Weinberg's difference method, applied to twin birth statistics, usually shows a dependence of the MZ rate on maternal age, like a thin shadow of the DZ rate. Some of this MZ variation could be explained away by James' finding of more same-sex (SS) than opposite-sex (OS) DZ twins, the excess being mistakenly classified as MZ by Weinberg's assumption of equal numbers. By several methods one can extract a Constant value for the MZ rate and a Constant or nearly Constant value for the DZ SS/OS ratio, but these “constants” are actually arbitrary and they vary between populations.

Dizygotic (DZ) World War II veteran twins who participated in the National Heart Lung and Blood Institute (NHLBI) Twin Study have been reported to have greater variance than monozygotic (MZ) twins for plasma high-density lipoprotein cholesterol (HDL-C), cholesterol in the low-density fraction of HDL (HDL2-C) and apolipoprotein A-I, a major protein component of HDL. It was hypothesized that a possible source of this difference in zygosity variance could be prenatal environmental influences related to placental type. Dermatoglyphics were used to provide a retrospective index of placental type in a subset of the NHLBI MZ twins aged 59-70. The MZ twins classified as dichorionic were found to have significantly greater within-pair variability than the monochorionic MZ twins for HDL-C, HDL2-C and Apo A-I. These findings indicate that intrauterine environmental influences on HDL are manifest later in life.

A sample of 583 twins, delivered in two English hospitals, and divided into groups of like sex and unlike sex, has been analysed as regards distributions, means and standard deviations of mother's age, order of birth, birth weight and length of gestation time.

In birth weight the unlike sex have greater means than the like sex, from which it may be deduced that dizygotic twins are heavier at birth than monozygotic.

The correlation of birth weight in twins is 0.727 for like and 0.676 for unlike sex.

Length of gestation time has a correlation with birth weight of 0.345 for like and 0.403 for unlike sex.

Neonatal mortality (including stillbirths) is high for twins, being 16.6% as compared with 4.5 % in single births. When these rates are sub-divided into groups of birthweight, it is found that the rates for the very low weight groups are a little better for twins than for single births, but those round the mean and above it are about 3 times those for single births of average or above average weight.

The mortality rates are a little lower for twins of unlike sex.

Based on Dutch twin incidence figures since the beginning of the current century, evidence is provided in support of the idea that the DZ unlike/like sexed ratio has gradually shifted (since 1900) from unity to less than unity. Opposing conclusions with regard to the justification of the use of Weingberg's differential rule are very probably correct in themselves but could depend on country and period of birth of the twin sample used. Furthermore, the fast drop and subsequent rise in DZ twinning rate between about 1963 and 1990 can very likely for the greater part be ascribed to a parallel shift in maternal age.

A statistical analysis has been carried out on the frequency of twin and triplet births in Poland in the years 1949-1971.

These frequencies appear to be higher than expected on the basis of Hellin's rule, there being 1 twin birth in 87 single births and 1 triplet birth in 118 twin births.

The share of MZ vs. DZ twins having been assessed through Weinberg's method, an analysis is carried out on maternal age, parity, and urban vs. rural environment. It is concluded that the frequency of polyovulation increases with parity, whereas it becomes reduced with age on account of menopause effect already at age 40.

The mechanism and the familial pattern of the late lepromin reaction (Mitsuda reaction) have been discussed. The interpretation of the familial correlation as mainly depending on a pair of allelic genes has been analysed thoroughly. A new technique has been described for identifying constitutional resistance or susceptibility to leprosy at an in vitro tissular level. The possibilities of this technique have been stressed.

The percentages of left-handed people among the males and females of Andhra Pradesh tribals in India were found to be 15.49 and 7.79, and in Hindus 6.9 and 4.65. The difference between the sexes among tribals is significant. The tribal people studied were Koya Doras, Sugalis {or Lambadis), and Konda Reddis. It is suggested that similar quantitative studies of left-handedness should be made in other tribal and aboriginal populations before they are culturally conditioned to right-handedness.

The study of developmental processes in twins furnishes a powerful resource for examining the role of gene-action systems in guiding the course of growth [2,9].

While there is a steady and rapid progression from birth onward, the growth rate is not entirely uniform for a given child, but rather moves in episodes of acceleration and lag. The timing of the growth spurts follows a distinctive pattern for each child, and consequently a child who may be smaller than average at one age may then enter a phase of rapid growth, and ultimately catch up with or surpass his peers at a later age.

