into additive genetic, nonadditive genetic, prenatal environmental, postnatal familial environmental, and postnatal extrafamilial environmental components. Estimations can also be made of additive genetic and residual correlations between spouses and of the correlation between parental additive genetic effect and progeny nonadditive genetic and environmental effect. The variance estimates are free of the biases that might result from these correlations. The statistical genetic analysis is being applied to a large group of MZ and DZ twins, their spouses, and their adult children who live in southern Sweden. Blood samples from each subject will be used to identify their genetic constitution for a number of biochemical polymorphisms, some of which may be considered a priori to have possible relationships to smoking. Associations and genetic linkages between biochemical marker loci and quantitative behavioral traits will be sought. Traits of interest include a wide array of tobacco-use variables, motives for smoking, personality and cognitive variables, and other variables associated with drug use and health. Zygosity determinations based on biochemical polymorphisms have indicated MZ to DZ and DZ to MZ misclassification rates of 0% and 6.15%, respectively, when based solely on external morphology and questionnaire data. The nonpaternity ratio of the fathers with respect to their supposedly biological children is estimated to be 0.28%. Gene frequency estimates for 21 marker loci show that the sample of twins and their relatives is quite representative of the Swedish population at large. All loci were in Hardy-Weinberg-Castle equilibrium, with no evidence of assortative mating for biochemical traits. The MZ twins are significantly more concordant than the DZ twins with respect to whether they have ever had a smoking habit.

Mean twin, triplet, and quadruplet birth rates in Japan from 1951 to 1968 were 6.41, 0.056, and 0.00094 per 1,000, respectively. In 1974 the corresponding figures were 5.83, 0.059, and 0.00329. No quintuplets were born in the former period, but a set was born in the latter year, the rate being 0.47 per million. From 1955 to 1966 the MZ twinning rate increased slightly, but decreased thereafter. This increase was limited to live-born MZ twins, particularly in the higher maternal age groups. The DZ twinning rate declined in the entire period, particularly in higher maternal age groups. This decline appeared to be essentially limited to fetal deaths. Among live births the MZ twinning rate underwent a nearly linear increase with maternal age, whereas the DZ twinning rate attained a mode in the maternal age group 35–39 years. The MZ and the DZ twinning rates among fetal deaths by maternal age had unimodal distributions with modes in maternal age groups 25–29 and 30–34 years, respectively. As to the DZ twinning rate, a geographical cline was noted, with a high rate in the northeast of Japan; the rate was positively correlated with latitude, which also positively correlated with the presence of multiple births among relatives. A negative but nonsignificant correlation was seen between the DZ twinning rate and the proportion of mothers treated with ovulation-inducing hormone. The proportion was higher in mothers of unlike-sexed twins than in those of like-sexed twins and in mothers of triplets than in those of twins. An association between DZ twinning rate and age-specific fertility per married woman is suggested to exist among higher maternal age groups in the northeast part of Japan in earlier years.

Estimation of the twin concordance rate for a disease often requires two stages. First, the disease is ascertained in a population or in a population sample, and such twins as are found with the disease become probands. Second, twin pairs with only one proband are further investigated and additional concordant pairs are thus discovered. A mathematical model is presented that allows for continuous variation in completeness of ascertainment in both stages, for correlation within pairs in the primary ascertainment process, and for correlation within pairs in occurrence of the disease. The concordance rate can be estimated by the proband method if secondary ascertainment is complete; other measures of concordance are accurate only if primary ascertainment is complete. A parameter analogous to the concordance rate but related to correlation in primary ascertainment can be estimated from the same data.

The genetical analysis of covariance structures is used to explore the genetical and environmental intercorrelations of impulsiveness and sensation seeking factors and their conformity to Eysenck's principal personality dimensions. The independent dimensions of psychoticism, extraversion, neuroticism, and lie scale are not found to give a very satisfactory account of the genetical factor structure. In particular, it is clear that impulsiveness and sensation seeking are not simple reflections of extraversion.

The comparison of 622 twin and 622 singleton births allowed us to confirm the association of dizygotic twinning with maternal age, parity, and maternal weight and height, as well as the occurrence of unlike-sex twins in the mother's family. It was also shown that none of these associations, except for maternal height, can be explained by the others. Dizygotic twinning was also found to be negatively correlated with previous use of oral contraceptives and with irregularity of menstrual cycle. Finally, the relationship between spontaneous abortion in previous pregnancies and maternal exposure to medical irradiation was investigated.