The effect of such individualized patterns of growth is that many children may change in relative size from one age to the next; and in this sense it may appear that the underlying developmental processes are erratic, rather than coherent. But if there is an underlying ground plan, a chronogenetic pattern, then the distinctive developmental gradients should unfold in synchrony for twins sharing the same genetic make-up. Episodes of acceleration and lag in growth would then occur in parallel for both twins and would presumably represent the activity of timed gene-action systems, which switch on and off according to a predetermined plan.

Weight and height growth has been examined in 400 twins and 229 singletons born 1975-1985. Periodical measurements have been taken from 0 to 7 years of age. As compared to values obtained in a previous study of twins and singletons born 1960-1974, though limited to the first year of age, twins appear to continue to do worse than singletons in their growth. However, height values appear to have become higher in the first year of age in both twins and singletons born 1975-1985.

A human thoracopagus of slightly unequal twins having a common umbilical cord with exomphalos is described, with special reference to the cardiovascular system. The larger twin had single umbilical artery. The smaller twin had a large left umbilical artery which was in direct continuity with the aorta, a small right umbilical artery with sole connection to the right external and internal iliac arteries (right common iliac artery was absent), missing umbilical vein, a rudimentary nonfunctional heart with atresia of the truncus arteriosus, and a right-sided aortic arch. Exomphalos of both twins is accounted by the possible defective growth of umbilical mesoderm caused by abnormal umbilical vessels. Reversal of systemic circulation in the upper part of the body of smaller twin is discussed. The importance of a careful study of umbilical veins is emphasized.

The formula H′C = 1 − CDZ/CMZ is suggested as a better summary of twin concordance data than the familiar Holzinger concordance formula, HC = (CMZ − CDZ)/ (1 − CDZ). The new formula better estimates degree of genetic determination, G, as calculated from a threshold model, but never exceeds unity, as G sometimes does. For high concordance rates, if CMZ + CDZ > 1, HC, may be more useful than either G or H′C.

In 1875, Sir Francis Galton was the first to propose that by comparing the concordance of a specific trait or disorder in monozygotic (MZ) twins (assumed to be genetically identical) and dizygotic (DZ) twins (which were assumed to be as similar or as different as any other pair of sibs), it would be possible to distinguish between environmental and heritable effects. DZ twins are derived from the fertilization of two eggs by two sperm. MZ twins are the result of the fertilization of one egg by one sperm that then divides to form two embryos.

Genetic research has made amazing progress since 1875. Advances in molecular DNA techniques and developmental genetics have made it possible to differentiate unequivocally between MZ and DZ twins [1, 14]. It is in this new light that this manuscript will review the recent knowledge about twins and the twinning process.

The incidence of DZ twins differs between population [11, 18]. A DZ twin birth in North America is estimated to occur 9–11 times in every 1000 live births or 1 in 100 births; i.e. 1 in every 50 people is a DZ twin.

The DZ twinning rate is closely related to maternal age, parity, height, weight and gonadotropin levels. An increased DZ twinning rate is seen with increasing maternal age and peaks around 35–39 years; higher parity is also associated with a higher DZ twinning rate [19]. Tall heavy women are more likely to give birth to DZ twins than short thin women. A higher incidence of DZ twins has also been reported with the use of new reproductive techniques.

As you all know, there is a growing concern with the Negro problem in America. I will look at one small aspect of that problem today: the possible hereditary difference in ability between Negro and White.

Fig. 1 is just one of many examples that could be chosen. It shows the extent of such differences in ability for different years, for male and female separately, between Negro and White students admitted to the University of Georgia, as reported by Hills (cf. Stanley and Porter, 1967). Ability was measured by the verbal score on the Scholastic Aptitude Test — a test widely used by college admission officers in the USA for selection of students.

Fig. 2 shows a comparison of the fertility of Negroes and Whites, indicating that less able Negroes have the highest fertility, while more able ones have lower fertility (Bogue, 1969).

Should the combined effect of these two factors (i.e., lower IQ,'s and greater fertility of low IQ, Negroes) lead us to worry about a gradual decline of the national level of intelligence in the USA? I believe not, and I will present some evidence which supports the thesis that the observed lower intelligence in US Negroes is due to an interaction between innate potential and unfavorable environmental conditions. If, as I believe, social conditions for the Negro are improving and if their greater fertility will level off as social conditions improve, we will not need to be concerned about a national decline in ability.