The formulas needed in the determination of monozygosity in twin pairs using genetic markers are derived and presented. A general formula for the calculation of concordance probabilities independent of gene frequencies and allele number is derived, enabling either manual computation or computer programming for any Mendelian markers usable in twin zygosity diagnosis.

To search for an association of twin zygosity with tooth size, 56 dental variables measured from 65 pairs of twins (43 MZ, 22 DZ) were studied. Results of the t′ test for equality of the means showed no association of zygosity with any of the variables in males or in females. Results of the F′ test for homogeneity of total variances between zygosities showed evidence for unequal total variances in 15 variables in males and 13 in females. Sex influence was further noted on the association of zygosity with the variance of tooth size. Where total variances were unequal, genetic variance estimates differed when only the within-pair mean squares were used and when combined estimates designed to be unbiased by differences in environmental variances were used.

Following previous formulations of a model of qualitative analysis of twin population data independent of zygosity, a new Bayesian approach has been developed. The present model can be applied to any qualitative genetic trait in twin population data, provided no specific source of variation be introduced by the twin condition, and allows not only estimation of the frequencies of mono- and dizygosity as well as the gene frequencies, but also verification of the trait's mode of inheritance.

Out of a total of 1025 twin births occurring in 11 South Moravian districts from 1972 to 1976, 638 occurred in the same time and space interval (P = 0.00037). The observed clustering suggests the existence of environmental factors influencing twinning rates.

The use of epidemiological studies of twins as a means of determining the existence of genetic factors in various conditions is well known. To be of value, the zygosity of the twin pairs must be identified with as great a degree of confidence as possible. A method of determining twin zygosity, using concordance in general likeness, hair colour, hair type, eye colour, ear form, tongue roll and phenylthiocarbamide (PTC) taste sensitivity, was developed for use in a survey of 244 pairs of like-sexed twins aged 5 to 15 years. The results obtained and the problems raised when applying this method are discussed. The method was simple to apply to a large group of twin pairs, and monozygotic pairs were identified with a degree of confidence greater than 95%.

The dependency of probabilities of phenotypic concordance of gene frequencies in three-allele genetic systems is presented. A graphical display enables the rapid comparison of the relative effectiveness of different systems, taking into account dominance relationships within each genetic system. Four or more allele systems can also be approximated, while two-allele systems are considered to be special cases of three-allele ones.

A mailed questionnaire dealing with similarity in childhood, and how often and by whom the twins were mixed up, was applied to 290 same-sex adult twin pairs. For about 75% of pairs ten genetic markers were analysed. The agreement in zygosity classification between questionnaire and blood typing was high. Irrespective of whether raw scores or discriminant function analysis was applied, the agreement was nearly 95%. When a decision tree was applied, the percentage rose to nearly 96%. Considering that probably four of the twin pairs were wrongly classified as MZ by genetic markers, the percentage rose to 96 and 98, respectively.

Twin methodology for the analysis of continuously distributed traits is presented as a guideline for initial steps in the analysis of twin data. Tests for heterogeneity of twin means and variances are reviewed along with estimates of genetic variance.

Analysis of twin data appears on the surface to be a rather simple task, but investigators may well be confused by the wealth of choices available for hypothesis testing. This paper is designed to help the less experienced investigator, with perhaps a modest amount of data, begin analysis with maximum attention to testing all testable assumptions of the twin model, and using recent examples of applications of the methods.

The dizygotic twinning rate in Western Nigeria is the highest on record (45—50 per mil maternities). It is suggested that, rather than to peculiarities in the population structure or to genetic factors, such a high incidence might be due to the presence in the diet of estrogen-like substances.

A study of sleep and dream characteristics has been carried out by questionnaire on a sample of 77 MZ and 76 DZ same-sex twin pairs of two age groups, 6–8 and 16–18 years. Genetic effects could not be detected in the younger age group and appeared to be rather limited in the older one, possibly as a result of the limited variability of the considered variables and of the levelling influence of the common environment.

More and more genetic markers usable for twin zygosity determination have become available. A relatively small number of markers is sufficient to achieve a satisfactory probability of correctly classifying a twin pair. Previously only the genetic properties of markers have been considered when choosing the markers to be determined. A cost-effectiveness analysis, which considers both genetic properties and relative determination costs of markers, is presented and illustrated with data from the Finnish Twin Registry studies.