The twinning rate remained nearly Constant up to 1968, then decreased in 1974 and gradually increased with the year. The triplet birth rate remained nearly Constant up to 1974, then increased up to 1982, where the rate was 1.8 times higher than in 1968, and decreased thereafter. On the other hand, the quadruplet birth rate remained nearly Constant up to 1968, was eightfold in 1975, then decreased until 1984 and suddenly increased to 8 per million births in 1985. The higher multiple birth rates since 1974 was attributed to the higher proportion of mothers treated with ovulation-inducing hormones in Japan. The stillbirth rates decreased to 2/5 for male twins and to 1/3 for female twins during the 25-year period from 1960, to 2/5 for triplets and to 1/5 for quadruplets during the 34-year period from 1951.

Taste thresholds for phenylthiourea were measured among 58 Caucasoid subjects affected with both tuberculosis and leprosy. The frequency of non-tasters among them (12.1%) is significantly lower than that found among healthy controls (24.7%) but not different from the frequency of non-tasters among tuberculous patients (14.1%) or leprous patients (19.6%). These results suggest a correlation between taste ability to phenylthiourea and the constitutional susceptibility to tuberculosis and leprosy.

We observed persistent first trimester growth disparity in a twin pregnancy following IVF-ET. The crown-rump length of the two fetuses was substantially different at 7 and 11 weeks and from the 20th week discordant growth was observed by intertwin differences in abdominal circumferences and estimated fetal weights. Birth weight discordance was 26.6% (1600/2180). This is apparently the first documentation of first trimester growth discordance persisting throughout pregnancy.

Many studies of embryogenesis and fate of twin pregnancies are invalidated because zygosity is not determined definitively, or is assumed on the basis of inadequate criteria. This paper briefly reviews methods of zygosity determination. It reports published results and a new series of twins in which zygosity was determined by DNA fingerprinting. Implications for methods of prenatal diagnosis of zygosity are discussed in the context of the occasional need for intervention in twin transfusion syndrome or in twins discordant for major malformations. Definitive zygosity and placental anatomy (number of chorions and amnions) is discussed as the firm substrate for studies of normal and abnormal twin development.

Over the last decades the frequency of multiple births has been on the decline in most developed countries. This trend, however, has been restricted to dizygotic twins, while monozygotic rates have remained stable or risen slowly. In more recent years, however, the fall in multiple dizygotic birth rates has ceased and a slight increase is observed. This trend and the increased frequency of triplets or higher-order births, registered from the early 1980s onwards, are essentially related to treatments for infertility. No single risk factor, such as maternal age, parity, oral contraceptive use, or declining fertility rates can explain the overall declining trends. In this paper we review the descriptive epidemiology and the main risk factors for multiple pregnancies.

Twinning rates in Nigeria are much higher than in Europe and USA, and appear to be highest in its western and eastern parts. The twinning incidence in a representative population in a town in Western Nigeria, Igbo-Ora, is approximately four times that in Europe and USA.

Zygosity determination has shown that only 8.6% of newborn twin pairs in Ibadan are MZ, compared to approximately 30% in Caucasian populations. Studies in Igbo-Ora have also given similar results, confirming the view that high twinning incidence in Nigeria is due to a very high DZ twinning rate, the MZ rate being the same as in Europe and USA.

The proportion of twins with monochorionic placentae in Ibadan is about 5%, against approximately 20% in UK and USA. The twinning rates in the two populations are 45‰ and 11‰ respectively. This inverse relation (between the twinning rate and the proportion of monochorionic placentae) may be used in calculating the twinning incidence in a population in which the proportion of twins with monochorionic placentation is known.

Findings from an earlier phase of this research project clearly pointed to the mother's psycological adaptation inherent to the specificities of triadic interaction. Mothers' attitudes were classified in a typology ranging from “early twinness” where the two babies are treated as though they were a single unit, to attempts to create two dyadic relationships. The current program deals with the effects of the type of parent-child relationship at one year on the emergence of twin bonding. Sixty-eight families of twins (26MZ, 24DZ same sex, 18DZ different sex) are followed up from birth to the age of 3 using a method based on clinical interviews, videotaped observations, and questionnaires. The data cover mothers' rearing attitudes towards organization of babycare and outward signs of twinness, maternal representations of the relationship between the twins and observation of the children's interaction in a standardized game situation. The findings are discussed in terms of zygosity, and parental SCS.